Null Modem cables for consoles Re: SRM troubles
> > Is there an easy way to tell, if a serial cable is
> a null modem cable?
> A nullmodem cable has two female plugs.
Usually were, should have been, two female plugs, but
some were specially built for non-conforming DTEs that
had female plugs on the Data Terminal Equipment, and
thus had one of each -- and even back then these were
impossible to recognize unless labelled. The
once-ubiquitous Laplink cable has two female DB25s and
one female DB9 at one end, like Zapod Beeblebrox, and
has a couple extra pin swaps besides the basic null
modem swaps, but works fine for most null-modem work.
(I've got one attached between an AlphaServer 400
4/233 and a ancient NCR notepad as console terminal at
If you have a Digital-Volt-Ohmmetter or continuity
tester, you can try ringing pin 2,3 to pin 2,3 at the
other end. 2-2, 3-3 is "normal", 2-3, 3-2 is Null
Modem. (If either device wants "hardware flow
control", other pins need to be crossed or jumpered as
well. Xon/Xoff flow control is much safer, same as
with cheap, 4 wire serial cables.)
In the old days, we had small DB25 M-F extenders with
LEDs wired in that would light up to indicate correct
wiring. Some had switches to compose a custom
A null-modem how-to is buried under the Linux Emacs
how-to, go figure.
Null modem adapters (to go on an existing cable) and
cables are still available in at better retailers.
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