I've rewritten my alternate ACIA servicing code for the CBM Plus/4 and posted it to my Github. In the end, I was not able to get any confirmation of the flaky bit 7 of the Status register. But since testing only involved one computer, I decide to bypass any possible problem by testing the transmit ...

Have you separately confirmed that the IO space works? It seems to me that your 55/AA /FE test with ram doesn't exclude the possibility that IO could be flaky (too). I mean it's technically possible just from that test that IO is a problem.

I guess I would try this ram chip in the previous project ...

I believe I have evidence that the bit 7 bug affects the 8551 ACIA found in the Plus/4, and does so at any baud rate. I got BSZ on the plus4world.powweb.com/forum to run a couple test programs on his Plus/4 from my repo on this problem:

https://github.com/gbhug5a/My_CBM_stuff/tree/main/Plus4_IRQ ...

Well, I've sent email to Craig Bruce asking if he remembers anything more about this issue. I'll report back if I get a response.
He did respond:

"For that article in C= Hacking, I was just editing and republishing preexisting documentation from CMD. I don't know if it's true or not."

I assume ...

Thanks for everyone's comments. Jeff, I agree that if the problem exists at all, it probably isn't dependent on baud rate. But of course I've been down this road before with the 6526, so I'm open to the possibility that the problem is real. And BigDumbDinosaur's report supports that. The remaining ...

I found it here:

http://csbruce.com/cbm/ftp/reference/swiftlink.txt

"Some programmers have reported problems with using bit #7 to verify ACIA
interrupts. At 9600 bps and higher, the ACIA generates interrupts properly,
and bits #3--#6 (described below) are set to reflect the cause of the ...

Thanks very much. Your IRQ code branches past all the ACIA stuff if bit 7 of the status register is cleared. If this worked at 19.2K, then at least the 6551 you tested it on did not have the bit 7 bug. And you have a real buffer for transmit, so the issue I have with the Plus/4's single-byte buffer ...

I'm new here. Hello everyone.

I recently decided to post all my Commodore stuff on Github, and ran across something from 30 years ago involving the Commodore Plus/4, which you may recall has a real 6551 ACIA (called the 8551 in the Plus/4). This computer's rom has an error in the IRQ servicing ...