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Recently, several new packeteers using PK-232s have appeared
on our local duplex repeater which is dedicated to packet
radio. This is one environment where the collision
frequency should be very low since there are no hidden
terminals. Almost immediately we noticed that the collision
frequency had risen dramatically. After some investigation
we discovered certain stations were almost guaranteed to be
involved in stepping on in-progress packets. These stations
were contacted and in all but 1 case they were new users of
PK-232s.
We were very puzzled as there have always been some stations
on the repeater using PK-232s with no apparent problems.
The new stations were asked how their station was configured
and what method was used to get the DCD operation adjusted.
We were very surprised at the answers we got. Every single
one of the offending stations had set their station up
exactly according to the instructions in the PK-232 manual.
However, contrary to the advice given in the manual, none of
these stations had configured their setup so that they could
hear what was going on on the channel when the PK-232 was
connected to the radio. None of these new operators knew
what DCD meant, what it did, or why it was important that it
should be working on a multiple access packet channel.
We obtained a PK-232 and manual to try and discover the
exact nature of the problem. What we found was that
although the manual is very complete and generally very
well written, there are some areas where it leaves something
to be desired. Specifically, in this case, the instructions
given on page 2-16 (we had manual PK232UG Rev. B 9/86 for
reference) for setting up the PK-232 and an FM radio for DCD
operation are simply incorrect. If set up exactly as
described, DCD will NEVER be asserted during a packet
transmission by another station on the channel!! We had
found the cause of our problem.
If you have a PK-232 and haven't already discovered this
problem for yourself, please disregard the instructions in
the manual for setting up a PK-232 and NBFM radio for 1200
baud packet operation and use the method presented here.
All your packet neighbors will appreciate it very much.
The manual is quite corect in all respects EXCEPT where they
discuss setting the DCD THRESHOLD control and receiver audio
output level for proper demodulation and DCD circuit
operation. The correct way to set these adjustments is as
follows:
1. At least temporarily, arrange to be able to hear
the receiver audio signal which is being sent to
the PK-232.
2. Set the squelch circuit on the radio for normal
squelched operation. The DCD circuit in the PK-232
is incapable of proper operation with unsquelched
audio from the receiver.
3. While monitoring incoming packets, adjust the
receiver audio level so that the tuning indicator
"spreads" fully when receiving a packet from the
station on the channel which produces the LEAST
amount of audio output level. There are several
limiters in the PK-232 demodulator so louder
stations will not be affected adversely by this.
4. Once the audio level is properly set, adjust the
DCD THRESHOLD control on the PK-232 so that the DCD
led lights when there is a packet being transmitted
by the station on channel which produces the LEAST
amount of audio output from the receiver. Make
sure, however, that the DCD LED is extinguished
when there is no signal and the radio's squelch
circuit has cut off all audio from the receiver.
If the above procedure is followed, the PK-232 will properly
hold off transmitting during a packet transmission from
another station and will not send acknowledgements to
individual frames of a maxframe greater than 1 packet while
it is still being transmitted.
We hope PK-232 owners will find this information useful and
take steps to assure that their DCD is operating properly.
Multiple access packet channel throughput is severely
degraded when DCD is not working.
73 and happy packeting,
Eric Gustafson N7CL