How to bit-bang 9600 baud on a Commodore 64 #Commodore #C64 #VintageComputing @pagetable
The user port of the Commodore 64 exposes a TTL-level RS-232 serial port that supports up to 1200 baud. In 1997, Daniel Dallmann came up with a very sophisticated trick that allowed sending and receiving at 9600 baud, using slightly different wiring and a dedicated driver. This “UP9600” wiring has become the de-facto standard for all modern accessories, like C64 WiFi modems. pagetable.com writes up how UP9600 works.
For cost saving reasons, the VIC-20 and its successors, the C64 and C128, did not contain a 6551 chip. Instead, Commodore included a bit-banging driver in the KERNAL that emulated the 6551 and exposed it as device #2. This emulator supports up to 2400 baud, but due to DMA from the VIC-II video chip (“badlines”), only speeds up to 1200 are stable on the C64 (and the C128 in 40 columns mode).
In 1997, Daniel Dallmann created UP9600, a solution that allowed 9600 baud on the user port. The idea is to use the hardware shift registers of the two CIA 6526 I/O controllers to do the timing-critical part of the transfer.
The highest baud rate that is possible on a C64 with the described algorithm is 9600 baud.
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