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CONNECTING TWO 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 9 pin to 9 pin null modem cable looks like this:
TD 03-----02 RD
RD 02-----03 TD
RTS 07-----08 CTS
CTS 08-----07 RTS
DSR ------06 06------ DSR
| |
ground | 05-----05 | ground
| |
CD ------01-----04 | DTR
|
DTR 04-----01------ CD
Note that pins 6 and 1 are jumpered together on each end.
This is a classy version; the minimal version would only
include the connections involving pins 3, 2, and 5.
See Also:
25 to 25 pin null modem
25 to 9 pin null modem
DCE/DTE
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Written by Dave Pearson