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A Null-Modem is a Null Modem, right?

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First post, by Jo22

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Good morning, everyone!

Just found this article at 8bit-micro.com, the online vintage computer magazine :

http://www.8bit-micro.com/full-null-modem.htm

I had no idea, but it seems that commercially made cables aren't as good as self-made ones.

In case you're wondering, the 3-wire null-modem cable was popular among hobbyists and is quite sufficient for most simple tasks.
The full version is better for 8bit computers, though, as it implements hardware-based handshaking techniques.

Edit: Site has disappeared (archived version).

Last edited by Jo22 on 2018-03-27, 23:20. Edited 1 time in total.

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Reply 1 of 4, by gdjacobs

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The two data pins as well as RTS and CTS will allow you to use hardware flow control.

All hail the Great Capacitor Brand Finder

Reply 2 of 4, by Zup

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AFAIK, there were three kinds of null-modem cables:

  • Three wired cable. TxD and RxD are crossed. This is the simplest cable, and only supports software flow control.
  • Three wired cable with shorts. TxD/RxD are crossed. DTR/DSR and RTS/CTS are shorted in each side, to make "fake" signals and allow you to use hardware flow control with only three wires. This cable can work on most computers, but it can easily run into trouble (so you'll have to switch to software flow control).
  • Seven wired cable. TxD/RxD, DTR/DSR and RTS/CTS are crossed. This is the best cable, allows any kind of flow control on any computer.

Note that these are PC to PC cables, you could also have un-crossed wires for printers, modems and other stuff. Also, some cables dealt with DCD signal (usually DTE-DCE cables).

If you can open the DB9 connectors, a 3 wired cable can be easily fitted with shorts to fake that hardware control.

DB9 connectors are big and cable is cheap, so if you need a specific kind of cable there is no reason to not make your own.

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Reply 3 of 4, by gdjacobs

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I remember messing with this. TxD, RxD, RTS, and CTS cross connected generally allows hardware flow control (although doing some reading, DTR/DSR is required for older stuff).

All hail the Great Capacitor Brand Finder

Reply 4 of 4, by Jo22

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Thx for all the feedback! 😀

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

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