If you have a Commodore 64, chances are you’ll eventually need to connect a modem or other serial device to it. You’ll find that’s not quite that easy since the C64 doesn’t have a standard RS-232 serial port. What it does have is called the “user port” and it can do serial over this port but it needs to be changed from TTL levels (0 to +5v) to RS-232 levels (-15v to +15v).

If you’ve ever attempted to purchase a VIC-1011a terminal type, SwiftLink or Turbo232 from eBay you’ll quickly find out that the price gets out of hand. Expect to pay upwards of $100 or more for these adapters.

Luckily, there’s an inexpensive way to get a RS-232 port on your C64 and it’ll cost you less than $15. Ready?

You’ll need these parts for the project.

Connect the RS232-TTL module directly to the C64 user port edge connector using the table below.

RS232-TTL Module C64 User Port GND A & N TXD M RXD B & C VCC 2

Update! Alwyz from 1200baud suggested that connecting VCC isn’t necessary and potentially dangerous. I’ve had zero problems with mine as listed here. I’m providing this warning so you can make your own decision!

Observe which side of the user port connector is the top (it’s the one with numbers– letters are on the bottom). It’s helpful to write on it with marker. For wire I used female jumper wire that I cut one end off. For GND and RXD you’ll need to jumper two of the pins together on the user port connector. I used a small bit of CAT5 solid core wire.

Once you’ve got it connected, add the null modem adapter and connect your modem. You may need to also use a gender changer and/or a 9 pin to 25 pin adapter depending on your modem.

Fire up CCGMS, Novaterm or Striketerm, set the baud rate to 2400, set the port to the user port and give it a few “AT” commands. You should see “OK” being returned. If it doesn’t, make sure you have a null modem adapter (test it on another machine to confirm) and double check your connections.

As with any tutorial you find online, be responsible and double check my work and your work before proceeding.