Connecting DB25 to DB9 serial connectors, and suggestion for MOD-TAP serial connectors (e.g. to use RJ45 connectors with WearComp to save space)

DB25  DB9
  8    1   CD -----  (this is an input)
  3    2   RXD    |
  2    3   TXD    |
 20    4   DTR-----  (this is an output)
  7    5   SGND   |
  6    6   DSR-----  (this is an input)
  4    7   RTS
  5    8   CTS
 22    9   RI

For IBM terminals, you need to connect pins 1+4+6 together and connect pins 7+8 together

Other common pinout for commercially sold DB25 to DB9 cables: 1-4 & 5, 2-2, 3-3, 4-6, 5-7, 6-20, 7 & 8-8, 9-22

A suggestion for MOD-TAP serial connectors; using RJ45 connectors with WearComp to save space

This proposed standard originates from the VT320 terminals described below:

Wiring to connect a VT320 to a DB9 male connector for use on waverider, etc. VT320 cable (looking at connector with wire facing away):

|6 2 5 5 3 4|
|           |
|  _________|
|__|

Wiring to connect a VT320 to a DB25 male connector for use on modem, etc. VT320 cable (looking at connector with wire facing away):

|6 3 7 7 2 20|
|            |
|  _________ |
|__|

The above connection strategy is also useful for MOD-TAP connectors, so that you can use a reversed-sense crimp-on phone wire as a null modem.

MOD-TAP connectors save space, e.g. all you need is RJ45 connectors on the WearComp instead of the much larger DB9 serial connectors. That means you can typically fit four serial ports into a small space, less space than even the two regular DB9 serial connectors.

Also, a 4 conductor phone wire will give you basic connectivity (Tx,Rx,SGND) while a 6 conductor phone wire will give full capability (DTR and DSR)