A Vintage "35-Cent Softmodem" Gets a Commodore 64 Talking to a Convergent WorkSlate at 300-Baud

Vintage computing enthusiast Cameron Kaiser has got two pieces of vintage technology, a Commodore 64 and a Convergent WorkSlate, talking together — over a "35-cent softmodem" that lets the two talk over a wire as easily as a telephone line.

"Of the many interesting posts from Usenet's more golden days, one of my favorites was John Iannetta's '35-cent modem,' where the [MOS Technology] SID chip provides one-way data modulation to a receiving modem connected via the [Commodore] C64's sound output," Kaiser explains. "While I remember him posting it back in 1998, I never actually tried it at the time."

Before the days of high-speed connections, Wi-Fi, and Ethernet, the gold standard for computerized telecommunications was hooking your micro up to a modem and having it dial up to a remote modem and scream at it — modulating the digital data from the computer into audio, sending it over the telephone wires originally designed for voice traffic, and demodulating it back into data at the other end.

The Commodore 64 had no shortage of hardware modems, but Iannetta's design was different: using the Sound Interface Device (SID) chip in the Commodore 64 to replace the bulk of the modem's hardware, requiring only a few cents in parts to interface with a real modem at the other end of the line. Perhaps surprisingly, it's a trick that works — and in Kaiser's case, meant a new and low-cost way to get a portable Commodore SX-64 talking to a Convergent WorkSlate.

"Officially the WorkSlate's only means of telecommunications is its 300 baud internal modem," Kaiser explains. "While we have a 9,600 bps way of wiring up a Workslate to a modern computer, it's always nice to have a simpler alternative, and I figured this would be a great challenge to see if John's old program could let my Commodore SX-64 talk to my WorkSlate. Spoiler alert: it works!"

Writing a modified version of Iannetta's original softmodem code, Kaiser was able to get the Commodore 64 to act as a modem talking to the one built into the WorkSlate — even simulating a dial tone and activating the WorkSlate's speaker phone function. The addition of an XMODEM protocols allows the software to transfer files to and from the WorkSlate, too. "John was a good guy with a clever programming style," Kaiser says, "and it was nice to see his code running again (and working, though that was a given). Plus, this is a great use for a Commodore to support your other systems, and a roadmap for doing something similar on other machines with sufficiently capable sound hardware."

The highly-detailed project write-up is available on the Old Vintage Computing Research blog; the source code is available on GitHub, along with a precompiled binary, under a public domain license; instructions for wiring a suitable cable are included. "Never connect this cable to a wall jack," Kaiser notes. "You could send 48V through your machine."