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CONNECTING 25 PIN TO 9 PIN SERIAL PORTS
Sometimes you wish to connect two computers together. One of
them has to pretend to be a modem (DCE) and one has to pretend
to be a terminal (DTE). Sometimes equipment is flexible and
sometimes they refuse to go in drag. When you have two pieces
of equipment both steadfastly insisting on being terminals or
two pieces both steadfastly insisting on being modems, what do
you do?
For two steadfast terminals, (the most common case) you build a
special null modem cable that logically looks like two modems
back to back over a very short phone line.
A typical 25 pin to 9 pin null modem cable looks like this:
25 pin end 9 pin end
TD 02-----02 RD
RD 03-----03 TD
RTS 04-----08 CTS
CTS 05-----07 RTS
DSR ------06 06------ DSR
| |
ground | 07-----05 | ground
| |
CD ------08-----04 | DTR
| |
DTR | 20-----01------ CD
|
SQ ------21
TC ------15
|
RC ------17
Note that pins 6, 8 and 21 are jumpered together on the 25 pin end,
pins 15 and 17 are jumpered to gether on the 25 pin end, and pins
6 and 1 are jumpered together on the 9 pin end.
This is a reasonably classy version. The connection to pin 21
is seldom required, and you often will not need the connections to
pins 15 and 17. The minimal version would only include the TD-RD,
RD-TD, and ground-ground connections.
See Also:
25 to 25 pin null modem
9 to 9 pin null modem
DCE/DTE
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Written by Dave Pearson