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Trying to establish a null modem connection between 2 C64s using 1 Commodore 1660 300 Baud modem and 1 Commodore 1670 1200 Baud modem.

Does anybody have detailed instructions on how to configure the modems for text exchange and file exchange?

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  • I'm unfamiliar with the Commodore 1630 modem. Is that a typo? I've heard of the 1600 VICmodem, the 1650 Automodem and the 1670 1200baud modem. Also, a null modem connection is used when you have no modems. You have 2 modems, so a null modem connection isn't what you want. I think you may want a PBX that lets you create 2 internal 'extensions', then dial one extension from the other. There is open-source PBX software available out there (e.g. Asterisk) so this does not need to be a terribly expensive option.
    – Ken Gober
    May 24, 2017 at 16:50
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    Or, possibly, to network the machines directly via their serial ports?
    – Tommy
    May 24, 2017 at 16:56
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    You may need a line voltage inducer or telephone line simulator to get them to dial and talk to each other. Or try this DIY RS-232 interface project that will give you a 2400 baud connection without the modems.
    – snips-n-snails
    May 24, 2017 at 17:12
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    Generally speaking, you have to present a modem with a dialling tone or at least an off-hook signal, before it will dial. Just wiring a telephone cable between them isn't enough and is not a null modem.. It would be a lot easier connecting the two computers together directly via RS232 - that is a null modem.
    – Chenmunka ♦
    May 24, 2017 at 17:59
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    @Tommy, RS-232 is designed for DTE-to-DCE communication; swapping wire pairs in a null-modem cable makes each computer think it is the DTE and the other is the DCE. This only works because the link is capable of full-duplex communication, with dedicated input and output lines. Commodore Bus is designed for host-to-device communication. It's half-duplex, with all lines being both input and output. Any sort of "null modem" cable would leave both computers fighting over which one is the host and which is the device. The fact that many C64 peripherals used 65xx CPUs is entirely irrelevant.
    – Mark
    May 24, 2017 at 23:03

2 Answers 2

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"Null modem cable" may not quite be a misnomer but is potentially misleading: it is a cable that connects two computers via their serial ports without modems. It's not a null cable for modems, it's the null example of one of those cables you use to connect a computer to a modem.

So if you want to make it look like there's a phone network between two modems that are really directly wired, you're not asking about null modem cables.

If you just want to network two C64s then the easiest thing is directly to connect them via their serial ports. That's not technically the same thing as the normal usage of the term "null modem cable" because that term is usually associated with full-duplex RS-232-style connections, whereas this will be half duplex but half duplex is no barrier whatsoever to networking. Ethernet is also half duplex.

This page contains a [German] write-up of networking via the Commodore serial bus, including the necessary software. Obviously you can use Google to read a translated-to-English version.

Although the software is customised, the cable you'll need isn't. Any standard Commodore serial cable will do — impedance is sufficient that the author had no issues even with five C64s and a total cable length of 25 metres. So if you have a 1541 then just use the cable from that.

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    Data transfer rates could be improved enormously by wiring up many of the pins in the user ports. If one was really careful, one could probably use a ribbon cable and a pair of IDC connectors if one was careful to cut all of the wires one didn't need and plug things in the right way, but mistakes could blow things up.
    – supercat
    Mar 8, 2019 at 22:48
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I know this is anecdotal, but here is my personal testimony:

I built many circuits interfacing with the User Port in my teens (mostly digital control circuits using the data lines), and I'm pretty sure I interfaced my 128 and 64 together by directly wiring the two user port's TX/RX TTL lines together in a crossover fashion(along with ground, of course), but it has been a long time, so I may be remembering incorrectly.

I do, however, specifically remember connecting two 300 baud modems to each other on many occasions, using a simple RJ-11 jumper cable. I would simply send the off-hook AT command to each modem (and I think another set of commands to initiate handshaking), and the computers were able to communicate...

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  • Back in the '90s I used the modem-to-modem technique to transfer some text files between my C=64 and my PC as well.
    – Wildcat Matt
    May 14, 2019 at 20:28

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