PACKET RADIO: TNC Settings
Steve Wolf, W8IZ@W8IZ
(This text from the W8IZ packet radio bulletin
board. It's formatted to fit a 80 character screen.)
In many cases, you TNC should work straight out of
the box. You set your call with the MYCALL command and
off you go.
In other cases, you are dead in the water. Nuthin'
The most common problem is that the TNC begins to
send data to the radio before the radio is fully in its
transmit mode. Sometimes you can hear this by listening
to your packet signal on another radio. The cure is a bit
of TXD-transmit delay. This lengthens the time between
when the TNC keys the radio and it starts sending data. In
an ideal world, you want this value as short as you can
possibly make it. The less time you take up, the more time
others have on the system. It may sound like you are the
only one on the PBBS but you might be surprised. This is
a multi-user system. While you are alone on two meters,
the board may be forwarding on 220, have users on 440 and
have messages getting forwarded on HF. If you fool with
TXD, please make it as short as possible.
Audio problems seems to be the second offender. If
you are getting a bunch of retries, set your audio
to a different level and see if that helps. In most cases
you need just enough audio to light your DCD light.
Timing problems are a bear to correct. There are
two different methods that are used.
If you find persistance and slot time are used in your
TNC, you are using the newer of the two methods.
Persistance is a formula used by the TNC to determine if
the TNC is going to transmit now or wait a period before
starting. Slot time is the time period that the TNC
waits before running the persistance formula again.
Persistance is a chance type of thing. A flip of the
coin if you will. Slot time is a wait period before
taking another chance.
The second method deals with txd and resptime. The
second is the delay that is imposed on the acknowledgement
packets. Dwait is the time that the TNC waits to avoid
collisions with digipeted packets.
If set wrong, these values will cause your TNC to
be out of sync with the rest of the network. You will
be heard and data may flow, but at a slow rate. You
will experience collisions with others and the station
you are talking to.
Setting these values requires you read and
understand your manual. If you can't or won't read, you
will end up messing the TNC up worse than it is.
You can set your TNC to be very agressive, to assert
itself on the network. Don't, the rest of amateur radio
Here in northern Ohio, you will find that the PBBS
stations are more assertive than a normal user's station.
This is as it should be. The network needs to function
above the users. Some activities, like the forwarding of
weather bulletins, will sometimes take over a channel.
Please don't mess with the aggressiveness of your
TNC. Let it lay back and draw from the network.
I GOOFED--MY TNC ISN'T WORKING-WHAT NOW?
Most TNCs depend on battery backed RAM to obtain
their operating parameters. You pull the battery, the
TNC defaults to its original state and you start over.
The manual will tell you how to do this. In most
cases the battery is hooked up through a jumper that can
be removed to allow the RAM to clear. Leave the battery
out of the system for a time, like fifteen minutes, to
insure that the RAM has reset itself. Some TNCs, like the
MFJ-1278, take quite a while to reset.
Keep the battery-pull advise in the back of your
mind. If nothing else works, pulling that battery will
get you started again.
This is true in other areas of packet radio. Keep a
virgin copy of all the programs you use. If nothing else
works, go back to the original and start over.
Also want to take a look at the two deviation files.
Return to Packet FAQuette Index