Over the years many have asked for the pinouts of the various cables used for 
telecommunications with the CoCo. The following is an attempt to gather all 
of that information in one source. This information comes from various forum 
messages. Most of the credit goes to Marty Goodman as most if not all of the 
messages were his. This information was not collected in electronic form but 
was collected from written copies. So there may possibly be a typo or so 
although the information should be correct. If you do spot a typo or if you 
have any problems using any of these cables please leave me a message so I 
can correct this file. This file covers the following cables.
 
CoCo 4 pin to DB25 modem cable
CoCo 4 pin to DB25 null modem cable (used to connect between two machines)
CoCo 4 pin to DB9  null modem cable (usually a PC and a CoCo with out using)
RS232 pak  to DB25 null modem cable (any modem though you still need)
RS232 pak  to DB9  null modem cable (terminal software on both machines)
 
CoCo 4 pin to DB25 modem cable
 1              8   
 2              3
 3              7
 4              2
 
CoCo 4 pin to DB25 and to DB9 null modem cable
 1          20,6,8       1,4,6   Short the multiple pins on DB and continue
 2             2           3     wire to pin 1 on CoCo end. Continuing  the 
 3             7           5     wire to pin 1 on the CoCo forces CD high
 4             3           2     just in case the CoCo term program cares.
Then short the following only on the DB end.
              4,5         7,8
 
Rs232 pak to DB25 and to DB9 null modem cable.
 7             7          5
 3             2          3
 2             3          2
Then at each end short the following but don't connect to the other end.
6,8,20       6,8,20     6,1,4
 4,5          4,5        7,8
 
Marlin Simmons; Delphi username LINLEE
 
  Addendum by Marty Goodman:
 
  Note:  When using the above information for null modem cables
between a CoCo and a PC compatible, the information is given
for connection of a Coco to EITHER a DB 25 PC serial port
OR to a DB 9 serial port.   If you put both the IBM DB 25 and the IBM DB 9
conectors on the cable, and the CoCo connector on the cable,
you will have a cable that will link in null modem fashion a CoCo to
EITHER a 25 or 9 pin DB serial port on a PC.   But you cannot use
that cable to link two PC compatibles in null modem fashion!  A
different pin out is required for that.   ---marty