Posted By
GeorgeHug
on 2021-12-03
18:11:41
|  Re: Could someone run test software for me?
@MMS, Yes, the 6551 opens up a lot of possibilities. In fact, you can actually run it faster than the computer can keep up with. Anyway, the +4 is still the only Commodore computer that doesn't bit-bang RS232. I don't think even the Amiga had an ACIA.
@BSZ, I really appreciate your help with the tests. With your results, I think we now know what's going on. There is a delay in triggering the second interrupt after a byte is written to the 6551, and the delay varies with the baud rate setting. This is confirmed by the fact that programs 2 and 4 do not work at 2400 baud, but do work at 9600 baud. The time taken by the processor instructions is enough to get past the short delay at 9600 baud, but not the longer delay at 2400 baud.
I'm still working on a program to determine how much delay is actually needed at each baud rate from 1200 through 19200, but, it will depend heavily on the standard kernal code. So I think it may be best if I ask Steven Combs (retroCombs on Youtube) to run it. He has unmodified NTSC Plus/4s to work with, and it would be less trouble for him to run the test.
I would say it is very encouraging that program 4 works at 9600 baud. That means at higher baud rates no delay loop is needed at all.
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