Forgot to mention that a null modem adapter converts a straight through cable
into a null modem cable, or it converts a null modem cable into the equivalent
of a straight through cable.
Serial ports on computers are supposed to use the DTE (Data Terminal Equipment)
pinout and modems and other serial devices are supposed to use the DCE (Data
Communications Equipment) pinout.
DTE to DCE uses a cable wired straight through.
Problem is some serial devices use the DTE pinout (for example old HP pen
plotters) and need a null modem cable with wires crossed to correctly route the
signals.
It's fairly simple to make your own null modem adaptor. I have a bunch of some
serial cable extenders that're simply a male 25 pin connector and a large phone
jack in a housing, meant to be connected to a small and flexible cable.
I combined that with a 25 pin connector and ribbon cable that originally was a
back panel serial port connector for an old AT style PC motherboard.
It also has a plastic housing.
After looking up the null modem wiring diagram, some soldering, a function test
then some hot glue to keep the wires from being yanked loose, I can use my old
HP 7470A Pen Plotter. :)
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