patch-pre2.0.5 linux/drivers/char/README.baycom

• Lines: 165
• Date: Thu May 16 16:35:39 1996
• Orig file: pre2.0.4/linux/drivers/char/README.baycom
• Orig date: Thu Jan 1 02:00:00 1970

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+		    LINUX DRIVER FOR BAYCOM MODEMS
+
+	     Thomas M. Sailer <hb9jnx@radio.amiv.ethz.ch>
+
+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.
+
+par97: 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.
+
+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
+
+The driver interfaces to the AX25 stack via a KISS interface. The driver
+can be accessed as a character device with major 60. Major 60 is the first
+number of the local/experimental range. I did no steps to coordinate a
+major number for this driver, but I plan to do so in the near future.
+The driver supports multiple modems (currently four, as defined with
+NR_PORTS). It therefore responds to minor numbers 0 to 3. I recommend
+to access the driver via the special device files /dev/bc[0-3], which
+can be created with 'make bc'.
+
+
+Compiling and installing the driver
+
+First unpack the source files into a directory. Then enter the following: (you
+must be root to do it)
+
+  make dep
+  make
+
+This will create the files baycom.o and setbaycom. baycom.o is as well copied
+to /lib/modules/`uname -n`/misc. If you plan to use kerneld, do the following:
+
+  depmod -a
+
+Do not forget to create the device special files if you install the driver the
+first time. This can be done with:
+
+  make bc
+
+You are now ready to use the driver. You can now activate the driver manually
+by entering
+
+  insmod baycom
+
+or leave this task to kerneld (if installed). Add the following line to
+/etc/conf.modules
+
+  alias char-major-60 baycom
+
+
+
+Configuring the driver
+
+Everytime 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:
+  insmod baycom modem=1 iobase=0x3f8 irq=4 options=1
+  setbaycom -i /dev/bc0 -p ser12 0x3f8 4 1
+
+Both lines configure the first port to drive a ser12 modem at the first
+serial port (COM1 under DOS). options=1 instructs the driver to use
+the software DCD algorithm (see below).
+
+  insmod baycom modem=2 iobase=0x378 irq=7 options=1
+  setbaycom -i /dev/bc0 -p par96 0x378 7 1
+
+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).
+  
+The channel access parameters must be set through KISS parameter frames. The
+AX25 stack may be used to generate such frames. KA9Q NET derivatives such
+as WAMPES or TNOS offer the 'param' command for this purpose.
+
+
+
+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 (options=1) or use a DCD signal from
+the hardware (options=0).
+
+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.
+
+par97: as far as I know it is in this respect equivalent to par96.
+
+
+
+Compatibility with the rest of the Linux kernel
+
+The tty interface gave me some headaches. I did not find a reasonable
+documentation of its interfaces, so I'm not particularly sure if I implemented
+it the way I should. Perhaps someone with a more profound knowledge about
+tty drivers could check the interface routines.
+The driver produces a rather high interrupt load. par96/par97 generates 600
+interrupts per second, ser12 1200 while transmitting and 2400 if hardware
+DCD is used, 3600 otherwise. If other device drivers disable interrupts
+too long, the performance of the driver drops (the packet loss rate increases),
+especially with the ser12 modem.
+There were also reports that under rather high load situations the driver
+drops frames. This might be either an interrupt problem, or an AX25 stack
+running in user mode might not get enough CPU time to process the packets
+before the drivers internal buffers overflow. There is no way to throttle
+the other radio stations from this layer, throttling must be done in the
+AX25 layer.
+
+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.
+
+
+
+vy 73s de
+Tom Sailer, hb9jnx@radio.amiv.ethz.ch
+hb9jnx @ hb9w.ampr.org