LINUX DRIVER FOR BAYCOM MODEMS Thomas M. Sailer, HB9JNX/AE4WA, This document describes the Linux Kernel Driver for simple Baycom style amateur radio modems. The driver supports the following modems: ser12: This is a very simple 1200 baud AFSK modem. The modem consists only of a modulator/demodulator chip, usually a TI TCM3105. The computer is responsible for regenerating the receiver bit clock, as well as for handling the HDLC protocol. The modem connects to a serial port, hence the name. Since the serial port is not used as an async serial port, the kernel driver for serial ports cannot be used, and this driver only supports standard serial hardware (8250, 16450, 16550) par96: This is a modem for 9600 baud FSK compatible to the G3RUH standard. The modem does all the filtering and regenerates the receiver clock. Data is transferred from and to the PC via a shift register. The shift register is filled with 16 bits and an interrupt is signalled. The PC then empties the shift register in a burst. This modem connects to the parallel port, hence the name. The modem leaves the implementation of the HDLC protocol and the scrambler polynomial to the PC. picpar: This is a redesign of the par96 modem by Henning Rech, DF9IC. The modem is protocol compatible to par96, but uses only three low power ICs and can therefore be fed from the parallel port and does not require an additional power supply. Furthermore, it incorporates a carrier detect circuitry. All of the above modems only support half duplex communications. However, the driver supports the KISS (see below) fullduplex command. It then simply starts to send as soon as there's a packet to transmit and does not care about DCD, i.e. it starts to send even if there's someone else on the channel. This command is required by some implementations of the DAMA channel access protocol. The Interface of the driver Unlike previous drivers, the driver is no longer a character device, but it is now a true kernel network interface. Installation is therefore simple. Once installed, four interfaces named bc[0-3] are available. sethdlc from the ax25 utilities may be used to set driver states etc. Users of userland AX.25 stacks may use the net2kiss utility (also available in the ax25 utilities package) to converts packets of a network interface to a KISS stream on a pseudo tty. There's also a patch available from me for WAMPES which allows attaching a kernel network interface directly. Configuring the driver Every time the driver is inserted into the kernel, it has to know which modems it should access at which ports. This can be done with the setbaycom utility. If you are only using one modem, you can also configure the driver from the insmod command line (or by means of an option line in /etc/conf.modules). Examples: (use either method, not both) insmod baycom mode="ser12*" iobase=0x3f8 irq=4 sethdlc -i bc0 -p type "ser12*" io 0x3f8 irq 4 Both lines configure the first port to drive a ser12 modem at the first serial port (COM1 under DOS). The star ('*') instructs the driver to use the software DCD algorithm (see below). insmod baycom mode="par96*" iobase=0x378 irq=7 sethdlc -i bc0 -p type "par96*" io 0x378 irq 7 Both lines configure the first port to drive a par96 or par97 modem at the first parallel port (LPT1 under DOS). options=1 instructs the driver to use the software DCD algorithm (see below). insmod baycom mode="par96" iobase=0x278 irq=5 sethdlc -i bc0 -p type "par96" io 0x278 irq 5 Both lines configure the first port to drive a picpar modem at the second parallel port (LPT2 under DOS). The driver uses the hardware DCD signalled by the picpar modem (see below). The channel access parameters can be set with sethdlc -a or kissparms. Note that both utilities interpret the values slightly different. Hardware DCD versus Software DCD To avoid collisions on the air, the driver must know when the channel is busy. This is the task of the DCD circuitry/software. The driver may either utilise a software DCD algorithm (a star after the mode string) or use a DCD signal from the hardware (no star). ser12: if software DCD is utilised, the radio's squelch should always be open. It is highly recommended to use the software DCD algorithm, as it is much faster than most hardware squelch circuitry. The disadvantage is a slightly higher load on the system. par96: the software DCD algorithm for this type of modem is rather poor. The modem simply does not provide enough information to implement a reasonable DCD algorithm in software. Therefore, if your radio feeds the DCD input of the par96 modem, the use of the hardware DCD circuitry is recommended. picpar: the picpar modem features a builtin DCD hardware, which is highly recommended. Compatibility with the rest of the Linux kernel The serial driver, the line printer (lp) driver and the baycom driver compete for the same hardware resources. Of course only one driver can access a given interface at a time. The serial driver grabs all interfaces it can find at startup time. Therefore the baycom driver subsequently won't be able to access a serial port. You might therefore find it necessary to release a port owned by the serial driver with 'setserial /dev/ttyS# uart none', where # is the number of the interface. The baycom driver does not reserve any port at startup, unless one is specified on the 'insmod' command line. Another method to solve the problem is to compile all three drivers as modules and leave it to kerneld to load the correct driver depending on the application. Further reading Please take a look at http://www.ife.ee.ethz.ch/~sailer/ham/linux/hdlc.html for further informations on the driver. vy 73s de Tom Sailer, sailer@ife.ee.ethz.ch Packet Radio: hb9jnx@hb9w.che.eu