'The Home Computer Wars' has this to say about the development of the cartridge modem for the Vic-20:

The size of the case was another problem. Most modems came in bulky rectangular boxes about 10 X 4 inches. Too expensive. I suggested we put the modem on a cartridge and plug it straight into the VIC-20 user port. Again, they went back to the drawing board and somehow squeezed the circuitry into a case less than 3 X 6 inches. It was the first modem-on-a-cartridge. The best part was, our competitors couldn't duplicate it. Only the VIC-20 had the right combination of RS-232 interface and user port engineering.

I thought the RS-232 port on the Vic-20 was completely separate from the cartridge port, so a thing had to be plugged into one or the other, and if the modem was plugged into the latter then it could not use the former. What am I missing?

I have to admit I don't recall hearing about cartridge modems for any other machine. Is this why?

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    The modem plugged directly into the user port (the RS-232 port) without using a cable, just like cartridges plugged directly into the cartridge port. The C64 and C128 supported modems on their user ports as well. I have two, a third-party 300 baud "pocket" modem that is only about 3" x 2" and a 1200 baud Commodore 1670 modem that's about 3" x 6". – Ross Ridge Aug 18 '18 at 18:02
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    The author is using the word "cartridge" to refer to a cased PCB designed for easy user attachment directly to an externally accessible I/O port, no cables required. First modem I ever owned was a small'ish plastic "cartridge" that plugged into the C64 User Port and offered 300 baud speed and auto-answering for my BBS. – Brian H Aug 18 '18 at 19:06
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    I'd swear that I had a cartridge modem for my CoCo2... Wish I still had it so I could confirm. – Brian Knoblauch Aug 21 '18 at 15:28

I thought the RS-232 port on the Vic-20 was completely separate from the cartridge port, so a thing had to be plugged into one or the other, and if the modem was plugged into the latter then it could not use the former. What am I missing?

Or better what is mixed up. The device (Serial IEC) bus is often called a serial port, but that one was driven by VIA#2 and handled by all device IDs 4 and greater. The serial interface he is refering two is the one on the unser port, produced by VIA#1 and answering to device ID 2. It's basicly the whole lower row of port B lines. While not mentioned in the VIC20 manual, the ROM did already hold all routines to bitbang a RS232 alike interface on PB of VIA#1.

The Vicmodem itself was basicly an application of Motorolas MC14412 Low Speed Modem circuit. The whole setup was minimized for cost, so, for example it had no way to take over a phone line from a phone, but the User had to plug the handset cable into the mocem after dialing.

I have to admit I don't recall hearing about cartridge modems for any other machine. Is this why?

Technical there is no difference between a serial modem puged into the User port or one having a cable inbetween. Nor does the kind of connector (edge vs DB25 or whatsoever) make a difference. Maybe because of naming? TI's Modem was as well a box, this time pluged into the system bus, and having the same for an Apple II was called an internal modem/interface card. So just ask each companies marketing team why they went each way :))

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    The Canadian models came with a little switch - you'd plug the line into the switch, and the phone into the switch, and toggle the switch to go between modem and phone line. Canadian handset cords at this time were not yet detachable; phone companies provided phones to subscribers. That changed very shortly after, but Commodore, as far as I know, didn't sell the modem here without that switch. – Jim MacKenzie Aug 21 '18 at 17:18

The Atari's originally included a 12V feed that was passed out through SIO and could be used to drive RS-232. I think the only product that ever used it was the 835 modem, but I'm not sure, it might have had a separate power supply as well.

Starting from the 1200, the 12V was removed and people started to get creative. The later Atari modems were similar in size to the VIC's, especially the XM301:

enter image description here

The connector is the Atari SIO cable - not exactly a user port, but definitely not an RS-232 either. There were 3rd party versions without the cable that plugged directly into the port. The one on the left plugs directly into the cartridge port, so I guess it's an exact match for what you're looking for.

enter image description here

Other small-form, non-RS232 modems include the ADB examples from the Mac:

enter image description here

This was a particularly dumb solution, as the ABD was seriously limited in performance terms as it was a polled protocol and the CPU simply didn't spend a lot of time working it.

ps I always found it amusing that we would take 5V TTL, convert to 12V RS-232, and then immediately convert back to TTL again in the device. A complete waste of effort that was reinforced by the IEEE's ham handed standards efforts that resulted in monstrosities like RS-449.


So-called cartridge modems existed for a few different platforms:

  • Commodore - originally came out with the VICModem (1600) but also, later, the 1650, 1660, and 1670 modems, all of which fit onto the user port of the VIC-20, Commodore 64 or Commodore 128. While not technically cartridges, they were cartridge-like in format, and simply connected to the computer's user port without a cable in between. Third parties made modems also, that used a similar form factor, such as the Pocket Modem, HESmodem, Mitey Mo and others.
  • Atari - while Atari's own modem used a cable, third party modems such the Pocket Modem (in Atari version) plugged right into the data port on the back of the floppy drive.

Other companies may have had modems with this form factor, too, but I've not seen them.


The VIC-20 was aimed at the home gaming crowd who wanted to play video games but needed an excuse to justify to their wives why it was OK to drop $400 on a computer. The excuse was "could use it for the kids homework" or some such. That's why the thing had a full keyboard.

However later when the C64 came out Commodore dropped the price on the 20's and quite a lot were sold to the hobby crowd (that was when I bought mine) That crowd often owned older computers or bought used gear (such as modems) from company tech sales and so on and was highly resentful that Commodore expected them to fork over $100 for a modem that plugged into the user port. Quite rapidly 3rd parties began selling TTL-to-real RS232 signal converters and plans were in general circulation (I built one myself on a breadboard) for them so that the hobby crowd could spend only $10 for the converter and plug in their Wargames-style 300 baud modems. In addition many of the hobby crowd wanted to connect other serial devices not just a modem. The blurb you quoted was pure salesdroid rubbish that was dumped out when Commodore realized the level converters plugged into used 300 baud modems were causing them to have warehouses of cartridge modems collecting dust. Commodore later admitted that the engineers had wanted to put real +12 - -12v serial ports on the Vic-20 but the hardware UART chips simply were not ready when the VIC went into production so they hurriedly wrote up assembler to simulate a serial port on the user port.

  • That the engineers wanted to include 12V, I certainly believe. I was assuming the lack thereof, was to save money on parts. c64-wiki.com/wiki/RS-232 seems to support that. Do you have any references for the theory that it was just about time to market? – rwallace Aug 19 '18 at 23:55
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    @rwallace: Commodore could easily have added +/-12V serial ports to the VIC-20 if they'd been willing to increase the manufacturing cost by somewhere around $1-$3. I don't think there was any reason to consider such a thing. People who had other 300-baud modems lying around might have benefited from being able to use them, but I don't recall that Commodore's modem was particularly more expensive than anyone else's. – supercat Aug 20 '18 at 16:36
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    That modem worked on the Commodore 64 (and the later Commodore 128) too. I actually did computer science programming for university using my C64 and a VICmodem. – Jim MacKenzie Aug 21 '18 at 17:19

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