Universal Null Modem

07/20/83


Ever want to hook two computers together thru the modem ports,
but didn't want to use those slow modems to make things work?Here is the answer.
...If you have the appropriate modem cables
from each of the two computers. You can connect what's called
a NULL MODEM between the two systems using your standard cables.

I realize that this is nothing new to many of those computerists
who have been 'hacking' away ever since the Intel 8008 and S-100.
For those of you who are interested, here is a diagram for a
Universal Null Modem. It can consist of two DB-25S connectors
tied together using short jumper wires and spacers between the
mounting holes, or a cable as long as you need it to reach be-
tween the two computer modem cables.


GRD 1 )---------------------( 1 GRD

TX 2 )---------------------( 3 RX

RX 3 )---------------------( 2 TX

RTS 4 ) nc nc ( 4 RTS

------------------(20 DTR
|
CTS 5 )---| |---( 5 CTS
| |
DSR 6 )---| |---( 5 DSR
| |
DCD 8 )---| |---( 8 DCR
|
DTR 20)------------------

GND 7 )---------------------( 7 GND

Note: The signal generated on pin 20 (DTR) is used to drive
the opposite pins (5,6,& 8).

TX= Tx Data RX= Rx Data
RTS= Request t Send CTS= Clear to Send
DSR= Data Set Ready DCD= Data Carrier Detect
DTR= Data Terminal Ready GND= Ground

Dennis Recla Wa5KTC Garland, Texas