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## Chapter 18 Serial Communications 
[[!toc  levels=3]]

## Synopsis 



UNIX® has always had support for serial communications. In fact, the very first UNIX machines relied on serial lines for user input and output. Things have changed a lot from the days when the average ***terminal*** consisted of a 10-character-per-second serial printer and a keyboard. This chapter will cover some of the ways in which DragonFly uses serial communications.



After reading this chapter, you will know:




* How to connect terminals to your DragonFly system.


* How to use a modem to dial out to remote hosts.


* How to allow remote users to login to your system with a modem.


* How to boot your system from a serial console.



Before reading this chapter, you should:




* Know how to configure and install a new kernel ([kernelconfig.html Chapter 10]).


* Understand UNIX permissions and processes ([basics.html Chapter 3]).


* Have access to the technical manual for the serial hardware (modem or multi-port card) that you would like to use with DragonFly.


***


## 18.1 Introduction 



### 18.1.1 Terminology 



bps:: Bits per Second -- the rate at which data is transmitted;

DTE:: Data Terminal Equipment -- for example, your computer;

DCE:: Data Communications Equipment -- your modem;

RS-232:: EIA standard for hardware serial communications.



When talking about communications data rates, this section does not use the term ***baud***. Baud refers to the number of electrical state transitions that may be made in a period of time, while ***bps*** (bits per second) is the ***correct*** term to use (at least it does not seem to bother the curmudgeons quite as much).



### 18.1.2 Cables and Ports 



To connect a modem or terminal to your DragonFly system, you will need a serial port on your computer and the proper cable to connect to your serial device. If you are already familiar with your hardware and the cable it requires, you can safely skip this section.



#### 18.1.2.1 Cables 



There are several different kinds of serial cables. The two most common types for our purposes are null-modem cables and standard (***straight***) RS-232 cables. The documentation for your hardware should describe the type of cable required.



##### 18.1.2.1.1 Null-modem Cables 



A null-modem cable passes some signals, such as ***signal ground***, straight through, but switches other signals. For example, the ***send data*** pin on one end goes to the ***receive data*** pin on the other end.



If you like making your own cables, you can construct a null-modem cable for use with terminals. This table shows the RS-232C signal names and the pin numbers on a DB-25 connector.



[[!table  data="""
| Signal | Pin # |  | Pin # | Signal 
 SG | 7 | connects to | 7 | SG 
 TxD | 2 | connects to | 3 | RxD 
 RxD | 3 | connects to | 2 | TxD 
 RTS | 4 | connects to | 5 | CTS 
 CTS | 5 | connects to | 4 | RTS 
 DTR | 20 | connects to | 6 | DSR 
 DCD | 8 |  | 6 | DSR 
 DSR | 6 | connects to | 20 | DTR |

"""]]

 **Note:** Connect ***Data Set Ready*** (DSR) and ***Data Carrier Detect*** (DCD) internally in the connector hood, and then to ***Data Terminal Ready*** (DTR) in the remote hood.



##### 18.1.2.1.2 Standard RS-232C Cables 



A standard serial cable passes all the RS-232C signals straight-through. That is, the ***send data*** pin on one end of the cable goes to the ***send data*** pin on the other end. This is the type of cable to use to connect a modem to your DragonFly system, and is also appropriate for some terminals.



#### 18.1.2.2 Ports 



Serial ports are the devices through which data is transferred between the DragonFly host computer and the terminal. This section describes the kinds of ports that exist and how they are addressed in DragonFly.



##### 18.1.2.2.1 Kinds of Ports 



Several kinds of serial ports exist. Before you purchase or construct a cable, you need to make sure it will fit the ports on your terminal and on the DragonFly system.



Most terminals will have DB25 ports. Personal computers, including PCs running DragonFly, will have DB25 or DB9 ports. If you have a multiport serial card for your PC, you may have RJ-12 or RJ-45 ports.



See the documentation that accompanied the hardware for specifications on the kind of port in use. A visual inspection of the port often works too.



##### 18.1.2.2.2 Port Names 



In DragonFly, you access each serial port through an entry in the `/dev` directory. There are two different kinds of entries:




* Call-in ports are named `/dev/ttyd`***N****** where `***N***` is the port number, starting from zero. Generally, you use the call-in port for terminals. Call-in ports require that the serial line assert the data carrier detect (DCD) signal to work correctly.


* Call-out ports are named `/dev/cuaa`***N******. You usually do not use the call-out port for terminals, just for modems. You may use the call-out port if the serial cable or the terminal does not support the carrier detect signal.



If you have connected a terminal to the first serial port (`COM1` in MS-DOS®), then you will use `/dev/ttyd0` to refer to the terminal. If the terminal is on the second serial port (also known as `COM2`), use `/dev/ttyd1`, and so forth.



### 18.1.3 Kernel Configuration 



DragonFly supports four serial ports by default. In the MS-DOS world, these are known as `COM1`, `COM2`, `COM3`, and `COM4`. DragonFly currently supports ***dumb*** multiport serial interface cards, such as the BocaBoard 1008 and 2016, as well as more intelligent multi-port cards such as those made by Digiboard and Stallion Technologies. However, the default kernel only looks for the standard COM ports.



To see if your kernel recognizes any of your serial ports, watch for messages while the kernel is booting, or use the `/sbin/dmesg` command to replay the kernel's boot messages. In particular, look for messages that start with the characters `sio`.



 **Tip:** To view just the messages that have the word `sio`, use the command:



    

    # /sbin/dmesg | grep 'sio'





For example, on a system with four serial ports, these are the serial-port specific kernel boot messages:



    

    sio0 at 0x3f8-0x3ff irq 4 on isa

    sio0: type 16550A

    sio1 at 0x2f8-0x2ff irq 3 on isa

    sio1: type 16550A

    sio2 at 0x3e8-0x3ef irq 5 on isa

    sio2: type 16550A

    sio3 at 0x2e8-0x2ef irq 9 on isa

    sio3: type 16550A





If your kernel does not recognize all of your serial ports, you will probably need to configure a custom DragonFly kernel for your system. For detailed information on configuring your kernel, please see [kernelconfig.html Chapter 10].



The relevant device lines for your kernel configuration file would look like this:



    

    device              sio0    at isa? port IO_COM1 irq 4

    device              sio1    at isa? port IO_COM2 irq 3

    device              sio2    at isa? port IO_COM3 irq 5

    device              sio3    at isa? port IO_COM4 irq 9





You can comment-out or completely remove lines for devices you do not have. Please refer to the [sio(4)](http://leaf.dragonflybsd.org/cgi/web-man?command#sio&section4) manual page for more information on serial ports and multiport boards configuration.



 **Note:** `port IO_COM1` is a substitution for `port 0x3f8`, `IO_COM2` is `0x2f8`, `IO_COM3` is `0x3e8`, and `IO_COM4` is `0x2e8`, which are fairly common port addresses for their respective serial ports; interrupts 4, 3, 5, and 9 are fairly common interrupt request lines. Also note that regular serial ports ***cannot*** share interrupts on ISA-bus PCs (multiport boards have on-board electronics that allow all the 16550A's on the board to share one or two interrupt request lines).



### 18.1.4 Device Special Files 



Most devices in the kernel are accessed through ***device special files***, which are located in the `/dev` directory. The `sio` devices are accessed through the `/dev/ttyd`***N****** (dial-in) and `/dev/cuaa`***N****** (call-out) devices. DragonFly also provides initialization devices (`/dev/ttyid`***N****** and `/dev/cuaia`***N******) and locking devices (`/dev/ttyld`***N****** and `/dev/cuala`***N******). The initialization devices are used to initialize communications port parameters each time a port is opened, such as `crtscts` for modems which use `RTS/CTS` signaling for flow control. The locking devices are used to lock flags on ports to prevent users or programs changing certain parameters; see the manual pages [termios(4)](http://leaf.dragonflybsd.org/cgi/web-man?command#termios&section4), [sio(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=sio&section=4), and [stty(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=stty&section=1) for information on the terminal settings, locking and initializing devices, and setting terminal options, respectively.



#### 18.1.4.1 Making Device Special Files 



A shell script called `MAKEDEV` in the `/dev` directory manages the device special files. To use `MAKEDEV` to make dial-up device special files for `COM1` (port 0), `cd` to `/dev` and issue the command `MAKEDEV ttyd0`. Likewise, to make dial-up device special files for `COM2` (port 1), use `MAKEDEV ttyd1`.



`MAKEDEV` not only creates the `/dev/ttyd`***N****** device special files, but also the `/dev/cuaa`***N******, `/dev/cuaia`***N******, `/dev/cuala`***N******, `/dev/ttyld`***N******, and `/dev/ttyid`***N****** nodes.



After making new device special files, be sure to check the permissions on the files (especially the `/dev/cua*` files) to make sure that only users who should have access to those device special files can read and write on them -- you probably do not want to allow your average user to use your modems to dial-out. The default permissions on the `/dev/cua*` files should be sufficient:



    

    crw-rw----    1 uucp     dialer    28, 129 Feb 15 14:38 /dev/cuaa1

    crw-rw----    1 uucp     dialer    28, 161 Feb 15 14:38 /dev/cuaia1

    crw-rw----    1 uucp     dialer    28, 193 Feb 15 14:38 /dev/cuala1





These permissions allow the user `uucp` and users in the group `dialer` to use the call-out devices.



### 18.1.5 Serial Port Configuration 



The `ttyd`***N****** (or `cuaa`***N******) device is the regular device you will want to open for your applications. When a process opens the device, it will have a default set of terminal I/O settings. You can see these settings with the command



    

    # stty -a -f /dev/ttyd1





When you change the settings to this device, the settings are in effect until the device is closed. When it is reopened, it goes back to the default set. To make changes to the default set, you can open and adjust the settings of the ***initial state*** device. For example, to turn on `CLOCAL` mode, 8 bit communication, and `XON/XOFF` flow control by default for `ttyd5`, type:



    

    # stty -f /dev/ttyid5 clocal cs8 ixon ixoff





System-wide initialization of the serial devices is controlled in `/etc/rc.serial`. This file affects the default settings of serial devices.



To prevent certain settings from being changed by an application, make adjustments to the ***lock state*** device. For example, to lock the speed of `ttyd5` to 57600 bps, type:



    

    # stty -f /dev/ttyld5 57600





Now, an application that opens `ttyd5` and tries to change the speed of the port will be stuck with 57600 bps.



Naturally, you should make the initial state and lock state devices writable only by the `root` account.

***

## 18.2 Terminals 


Terminals provide a convenient and low-cost way to access your DragonFly system when you are not at the computer's console or on a connected network. This section describes how to use terminals with DragonFly.



### 18.2.1 Uses and Types of Terminals 



The original UNIX® systems did not have consoles. Instead, people logged in and ran programs through terminals that were connected to the computer's serial ports. It is quite similar to using a modem and terminal software to dial into a remote system to do text-only work.



Today's PCs have consoles capable of high quality graphics, but the ability to establish a login session on a serial port still exists in nearly every UNIX style operating system today; DragonFly is no exception. By using a terminal attached to an unused serial port, you can log in and run any text program that you would normally run on the console or in an `xterm` window in the X Window System.



For the business user, you can attach many terminals to a DragonFly system and place them on your employees' desktops. For a home user, a spare computer such as an older IBM PC or a Macintosh® can be a terminal wired into a more powerful computer running DragonFly. You can turn what might otherwise be a single-user computer into a powerful multiple user system.



For DragonFly, there are three kinds of terminals:


* [ Dumb terminals](term.html#TERM-DUMB)


* [ PCs acting as terminals](term.html#TERM-PCS)


* [ X terminals](term.html#TERM-X)


The remaining subsections describe each kind.



#### 18.2.1.1 Dumb Terminals 



Dumb terminals are specialized pieces of hardware that let you connect to computers over serial lines. They are called ***dumb*** because they have only enough computational power to display, send, and receive text. You cannot run any programs on them. It is the computer to which you connect them that has all the power to run text editors, compilers, email, games, and so forth.



There are hundreds of kinds of dumb terminals made by many manufacturers, including Digital Equipment Corporation's VT-100 and Wyse's WY-75. Just about any kind will work with DragonFly. Some high-end terminals can even display graphics, but only certain software packages can take advantage of these advanced features.



Dumb terminals are popular in work environments where workers do not need access to graphical applications such as those provided by the X Window System.



#### 18.2.1.2 PCs Acting as Terminals 



If a [ dumb terminal](term.html#TERM-DUMB) has just enough ability to display, send, and receive text, then certainly any spare personal computer can be a dumb terminal. All you need is the proper cable and some ***terminal emulation*** software to run on the computer.



Such a configuration is popular in homes. For example, if your spouse is busy working on your DragonFly system's console, you can do some text-only work at the same time from a less powerful personal computer hooked up as a terminal to the DragonFly system.



#### 18.2.1.3 X Terminals 



X terminals are the most sophisticated kind of terminal available. Instead of connecting to a serial port, they usually connect to a network like Ethernet. Instead of being relegated to text-only applications, they can display any X application.



We introduce X terminals just for the sake of completeness. However, this chapter does ***not*** cover setup, configuration, or use of X terminals.



### 18.2.2 Configuration 



This section describes what you need to configure on your DragonFly system to enable a login session on a terminal. It assumes you have already configured your kernel to support the serial port to which the terminal is connected--and that you have connected it.



Recall from [boot.html Chapter 7] that the `init` process is responsible for all process control and initialization at system startup. One of the tasks performed by `init` is to read the `/etc/ttys` file and start a `getty` process on the available terminals. The `getty` process is responsible for reading a login name and starting the `login` program.



Thus, to configure terminals for your DragonFly system the following steps should be taken as `root`:



  1. Add a line to `/etc/ttys` for the entry in the `/dev` directory for the serial port if it is not already there.

  1. Specify that `/usr/libexec/getty` be run on the port, and specify the appropriate `***getty***` type from the `/etc/gettytab` file.

  1. Specify the default terminal type.

  1. Set the port to ***on.***

  1. Specify whether the port should be ***secure.***

  1. Force `init` to reread the `/etc/ttys` file.



As an optional step, you may wish to create a custom `***getty***` type for use in step 2 by making an entry in `/etc/gettytab`. This chapter does not explain how to do so; you are encouraged to see the [gettytab(5)](http://leaf.dragonflybsd.org/cgi/web-man?command#gettytab&section5) and the [getty(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=getty&section=8) manual pages for more information.



#### 18.2.2.1 Adding an Entry to `/etc/ttys` 



The `/etc/ttys` file lists all of the ports on your DragonFly system where you want to allow logins. For example, the first virtual console `ttyv0` has an entry in this file. You can log in on the console using this entry. This file also contains entries for the other virtual consoles, serial ports, and pseudo-ttys. For a hardwired terminal, just list the serial port's `/dev` entry without the `/dev` part (for example, `/dev/ttyv0` would be listed as `ttyv0`).



A default DragonFly install includes an `/etc/ttys` file with support for the first four serial ports: `ttyd0` through `ttyd3`. If you are attaching a terminal to one of those ports, you do not need to add another entry.



 **Example 17-1. Adding Terminal Entries to `/etc/ttys`** 



Suppose we would like to connect two terminals to the system: a Wyse-50 and an old 286 IBM PC running  **Procomm**  terminal software emulating a VT-100 terminal. We connect the Wyse to the second serial port and the 286 to the sixth serial port (a port on a multiport serial card). The corresponding entries in the `/etc/ttys` file would look like this:



    

    ttyd1./imagelib/callouts/1.png  "/usr/libexec/getty std.38400"./imagelib/callouts/2.png  wy50./imagelib/callouts/3.png  on./imagelib/callouts/4.png  insecure./imagelib/callouts/5.png

    ttyd5   "/usr/libexec/getty std.19200"  vt100  on  insecure

    





[ ./imagelib/callouts/1.png](term.html#CO-TTYS-LINE1COL1):: The first field normally specifies the name of the terminal special file as it is found in `/dev`.[ ./imagelib/callouts/2.png](term.html#CO-TTYS-LINE1COL2):: The second field is the command to execute for this line, which is usually [getty(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#getty&section8). `getty` initializes and opens the line, sets the speed, prompts for a user name and then executes the [login(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=login&section=1) program.The `getty` program accepts one (optional) parameter on its command line, the `***getty***` type. A `***getty***` type configures characteristics on the terminal line, like bps rate and parity. The `getty` program reads these characteristics from the file `/etc/gettytab`.The file `/etc/gettytab` contains lots of entries for terminal lines both old and new. In almost all cases, the entries that start with the text `std` will work for hardwired terminals. These entries ignore parity. There is a `std` entry for each bps rate from 110 to 115200. Of course, you can add your own entries to this file. The [gettytab(5)](http://leaf.dragonflybsd.org/cgi/web-man?command=gettytab&section=5) manual page provides more information.When setting the `***getty***` type in the `/etc/ttys` file, make sure that the communications settings on the terminal match.For our example, the Wyse-50 uses no parity and connects at 38400 bps. The 286 PC uses no parity and connects at 19200 bps.[ ./imagelib/callouts/3.png](term.html#CO-TTYS-LINE1COL3):: The third field is the type of terminal usually connected to that tty line. For dial-up ports, `unknown` or `dialup` is typically used in this field since users may dial up with practically any type of terminal or software. For hardwired terminals, the terminal type does not change, so you can put a real terminal type from the [termcap(5)](http://leaf.dragonflybsd.org/cgi/web-man?command=termcap&section=5) database file in this field.For our example, the Wyse-50 uses the real terminal type while the 286 PC running  **Procomm**  will be set to emulate at VT-100.[ ./imagelib/callouts/4.png](term.html#CO-TTYS-LINE1COL4):: The fourth field specifies if the port should be enabled. Putting `on` here will have the `init` process start the program in the second field, `getty`. If you put `off` in this field, there will be no `getty`, and hence no logins on the port.[ ./imagelib/callouts/5.png](term.html#CO-TTYS-LINE1COL5):: The final field is used to specify whether the port is secure. Marking a port as secure means that you trust it enough to allow the `root` account (or any account with a user ID of 0) to login from that port. Insecure ports do not allow `root` logins. On an insecure port, users must login from unprivileged accounts and then use [su(1)](http://leaf.dragonflybsd.org/cgi/web-man?command=su&section=1) or a similar mechanism to gain superuser privileges.It is highly recommended that you use ***insecure*** even for terminals that are behind locked doors. It is quite easy to login and use `su` if you need superuser privileges.



#### 18.2.2.2 Force `init` to Reread `/etc/ttys` 



After making the necessary changes to the `/etc/ttys` file you should send a SIGHUP (hangup) signal to the `init` process to force it to re-read its configuration file. For example:



    

    # kill -HUP 1





 **Note:** `init` is always the first process run on a system, therefore it will always have PID 1.



If everything is set up correctly, all cables are in place, and the terminals are powered up, then a `getty` process should be running on each terminal and you should see login prompts on your terminals at this point.



### 18.2.3 Troubleshooting Your Connection 



Even with the most meticulous attention to detail, something could still go wrong while setting up a terminal. Here is a list of symptoms and some suggested fixes.



#### 18.2.3.1 No Login Prompt Appears 



Make sure the terminal is plugged in and powered up. If it is a personal computer acting as a terminal, make sure it is running terminal emulation software on the correct serial port.



Make sure the cable is connected firmly to both the terminal and the DragonFly computer. Make sure it is the right kind of cable.



Make sure the terminal and DragonFly agree on the bps rate and parity settings. If you have a video display terminal, make sure the contrast and brightness controls are turned up. If it is a printing terminal, make sure paper and ink are in good supply.



Make sure that a `getty` process is running and serving the terminal. For example, to get a list of running `getty` processes with `ps`, type:



    

    # ps -axww|grep getty





You should see an entry for the terminal. For example, the following display shows that a `getty` is running on the second serial port `ttyd1` and is using the `std.38400` entry in `/etc/gettytab`:



    

    22189  d1  Is+    0:00.03 /usr/libexec/getty std.38400 ttyd1





If no `getty` process is running, make sure you have enabled the port in `/etc/ttys`. Also remember to run `kill -HUP 1` after modifying the `ttys` file.



If the `getty` process is running but the terminal still does not display a login prompt, or if it displays a prompt but will not allow you to type, your terminal or cable may not support hardware handshaking. Try changing the entry in `/etc/ttys` from `std.38400` to `3wire.38400` remember to run `kill -HUP 1` after modifying `/etc/ttys`). The `3wire` entry is similar to `std`, but ignores hardware handshaking. You may need to reduce the baud rate or enable software flow control when using `3wire` to prevent buffer overflows.



#### 18.2.3.2 If Garbage Appears Instead of a Login Prompt 


Make sure the terminal and DragonFly agree on the bps rate and parity settings. Check the `getty` processes to make sure the correct `***getty***` type is in use. If not, edit `/etc/ttys` and run `kill -HUP 1`.



#### 18.2.3.3 Characters Appear Doubled, the Password Appears When Typed 



Switch the terminal (or the terminal emulation software) from ***half duplex*** or ***local echo*** to ***full duplex.***

***

## 18.3 Dial-in Service 


Configuring your DragonFly system for dial-in service is very similar to connecting terminals except that you are dealing with modems instead of terminals.



### 18.3.1 External vs. Internal Modems 



External modems seem to be more convenient for dial-up, because external modems often can be semi-permanently configured via parameters stored in non-volatile RAM and they usually provide lighted indicators that display the state of important RS-232 signals. Blinking lights impress visitors, but lights are also very useful to see whether a modem is operating properly.



Internal modems usually lack non-volatile RAM, so their configuration may be limited only to setting DIP switches. If your internal modem has any signal indicator lights, it is probably difficult to view the lights when the system's cover is in place.



#### 18.3.1.1 Modems and Cables 



If you are using an external modem, then you will of course need the proper cable. A standard RS-232C serial cable should suffice as long as all of the normal signals are wired:




* Transmitted Data (SD)


* Received Data (RD)


* Request to Send (RTS)


* Clear to Send (CTS)


* Data Set Ready (DSR)


* Data Terminal Ready (DTR)


* Carrier Detect (CD)


* Signal Ground (SG)



DragonFly needs the RTS and CTS signals for flow-control at speeds above 2400 bps, the CD signal to detect when a call has been answered or the line has been hung up, and the DTR signal to reset the modem after a session is complete. Some cables are wired without all of the needed signals, so if you have problems, such as a login session not going away when the line hangs up, you may have a problem with your cable.



Like other UNIX® like operating systems, DragonFly uses the hardware signals to find out when a call has been answered or a line has been hung up and to hangup and reset the modem after a call. DragonFly avoids sending commands to the modem or watching for status reports from the modem. If you are familiar with connecting modems to PC-based bulletin board systems, this may seem awkward.



### 18.3.2 Serial Interface Considerations 



DragonFly supports NS8250-, NS16450-, NS16550-, and NS16550A-based EIA RS-232C (CCITT V.24) communications interfaces. The 8250 and 16450 devices have single-character buffers. The 16550 device provides a 16-character buffer, which allows for better system performance. (Bugs in plain 16550's prevent the use of the 16-character buffer, so use 16550A's if possible). Because single-character-buffer devices require more work by the operating system than the 16-character-buffer devices, 16550A-based serial interface cards are much preferred. If the system has many active serial ports or will have a heavy load, 16550A-based cards are better for low-error-rate communications.



### 18.3.3 Quick Overview 



As with terminals, `init` spawns a `getty` process for each configured serial port for dial-in connections. For example, if a modem is attached to `/dev/ttyd0`, the command `ps ax` might show this:



    

     4850 ??  I      0:00.09 /usr/libexec/getty V19200 ttyd0





When a user dials the modem's line and the modems connect, the CD (Carrier Detect) line is reported by the modem. The kernel notices that carrier has been detected and completes `getty`'s open of the port. `getty` sends a login: prompt at the specified initial line speed. `getty` watches to see if legitimate characters are received, and, in a typical configuration, if it finds junk (probably due to the modem's connection speed being different than `getty`'s speed), `getty` tries adjusting the line speeds until it receives reasonable characters.



After the user enters his/her login name, `getty` executes `/usr/bin/login`, which completes the login by asking for the user's password and then starting the user's shell.



### 18.3.4 Configuration Files 



There are three system configuration files in the `/etc` directory that you will probably need to edit to allow dial-up access to your DragonFly system. The first, `/etc/gettytab`, contains configuration information for the `/usr/libexec/getty` daemon. Second, `/etc/ttys` holds information that tells `/sbin/init` what `tty` devices should have `getty` processes running on them. Lastly, you can place port initialization commands in the `/etc/rc.serial` script.



There are two schools of thought regarding dial-up modems on UNIX. One group likes to configure their modems and systems so that no matter at what speed a remote user dials in, the local computer-to-modem RS-232 interface runs at a locked speed. The benefit of this configuration is that the remote user always sees a system login prompt immediately. The downside is that the system does not know what a user's true data rate is, so full-screen programs like Emacs will not adjust their screen-painting methods to make their response better for slower connections.



The other school configures their modems' RS-232 interface to vary its speed based on the remote user's connection speed. For example, V.32bis (14.4 Kbps) connections to the modem might make the modem run its RS-232 interface at 19.2 Kbps, while 2400 bps connections make the modem's RS-232 interface run at 2400 bps. Because `getty` does not understand any particular modem's connection speed reporting, `getty` gives a login: message at an initial speed and watches the characters that come back in response. If the user sees junk, it is assumed that they know they should press the Enter key until they see a recognizable prompt. If the data rates do not match, `getty` sees anything the user types as ***junk***, tries going to the next speed and gives the login: prompt again. This procedure can continue ad nauseam, but normally only takes a keystroke or two before the user sees a good prompt. Obviously, this login sequence does not look as clean as the former ***locked-speed*** method, but a user on a low-speed connection should receive better interactive response from full-screen programs.



This section will try to give balanced configuration information, but is biased towards having the modem's data rate follow the connection rate.



#### 18.3.4.1 `/etc/gettytab` 



`/etc/gettytab` is a [termcap(5)](http://leaf.dragonflybsd.org/cgi/web-man?command#termcap&section5)-style file of configuration information for [getty(8)](http://leaf.dragonflybsd.org/cgi/web-man?command=getty&section=8). Please see the [gettytab(5)](http://leaf.dragonflybsd.org/cgi/web-man?command=gettytab&section=5) manual page for complete information on the format of the file and the list of capabilities.



##### 18.3.4.1.1 Locked-speed Config 



If you are locking your modem's data communications rate at a particular speed, you probably will not need to make any changes to `/etc/gettytab`.



##### 18.3.4.1.2 Matching-speed Config 



You will need to set up an entry in `/etc/gettytab` to give `getty` information about the speeds you wish to use for your modem. If you have a 2400 bps modem, you can probably use the existing `D2400` entry.



    

    #

    # Fast dialup terminals, 2400/1200/300 rotary (can start either way)

    #

    D2400|d2400|Fast-Dial-2400:\

            :nx#D1200:tc2400-baud:

    3|D1200|Fast-Dial-1200:\

            :nx#D300:tc1200-baud:

    5|D300|Fast-Dial-300:\

            :nx#D2400:tc300-baud:





If you have a higher speed modem, you will probably need to add an entry in `/etc/gettytab`; here is an entry you could use for a 14.4 Kbps modem with a top interface speed of 19.2 Kbps:



    

    #

    # Additions for a V.32bis Modem

    #

    um|V300|High Speed Modem at 300,8-bit:\

            :nx#V19200:tcstd.300:

    un|V1200|High Speed Modem at 1200,8-bit:\

            :nx#V300:tcstd.1200:

    uo|V2400|High Speed Modem at 2400,8-bit:\

            :nx#V1200:tcstd.2400:

    up|V9600|High Speed Modem at 9600,8-bit:\

            :nx#V2400:tcstd.9600:

    uq|V19200|High Speed Modem at 19200,8-bit:\

            :nx#V9600:tcstd.19200:





This will result in 8-bit, no parity connections.



The example above starts the communications rate at 19.2 Kbps (for a V.32bis connection), then cycles through 9600 bps (for V.32), 2400 bps, 1200 bps, 300 bps, and back to 19.2 Kbps. Communications rate cycling is implemented with the `nx#` (***next table***) capability. Each of the lines uses a `tc` (***table continuation***) entry to pick up the rest of the ***standard*** settings for a particular data rate.



If you have a 28.8 Kbps modem and/or you want to take advantage of compression on a 14.4 Kbps modem, you need to use a higher communications rate than 19.2 Kbps. Here is an example of a `gettytab` entry starting a 57.6 Kbps:



    

    #

    # Additions for a V.32bis or V.34 Modem

    # Starting at 57.6 Kbps

    #

    vm|VH300|Very High Speed Modem at 300,8-bit:\

            :nx#VH57600:tcstd.300:

    vn|VH1200|Very High Speed Modem at 1200,8-bit:\

            :nx#VH300:tcstd.1200:

    vo|VH2400|Very High Speed Modem at 2400,8-bit:\

            :nx#VH1200:tcstd.2400:

    vp|VH9600|Very High Speed Modem at 9600,8-bit:\

            :nx#VH2400:tcstd.9600:

    vq|VH57600|Very High Speed Modem at 57600,8-bit:\

            :nx#VH9600:tcstd.57600:





If you have a slow CPU or a heavily loaded system and do not have 16550A-based serial ports, you may receive ***`sio`*** ***silo*** errors at 57.6 Kbps.



#### 18.3.4.2 `/etc/ttys` 



Configuration of the `/etc/ttys` file was covered in [ Example 17-1](term.html#EX-ETC-TTYS). Configuration for modems is similar but we must pass a different argument to `getty` and specify a different terminal type. The general format for both locked-speed and matching-speed configurations is:



    

    ttyd0   "/usr/libexec/getty `***xxx***`"   dialup on





The first item in the above line is the device special file for this entry -- `ttyd0` means `/dev/ttyd0` is the file that this `getty` will be watching. The second item, `"/usr/libexec/getty `***xxx***`"` (`***xxx***` will be replaced by the initial `gettytab` capability) is the process `init` will run on the device. The third item, `dialup`, is the default terminal type. The fourth parameter, `on`, indicates to `init` that the line is operational. There can be a fifth parameter, `secure`, but it should only be used for terminals which are physically secure (such as the system console).



The default terminal type (`dialup` in the example above) may depend on local preferences. `dialup` is the traditional default terminal type on dial-up lines so that users may customize their login scripts to notice when the terminal is `dialup` and automatically adjust their terminal type. However, the author finds it easier at his site to specify `vt102` as the default terminal type, since the users just use VT102 emulation on their remote systems.



After you have made changes to `/etc/ttys`, you may send the `init` process a HUP signal to re-read the file. You can use the command



    

    # kill -HUP 1





 to send the signal. If this is your first time setting up the system, you may want to wait until your modem(s) are properly configured and connected before signaling `init`.



##### 18.3.4.2.1 Locked-speed Config 



For a locked-speed configuration, your `ttys` entry needs to have a fixed-speed entry provided to `getty`. For a modem whose port speed is locked at 19.2 Kbps, the `ttys` entry might look like this:



    

    ttyd0   "/usr/libexec/getty std.19200"   dialup on





If your modem is locked at a different data rate, substitute the appropriate value for `std.`***speed****** instead of `std.19200`. Make sure that you use a valid type listed in `/etc/gettytab`.



##### 18.3.4.2.2 Matching-speed Config 



In a matching-speed configuration, your `ttys` entry needs to reference the appropriate beginning ***auto-baud*** (sic) entry in `/etc/gettytab`. For example, if you added the above suggested entry for a matching-speed modem that starts at 19.2 Kbps (the `gettytab` entry containing the `V19200` starting point), your `ttys` entry might look like this:



    

    ttyd0   "/usr/libexec/getty V19200"   dialup on





#### 18.3.4.3 `/etc/rc.serial` 



High-speed modems, like V.32, V.32bis, and V.34 modems, need to use hardware (`RTS/CTS`) flow control. You can add `stty` commands to `/etc/rc.serial` to set the hardware flow control flag in the DragonFly kernel for the modem ports.



For example to set the `termios` flag `crtscts` on serial port #1's (`COM2`) dial-in and dial-out initialization devices, the following lines could be added to `/etc/rc.serial`:



    

    # Serial port initial configuration

    stty -f /dev/ttyid1 crtscts

    stty -f /dev/cuaia1 crtscts





### 18.3.5 Modem Settings 



If you have a modem whose parameters may be permanently set in non-volatile RAM, you will need to use a terminal program (such as Telix under MS-DOS® or `tip` under DragonFly) to set the parameters. Connect to the modem using the same communications speed as the initial speed `getty` will use and configure the modem's non-volatile RAM to match these requirements:




* CD asserted when connected


* DTR asserted for operation; dropping DTR hangs up line and resets modem


* CTS transmitted data flow control


* Disable XON/XOFF flow control


* RTS received data flow control


* Quiet mode (no result codes)


* No command echo



Please read the documentation for your modem to find out what commands and/or DIP switch settings you need to give it.



For example, to set the above parameters on a U.S. Robotics® Sportster® 14,400 external modem, one could give these commands to the modem:



    

    ATZ

    AT&C1&D2&H1&I0&R2&W





You might also want to take this opportunity to adjust other settings in the modem, such as whether it will use V.42bis and/or MNP5 compression.



The U.S. Robotics Sportster 14,400 external modem also has some DIP switches that need to be set; for other modems, perhaps you can use these settings as an example:




* Switch 1: UP -- DTR Normal


* Switch 2: N/A (Verbal Result Codes/Numeric Result Codes)


* Switch 3: UP -- Suppress Result Codes


* Switch 4: DOWN -- No echo, offline commands


* Switch 5: UP -- Auto Answer


* Switch 6: UP -- Carrier Detect Normal


* Switch 7: UP -- Load NVRAM Defaults


* Switch 8: N/A (Smart Mode/Dumb Mode)



Result codes should be disabled/suppressed for dial-up modems to avoid problems that can occur if `getty` mistakenly gives a login: prompt to a modem that is in command mode and the modem echoes the command or returns a result code. This sequence can result in a extended, silly conversation between `getty` and the modem.



#### 18.3.5.1 Locked-speed Config 



For a locked-speed configuration, you will need to configure the modem to maintain a constant modem-to-computer data rate independent of the communications rate. On a U.S. Robotics Sportster 14,400 external modem, these commands will lock the modem-to-computer data rate at the speed used to issue the commands:



    

    ATZ

    AT&B1&W





#### 18.3.5.2 Matching-speed Config 



For a variable-speed configuration, you will need to configure your modem to adjust its serial port data rate to match the incoming call rate. On a U.S. Robotics Sportster 14,400 external modem, these commands will lock the modem's error-corrected data rate to the speed used to issue the commands, but allow the serial port rate to vary for non-error-corrected connections:
    

    ATZ

    AT&B2&W


#### 18.3.5.3 Checking the Modem's Configuration 



Most high-speed modems provide commands to view the modem's current operating parameters in a somewhat human-readable fashion. On the U.S. Robotics Sportster 14,400 external modems, the command `ATI5` displays the settings that are stored in the non-volatile RAM. To see the true operating parameters of the modem (as influenced by the modem's DIP switch settings), use the commands `ATZ` and then `ATI4`.



If you have a different brand of modem, check your modem's manual to see how to double-check your modem's configuration parameters.



### 18.3.6 Troubleshooting 



Here are a few steps you can follow to check out the dial-up modem on your system.



#### 18.3.6.1 Checking Out the DragonFly System 



Hook up your modem to your DragonFly system, boot the system, and, if your modem has status indication lights, watch to see whether the modem's DTR indicator lights when the login: prompt appears on the system's console -- if it lights up, that should mean that DragonFly has started a `getty` process on the appropriate communications port and is waiting for the modem to accept a call.



If the DTR indicator does not light, login to the DragonFly system through the console and issue a `ps ax` to see if DragonFly is trying to run a `getty` process on the correct port. You should see lines like these among the processes displayed:



    

      114 ??  I      0:00.10 /usr/libexec/getty V19200 ttyd0

      115 ??  I      0:00.10 /usr/libexec/getty V19200 ttyd1




If you see something different, like this:


    

      114 d0  I      0:00.10 /usr/libexec/getty V19200 ttyd0





and the modem has not accepted a call yet, this means that `getty` has completed its open on the communications port. This could indicate a problem with the cabling or a mis-configured modem, because `getty` should not be able to open the communications port until CD (carrier detect) has been asserted by the modem.



If you do not see any `getty` processes waiting to open the desired `ttyd`***N****** port, double-check your entries in `/etc/ttys` to see if there are any mistakes there. Also, check the log file `/var/log/messages` to see if there are any log messages from `init` or `getty` regarding any problems. If there are any messages, triple-check the configuration files `/etc/ttys` and `/etc/gettytab`, as well as the appropriate device special files `/dev/ttydN`, for any mistakes, missing entries, or missing device special files.



#### 18.3.6.2 Try Dialing In 



Try dialing into the system; be sure to use 8 bits, no parity, and 1 stop bit on the remote system. If you do not get a prompt right away, or get garbage, try pressing Enter about once per second. If you still do not see a login: prompt after a while, try sending a `BREAK`. If you are using a high-speed modem to do the dialing, try dialing again after locking the dialing modem's interface speed (via `AT&B1` on a U.S. Robotics Sportster modem, for example).


If you still cannot get a login: prompt, check `/etc/gettytab` again and double-check that



* The initial capability name specified in `/etc/ttys` for the line matches a name of a capability in `/etc/gettytab`


* Each `nx=` entry matches another `gettytab` capability name


* Each `tc=` entry matches another `gettytab` capability name


If you dial but the modem on the DragonFly system will not answer, make sure that the modem is configured to answer the phone when DTR is asserted. If the modem seems to be configured correctly, verify that the DTR line is asserted by checking the modem's indicator lights (if it has any).



If you have gone over everything several times and it still does not work, take a break and come back to it later. If it still does not work, perhaps you can send an electronic mail message to the [DragonFly User related mailing list](http://leaf.dragonflybsd.org/mailarchive/) describing your modem and your problem, and the good folks on the list will try to help.

***