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
 units.

     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 
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 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