Configuring the Serial Hardware

The Internet is growing at an incredible rate. Much of this growth is attributed to Internet users who can't afford high-speed permanent network connections and who use protocols such as SLIP, PPP, or UUCP to dial in to a network provider to retrieve their daily dose of email and news.

This chapter is intended to help all people who rely on modems to maintain their link to the outside world. We won't cover the mechanics of how to configure your modem (the manual that came with it will tell you more about it than we can), but we will cover most of the Linux-specific aspects of managing devices that use serial ports. Topics include serial communications software, creating the serial device files, serial hardware, and configuring serial devices using the setserial and stty commands. Many other related topics are covered in the Serial HOWTO by David Lawyer.[1]

1. Communications Software for Modem Links

There are a number of communications packages available for Linux. Many of these packages are terminal programs, which allow a user to dial in to another computer as if she were sitting in front of a simple terminal. The traditional terminal program for Unix-like environments is kermit. It is, however, fairly ancient now, and would probably be considered difficult to use. There are more comfortable programs available that support features, like telephone-dialing dictionaries, script languages to automate dialing and logging in to remote computer systems, and a variety of file exchange protocols. One of these programs is minicom, which was modeled after some of the most popular DOS terminal programs. X11 users are accommodated, too. seyon is a fully featured X11-based communications program.

Terminal programs aren't the only type of serial communication programs available. Other programs let you connect to a host and download news and email in a single bundle, to read and reply later at your leisure. This can save a lot of time, and is especially useful if you are unfortunate enough to live in an area where your local calls are time-charged. All of the reading and replying time can be spent offline, and when you are ready, you can redial and upload your responses in a single bundle. This all consumes a bit more hard disk because all of the messages have to be stored to your disk before you can read them, but this could be a reasonable trade-off at today's hard drive prices.

UUCP epitomizes this communication software style. It is a program suite that copies files from one host to another and executes programs on a remote host. It is frequently used to transport mail or news in private networks. Ian Taylor's UUCP package, which also runs under Linux, is described in detail in Chapter 16. Other noninteractive communications software is used throughout networks such as Fidonet. Fidonet application ports like ifmail are also available, although we expect that not many people still use them.

PPP and SLIP are in between, allowing both interactive and noninteractive use. Many people use PPP or SLIP to dial in to their campus network or other Internet Service Provider to run FTP and read web pages. PPP and SLIP are also, however, commonly used over permanent or semipermanent connections for LAN-to-LAN coupling, although this is really only interesting with ISDN or other high-speed network connections.

2. Introduction to Serial Devices

The Unix kernel provides devices for accessing serial hardware, typically called tty devices (pronounced as it is spelled: T-T-Y). This is an abbreviation for Teletype device, which used to be one of the major manufacturers of terminal devices in the early days of Unix. The term is used now for any character-based data terminal. Throughout this chapter, we use the term to refer exclusively to the Linux device files rather than the physical terminal.

Linux provides three classes of tty devices: serial devices, virtual terminals (all of which you can access in turn by pressing Alt-F1 through Alt-Fnn on the local console), and pseudo-terminals (similar to a two-way pipe, used by applications such as X11). The former were called tty devices because the original character-based terminals were connected to the Unix machine by a serial cable or telephone line and modem. The latter two were named after the tty device because they were created to behave in a similar fashion from the programmer's perspective.

SLIP and PPP are most commonly implemented in the kernel. The kernel doesn't really treat the tty device as a network device that you can manipulate like an Ethernet device, using commands such as ifconfig. However, it does treat tty devices as places where network devices can be bound. To do this, the kernel changes what is called the "line discipline" of the tty device. Both SLIP and PPP are line disciplines that may be enabled on tty devices. The general idea is that the serial driver handles data given to it differently, depending on the line discipline it is configured for. In its default line discipline, the driver simply transmits each character it is given in turn. When the SLIP or PPP line discipline is selected, the driver instead reads a block of data, wraps a special header around it that allows the remote end to identify that block of data in a stream, and transmits the new data block. It isn't too important to understand this yet; we'll cover both SLIP and PPP in later chapters, and it all happens automatically for you anyway.

3. Accessing Serial Devices

Like all devices in a Unix system, serial ports are accessed through device special files, located in the /dev directory. There are two varieties of device files related to serial drivers, and there is one device file of each type for each port. The device will behave slightly differently, depending on which of its device files we open. We'll cover the differences because it will help you understand some of the configurations and advice that you might see relating to serial devices, but in practice you need to use only one of these. At some point in the future, one of them may even disappear completely.

The most important of the two classes of serial device has a major number of 4, and its device special files are named ttyS0, ttyS1, etc. The second variety has a major number of 5, and was designed for use when dialing out (calling out) through a port; its device special files are called cua0, cua1, etc. In the Unix world, counting generally starts at zero, while laypeople tend to start at one. This creates a small amount of confusion for people because COM1: is represented by /dev/ttyS0, COM2: by /dev/ttyS1, etc. Anyone familiar with IBM PC-style hardware knows that COM3: and greater were never really standardized anyway.

The cua, or "callout," devices were created to solve the problem of avoiding conflicts on serial devices for modems that have to support both incoming and outgoing connections. Unfortunately, they've created their own problems and are now likely to be discontinued. Let's briefly look at the problem.

Linux, like Unix, allows a device, or any other file, to be opened by more than one process simultaneously. Unfortunately, this is rarely useful with tty devices, as the two processes will almost certainly interfere with each other. Luckily, a mechanism was devised to allow a process to check if a tty device had already been opened by another device before opening it. The mechanism uses what are called lock files. The idea was that when a process wanted to open a tty device, it would check for the existence of a file in a special location, named similarly to the device it intends to open. If the file does not exist, the process creates it and opens the tty device. If the file does exist, the process assumes another process already has the tty device open and takes appropriate action. One last clever trick to make the lock file management system work was writing the process ID (pid) of the process that had created the lock file into the lock file itself; we'll talk more about that in a moment.

The lock file mechanism works perfectly well in circumstances in which you have a defined location for the lock files and all programs know where to find them. Alas, this wasn't always the case for Linux. It wasn't until the Linux Filesystem Standard defined a standard location for lock files when tty lock files began to work correctly. At one time there were at least four, and possibly more locations chosen by software developers to store lock files: /usr/spool/locks/, /var/spool/locks/, /var/lock/, and /usr/lock/. Confusion caused chaos. Programs were opening lock files in different locations that were meant to control a single tty device; it was as if lock files weren't being used at all.

The cua devices were created to provide a solution to this problem. Rather than relying on the use of lock files to prevent clashes between programs wanting to use the serial devices, it was decided that the kernel could provide a simple means of arbitrating who should be given access. If the ttyS device were already opened, an attempt to open the cua would result in an error that a program could interpret to mean the device was already being used. If the cua device were already open and an attempt was made to open the ttyS, the request would block; that is, it would be put on hold and wait until the cua device was closed by the other process. This worked quite well if you had a single modem that you had configured for dial-in access and you occasionally wanted to dial out on the same device. But it did not work very well in environments where you had multiple programs wanting to call out on the same device. The only way to solve the contention problem was to use lock files! Back to square one.

Suffice it to say that the Linux Filesystem Standard came to the rescue and now mandates that lock files be stored in the /var/lock directory, and that by convention, the lock file name for the ttyS1 device, for instance, is LCK..ttyS1. The cua lock files should also go in this directory, but use of cua devices is now discouraged.

The cua devices will probably still be around for some time to provide a period of backward compatibility, but in time they will be retired. If you are wondering what to use, stick to the ttyS device and make sure that your system is Linux FSSTND compliant, or at the very least that all programs using the serial devices agree on where the lock files are located. Most software dealing with serial tty devices provides a compile-time option to specify the location of the lock files. More often than not, this will appear as a variable called something like LOCKDIR in the Makefile or in a configuration header file. If you're compiling the software yourself, it is best to change this to agree with the FSSTND-specified location. If you're using a precompiled binary and you're not sure where the program will write its lock files, you can use the following command to gain a hint:

If the location found does not agree with the rest of your system, you can try creating a symbolic link from the lock directory that the foreign executable wants to use back to /var/lock/. This is ugly, but it will work.

3.1. The Serial Device Special Files

Minor numbers are identical for both types of serial devices. If you have your modem on one of the ports COM1: through COM4:, its minor number will be the COM port number plus 63. If you are using special serial hardware, such as a high-performance multiple port serial controller, you will probably need to create special device files for it; it probably won't use the standard device driver. The Serial-HOWTO should be able to assist you in finding the appropriate details.

Assume your modem is on COM2:. Its minor number will be 65, and its major number will be 4 for normal use. There should be a device called ttyS1 that has these numbers. List the serial ttys in the /dev/ directory. The fifth and sixth columns show the major and minor numbers, respectively:

If there is no device with major number 4 and minor number 65, you will have to create one. Become the superuser and type:

The various Linux distributions use slightly differing strategies for who should own the serial devices. Sometimes they will be owned by root, and other times they will be owned by another user, such as uucp in our example. Modern distributions have a group specifically for dial-out devices, and any users who are allowed to use them are added to this group.

Some people suggest making /dev/modem a symbolic link to your modem device so that casual users don't have to remember the somewhat unintuitive ttyS1. However, you cannot use modem in one program and the real device file name in another. Their lock files would have different names and the locking mechanism wouldn't work.

Serial Hardware

RS-232 is currently the most common standard for serial communications in the PC world. It uses a number of circuits for transmitting single bits, as well as for synchronization. Additional lines may be used for signaling the presence of a carrier (used by modems) and for handshaking. Linux supports a wide variety of serial cards that use the RS-232 standard.

Hardware handshake is optional, but very useful. It allows either of the two stations to signal whether it is ready to receive more data, or if the other station should pause until the receiver is done processing the incoming data. The lines used for this are called "Clear to Send" (CTS) and "Ready to Send" (RTS), respectively, which explains the colloquial name for hardware handshake: "RTS/CTS." The other type of handshake you might be familiar with is called "XON/XOFF" handshaking. XON/XOFF uses two nominated characters, conventionally Ctrl-S and Ctrl-Q, to signal to the remote end that it should stop and start transmitting data, respectively. While this method is simple to implement and okay for use by dumb terminals, it causes great confusion when you are dealing with binary data, as you may want to transmit those characters as part of your data stream, and not have them interpreted as flow control characters. It is also somewhat slower to take effect than hardware handshake. Hardware handshake is clean, fast, and recommended in preference to XON/XOFF when you have a choice.

In the original IBM PC, the RS-232 interface was driven by a UART chip called the 8250. PCs around the time of the 486 used a newer version of the UART called the 16450. It was slightly faster than the 8250. Nearly all Pentium-based machines have been supplied with an even newer version of the UART called the 16550. Some brands (most notably internal modems equipped with the Rockwell chip set) use completely different chips that emulate the behavior of the 16550 and can be treated similarly. Linux supports all of these in its standard serial port driver.[1]

The 16550 was a significant improvement over the 8250 and the 16450 because it offered a 16-byte FIFO buffer. The 16550 is actually a family of UART devices, comprising the 16550, the 16550A, and the 16550AFN (later renamed PC16550DN). The differences relate to whether the FIFO actually works; the 16550AFN is the one that is sure to work. There was also an NS16550, but its FIFO never really worked either.

The 8250 and 16450 UARTs had a simple 1-byte buffer. This means that a 16450 generates an interrupt for every character transmitted or received. Each interrupt takes a short period of time to service, and this small delay limits 16450s to a reliable maximum bit speed of about 9,600 bps in a typical ISA bus machine.

In the default configuration, the kernel checks the four standard serial ports, COM1: through COM4:. The kernel is also able to automatically detect what UART is used for each of the standard serial ports, and will make use of the enhanced FIFO buffer of the 16550, if it is available.

5. Using the Configuration Utilities

Now let's spend some time looking at the two most useful serial device configuration utilities: setserial and stty.

5.1. The setserial Command

The kernel will make its best effort to correctly determine how your serial hardware is configured, but the variations on serial device configuration makes this determination difficult to achieve 100 percent reliably in practice. A good example of where this is a problem is the internal modems we talked about earlier. The UART they use has a 16-byte FIFO buffer, but it looks like a 16450 UART to the kernel device driver: unless we specifically tell the driver that this port is a 16550 device, the kernel will not make use of the extended buffer. Yet another example is that of the dumb 4-port cards that allow sharing of a single IRQ among a number of serial devices. We may have to specifically tell the kernel which IRQ port it's supposed to use, and that IRQs may be shared.

setserial was created to configure the serial driver at runtime. The setserial command is most commonly executed at boot time from a script called 0setserial on some distributions, and rc.serial on others. This script is charged with the responsibility of initializing the serial driver to accommodate any nonstandard or unusual serial hardware in the machine.

The general syntax for the setserial command is:

in which the device is one of the serial devices, such as ttyS0.

The setserial command has a large number of parameters. The most common of these are described in Table 4-1. For information on the remainder of the parameters, you should refer to the setserial manual page.

Table 4-1. setserial Command-Line Parameters

A typical and simple rc file to configure your serial ports at boot time might look something like that shown in Example 4-1. Most Linux distributions will include something slightly more sophisticated than this one.

Example 4-1. Example rc.serial setserial Commands

The -bg /dev/ttyS* argument in the last command will print a neatly formatted summary of the hardware configuration of all active serial devices. The output will look like that shown in Example 4-2.

Example 4-2. Output of setserial -bg /dev/ttyS Command

5.2. The stty Command

The name stty probably means "set tty," but the stty command can also be used to display a terminal's configuration. Perhaps even more so than setserial, the stty command provides a bewildering number of characteristics you can configure. We'll cover the most important of these in a moment. You can find the rest described in the stty manual page.

The stty command is most commonly used to configure terminal parameters, such as whether characters will be echoed or what key should generate a break signal. We explained earlier that serial devices are tty devices and the stty command is therefore equally applicable to them.

One of the more important uses of the stty for serial devices is to enable hardware handshaking on the device. We talked briefly about hardware handshaking earlier. The default configuration for serial devices is for hardware handshaking to be disabled. This setting allows "three wire" serial cables to work; they don't support the necessary signals for hardware handshaking, and if it were enabled by default, they'd be unable to transmit any characters to change it.

Surprisingly, some serial communications programs don't enable hardware handshaking, so if your modem supports hardware handshaking, you should configure the modem to use it (check your modem manual for what command to use), and also configure your serial device to use it. The stty command has a crtscts flag that enables hardware handshaking on a device; you'll need to use this. The command is probably best issued from the rc.serial file (or equivalent) at boot time using commands like those shown in Example 4-3.

Example 4-3. Example rc.serial stty Commands

The stty command works on the current terminal by default, but by using the input redirection ("<") feature of the shell, we can have stty manipulate any tty device. It's a common mistake to forget whether you are supposed to use "<" or ">"; modern versions of the stty command have a much cleaner syntax for doing this. To use the new syntax, we'd rewrite our sample configuration to look like that shown in Example 4-

Example 4- Example rc.serial stty Commands Using Modern Syntax

We mentioned that the stty command can be used to display the terminal configuration parameters of a tty device. To display all of the active settings on a tty device, use:

The output of this command, shown in Example 4-5, gives you the status of all flags for that device; a flag shown with a preceding minus, as in -crtscts, means that the flag has been turned off.

Example 4-5. Output of stty -a Command

A description of the most important of these flags is given in Table 4-2. Each of these flags is enabled by supplying it to stty and disabled by supplying it to stty with the - character in front of it. Thus, to disable hardware handshaking on the ttyS0 device, you would use:

Table 4-2. stty Flags Most Relevant to Configuring Serial Devices

The next example combines some of these flags and sets the ttyS0 device to 19,200 bps, 8 data bits, no parity, and hardware handshaking with echo disabled:

6. Serial Devices and the login: Prompt

It was once very common that a Unix installation involved one server machine and many "dumb" character mode terminals or dial-up modems. Today that sort of installation is less common, which is good news for many people interested in operating this way, because the "dumb" terminals are now very cheap to acquire. Dial-up modem configurations are no less common, but these days they would probably be used to support a SLIP or PPP login (discussed in Chapter 7 and Chapter 8 ) than to be used for a simple login. Nevertheless, each of these configurations can make use of a simple program called a getty program.

The term getty is probably a contraction of "get tty." A getty program opens a serial device, configures it appropriately, optionally configures a modem, and waits for a connection to be made. An active connection on a serial device is usually indicated by the Data Carrier Detect (DCD) pin on the serial device being raised. When a connection is detected, the getty program issues a login: prompt, and then invokes the login program to handle the actual system login. Each of the virtual terminals (e.g., /dev/tty1) in Linux has a getty running against it.

There are a number of different getty implementations, each designed to suit some configurations better than others. The getty that we'll describe here is called mgetty. It is quite popular because it has all sorts of features that make it especially modem-friendly, including support for automatic fax programs and voice modems. We'll concentrate on configuring mgetty to answer conventional data calls and leave the rest for you to explore at your convenience.

6.1. Configuring the mgetty Daemon

The mgetty daemon is available in source form from ftp://alpha.greenie.net/pub/mgetty/source/, and is available in just about all Linux distributions in prepackaged form. The mgetty daemon differs from most other getty implementations in that it has been designed specifically for Hayes-compatible modems. It still supports direct terminal connections, but is best suited for dialup applications. Rather than using the DCD line to detect an incoming call, it listens for the RING message generated by modern modems when they detect an incoming call and are not configured for auto-answer.

The main executable program is called /usr/sbin/mgetty, and its main configuration file is called /etc/mgetty/mgetty.config. There are a number of other binary programs and configuration files that cover other mgetty features.

For most installations, configuration is a matter of editing the /etc/mgetty/ mgetty.config file and adding appropriate entries to the /etc/inittab file to execute mgetty automatically.

Example 4-6 shows a very simple mgetty configuration file. This example configures two serial devices. The first, /dev/ttyS0, supports a Hayes-compatible modem at 38,400 bps. The second, /dev/ttyS0, supports a directly connected VT100 terminal at 19,200 bps.

Example 4-6. Sample /etc/mgetty/mgetty.config File

The configuration file supports global and port-specific options. In our example we used a global option to set the speed to 38,400 bps. This value is inherited by the ttyS0 port. Ports we apply mgetty to use this speed setting unless it is overwritten by a port-specific speed setting, as we have done in the ttyS1 configuration.

The debug keyword controls the verbosity of mgetty logging. The data-only keyword in the ttyS0 configuration causes mgetty to ignore any modem fax features, to operate just as a data modem. The direct keyword in the ttyS1 configuration instructs mgetty not to attempt any modem initialization on the port. Finally, the toggle-dtr keyword instructs mgetty not to attempt to hang up the line by dropping the DTR (Data Terminal Ready) pin on the serial interface; some terminals don't like this to happen.

You can also choose to leave the mgetty.config file empty and use command-line arguments to specify most of the same parameters. The documentation accompanying the application includes a complete description of the mgetty configuration file parameters and command-line arguments. See the following example.

We need to add two entries to the /etc/inittab file to activate this configuration. The inittab file is the configuration file of the Unix System V init command. The init command is responsible for system initialization; it provides a means of automatically executing programs at boot time and re-executing them when they terminate. This is ideal for the goals of running a getty program.

Each line of the /etc/inittab file contains four fields, separated by colons. The first field is an identifier that uniquely labels an entry in the file; traditionally it is two characters, but modern versions allow four. The second field is the list of run levels at which this entry should be active. A run level is a means of providing alternate machine configurations and is implemented using trees of startup scripts stored in directories called /etc/rc1.d, /etc/rc2.d, etc. This feature is typically implemented very simply, and you should model your entries on others in the file or refer to your system documentation for more information. The third field describes when to take action. For the purposes of running a getty program, this field should be set to respawn, meaning that the command should be re-executed automatically when it dies. There are several other options, as well, but they are not useful for our purposes here. The fourth field is the actual command to execute; this is where we specify the mgetty command and any arguments we wish to pass it. In our simple example we're starting and restarting mgetty whenever the system is operating at either of run levels two or three, and are supplying as an argument just the name of the device we wish it to use. The mgetty command assumes the /dev/, so we don't need to supply it.

This chapter was a quick introduction to mgetty and how to offer login prompts to serial devices. You can find more extensive information in the Serial-HOWTO.

After you've edited the configuration files, you need to reload init to make the changes take effect. Simply send a hangup signal to the init process; it always has a process ID of one, so you can use the following command safely: