This is a very cool way to actually use the CIA's serial port. But this also means it is technically not bit-banging.

Ironically, bit-banging is what the C64's "KERNAL" does by default in its rs232 implementation.

> TTL-level RS-232

I ran 115k2 for a couple years directly between an atmel 8515 (sending TTL) and a Pentium2 motherboard (sending +12/-12) 24/7 for a couple years with no ill effects.

atmels are known to be tanks wrt to overvoltage protection. IMO half the success of the arduinos is that their atmel's pads can even handle children. Few chips are so tolerant.

Least you could have done is a voltage divider with a couple high value resistors.

Interestingly, the 6551 (and later 8551) are used on the Commodore Amiga A2232 seven port serial card. Seven of them are run by a 3.58 MHz 65CE02 at up to 19200 baud, although the card can be modified to run all ports simultaneously at 57600 baud, or two ports at 115200 baud. Because the 65CE02 does all the work, there's almost no overhead for the host Amiga. It's a versatile chip!

The fact that the C64 and friends can emulate a 6551 is impressive. That they can emulate it up to 9600 baud with unused shift registers, on the other hand, is awe inspiring.