Here is the modification I made to my PAC-COMM NB-9600 modem.  This mod
got rid of all the garbage calls in my mheard list both when using the
TNC in standard mode as well as while using net/nos software packages.
This modification makes sure that the modem will send all zeros to the
TNC until the modems PLL is locked on a signal.  This means that the
TNC will not be reading garbage comming in to the UART all the time.


I removed the socket for U11 (74HC14) and inserted the 74HC14 directly into
the PC board.  I then lifted all the pins of a 74HC00 except 7, 12 and 14.
These lifted pins were carefully bent so they were pointing straight out.
I cut back the bent pins so the 74HC00 can be piggybacked on the 74HC14.
Without removing the socket on  U11, I would not have ben able to slide
the modem and Tiny-2 back into the box (you may not have this problem).

I cut the trace from U11 pin 12 to U20 (74HC157) pin 13.  I soldered the
74HC00 on top of U11 (soldering pins 1, 12 and 14). I wired U10 pin
13 (LM339) to the 74HC00 pins 9 and 10.  I wired 74HC00 pin 8 to pin 13.
Pin 12 of U11 was connected to pin 12 of the 74HC00 when I piggybacked it.
I connected pin 11 of the 74HC00 to pin 1 and 2 of the 74HC00.  I connected
pin 3 of the 74HC00 to pin 13 of U20 (74HC157).  I think thats it.

Theory of operation:

The DCD signal comming from U10 (LM389) is low true so I invert it using
one of the nand gates (pins 8, 9 and 10).  This is used to qualify the 
RXD signal from the 74HC14 (pin 12). Pin 11 of the 74HC00 is the RXD signal 
(low true) qualified by the DCD signal which is LOCK DETECT.  One more
inversion of this signal (74HC00 pins 1, 2 and 3) and I have the qualified
RX DATA signal which is connected to U20 (74HC157) pin 13.

RX DATA is always zero until the PLL is locked to the data.  Then RX DATA
will be the true data comming from the unscrambler or zeros if we are not
locked.

Before making this change, my mheard was always full of random garbage.
After the change, my mheard is clean and my RXErrors is not increasing
at a rate of about 200 errors per minute.
PAC-COMM said that their firmware uses DCD to determine if the channel is
clear and not to determine if the data is valid.  This may not be true
for all versions of firmware available.  

Anyway, there you have it.  Three gates later and I am much happier.
The 9600 baud modem seems to work very well.  I have not tested it under
any marginal situations.  The other station (who also has the mod) is
about 4 miles away and the signal seems too strong to determine how well
the modem works under weak signals.  From this distance I seldom see
a retry.

Steve King  

Steve King   (KD7RO) king@hpdml57.boi.hp.com
             Hewlett Packard 
             Boise, Idaho