From: James S. Kaplan KG7FU (xtfh.vlsrxlxo@mlsd.org)
Date: Sun Jan 02 2000 - 17:31:18 EET
I just finished up testing software and hardware for the "An MX615
Packet Modem" project
article in the January 2000 QST by N9ART.
This is a "Baycom style" modem based on the MX-COM MX614 mod/demod IC.
It's a
relatively inexpensive replacement for designs using the TI TCM3105
modem chip and
the same board can be configured for either serial or parallel port
operation.
Linux kernel level drivers for Baycom modems using kernel 2.2.13 work
very well and
are simple to configure. I tested the modem using Baycom 1.6 for
MS-DOS as well
(note: Baycom 1.5 did not work for me with this modem).
Parts aren't critical, except maybe mylar or poly capacitors in the
audio paths. Besides
the IC and a 74HC04, the circuit board from FAR Circuits, all the
parts I used came
from my junkbox or from Radio Shack.
Setting up a Baycom device with 2.2.x kernels doesn't go exactly as
indicated in
the AX.25 HOW-TO, but that information was close enough to get me
started.
I am using the baycom_ser_fdx.o module. Since it worked so well I
haven't
bothered to try the baycom_ser_hdx.o module. Basically, you build your
kernel
and modules per the HOW-TO. The differences from previous builds are
mostly
minor syntax changes in configuration of the devices and networking
options.
Baycom devices are now known as bcsf[x] rather than bc[x]. I am using
SuSE 6.3b2
which doesn't seem to have some user utils, such as setbaycom.
My /etc/ax25/rc.ax25 is a shell script to start all of my AX.25 and
Ham Radio
utilities. The baycom section looks like this:
#!/bin/bash
## config baycom modem
echo "Preparing Baycom TNC device"
#remove stale locks & disable serial port
rm -rf /var/lock/LCK..ttyS0
rm -rf /var/lock/LCK..ptyq0
/sbin/setserial /dev/ttyS0 uart none
#/sbin/setserial /dev/cua0 uart none
#/sbin/setserial /dev/modem uart none
#
#install kernel modules
insmod hdlcdrv
#see man baycom for module options
insmod baycom_ser_fdx mode="ser12" iobase=0x3f8 irq=4
#set hsrdware port paramaters, half duplex
sethdlc -a -i bcsf0 txd 450 slot 100 ppersist 10 half
#setup ax.25 network device
ifconfig bcsf0 hw ax25 kg7fu-12 up
ifconfig bcsf0 44.26.1.150 netmask 255.255.255.0
/bin/sleep 1
#
#add kiss interface & set params
#this is for user programs which are baycom device unaware
#creates a device called ax0
net2kiss -s -f -i bcsf0 ptyq0 &
/bin/sleep 1
/usr/sbin/kissattach -m 512 ptyq0 1 44.26.1.150
/bin/sleep 1
#
# start the daemons
/usr/sbin/ax25d -l
/bin/sleep 1
/usr/sbin/mheardd
#for x window monkt & linkt
/usr/sbin/ax25spyd
#
All that's left is configuring netrom and/or rose devices and setting
up routing. The script
above will get you on-the-air.
It only took several hours to build the modem, cable it and begin
testing the drivers. It
took far more time to work out little details such as getting the ptt
interface to drive
my IC-W32A ht and getting clean audio out of the modem. The presence
of just a
little RF at 2m seemed to couple heavily in to the ht's mic in audio.
I suspect using
a "real" ground instead of the mic audio return line on a real radio
and proper shielding and
bypassing is all that's necessary for portable operation. In the
meantime, adding small
ferrite beads to all the audio and data i/o lines on the pcb cured the
audio blues. I also
found the TXA output adjustment range to be fairly narrow between
"off" and "full
distortion". Increasing the value of R8 or decreasing R4's value may
help here.
The PCB, MX614P IC and color-burst crystal are only $15. Pretty good
deal I say.
-----------------------------
James S. Kaplan KG7FU
Eugene Oregon USA
tti@metlife.com
http://www.rio.com/~kg7fu
ICQ # 1227639
Have YOU tried Linux today?
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