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CONNECTING TWO 25 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 25 pin null modem cable looks like this:
Frame 01-----01 Frame
TD 02-----03 RD
RD 03-----02 TD
RTS 04-----05 CTS
CTS 05-----04 RTS
DSR ------06 06------ DSR
| |
ground | 07-----07 | ground
| |
CD ------08-----20 | DTR
| |
TC | ---15 15--- | TC
| | | |
RC | ---17-----24 | | (TC)
| | |
(TC) | 24-----17--- | RC
| |
DTR | 20-----08-----| CD
| |
SQ ------21 21------ SQ
Notice that pins 6 and 8 and 21 are jumpered together on each end, and
15 and 17 are jumpered together on each end.
This is a reasonably classy version of the null modem cable.
You can often get by without connecting pins 15, 17 and 24 at
all. The jumper to pin 21 is seldom required. And the simplest version
of the null modem consists only of the pin 2, 3, and 7 connections.
See Also:
25 to 9 pin null modem
9 to 9 pin null modem
DCE/DTE
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Written by Dave Pearson