Pk232 DCD setup (howto)

Msg # 12990 Type:B Stat:$ To: ALL @ALLOH From: KA8TEF Date: 05-Sep/1912
Subject: Setting the DCD THRESHOLD of the 232
Bulletin ID: 5373_N8FIS
Path: AD8I!N8FIS

The following is a reprint of a article in the TAPER newsletter August
1988 Issue 32.

AEA PK-232 Notes:
by Eric Gustafson

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 verylow since there are no hidden
terminals. Almost immediatly we noticed that the collision frequency had risen
dramatically. After some investigation we discovered certain stations were
almost guarenteed to be involved in stepping on in progress packets. These
stations were contacted and in all but one case they were new users of PK-232s

We were very puzzled as there have always been some stations on the
repeater using PK232s 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 stations up 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 the manual to try to 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 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 trans-
mission by another station on the channel! We had found the cause of our

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 operations and use the method
presented here. All your packet neighbors will appreciate it very much.

The manual is quite correct EXCEPT where they discuss setting the DCD
THRESHOLD control and receiver audio output level for proper demodulation
and DCD circuit operation. The corect way to set these adjustments is :

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 in
capable of proper operation with unsqueched
audio from the receiver.

3. While monitoring incoming packets,
adjust the receiver audio level so
indicator “spreads” fully when
receiving a packet 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 the
channelwhich produces the LEAST amount
of audio output from the receiver.
Make sure 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 acknowlagements 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 the
steps to assure that their DCD is operating properly. Multiple access
packet channel throughput is severly degraded when DCD is not working.

Hope this helps out
73 Phil KA8TEF

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