[ikiwiki.git] / docs / docs / newhandbook / serial_communications / index.mdwn

## Chapter 18 Serial Communications 


## 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 transmittedDTE:: Data Terminal Equipment -- for example, your computerDCE:: Data Communications Equipment -- your modemRS-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.



***