Adding an UP9600 switch to the StrikeLink wifi modem

The retrocomputing community is ♥ enamored ♥ with the new breed of ESP 8266 based wifi adapters. The tiny ESP 8266 board features a 32-bit RISC microcontroller and 802.11 b/g/n wifi, and it can easily be connected to an 8-bit PIO port like the user port on Commodore’s 8-bit line.

The Commodore 128’s user port can handle 9600 baud natively, but its older brother the Commodore 64 (and the 128 in 64 mode) can only do 2400 baud without help. Daniel Dallmann’s UP9600 standard, which my research traces back to 1997, uses an additional data line to send data faster. But, this technique conflicts with the Commodore 128’s Burst IEC serial mode. And, since the 128 tries to boot from disk drive #8 on boot, and it uses Burst serial to talk to a 1571 disk drive, it freezes on boot if such a drive is connected.

In order to make the device work with my favorite 8-bit in all its modes, I had to modify my StrikeLink so I could disable UP9600.

The first step was to physically disable UP9600, using a small drill bit to cut through a trace on the board that connects to pin 5 (SP1) of the user port connector.

Commodore 64 user port pinout.

Next, I drilled some holes to mount three header pins for my switch. One header pin was routed back to pin 5 (SP1), and another to pin M (PA2) on the bottom of the user port connector. Putting a jumper over these pins enables UP9600, and moving it over to the unconnected pin disables it so the 128 can boot.

Hot glue holds the wires in place and anchors the header pins to the board.