Posted By
TLC
on 2022-09-07
10:53:52
|  Re: External User Port for the C16: Let's make it!
@Sukkopera
Do you know if that has been fixed in the W65C51S?
Looks like it has indeed. https://www.westerndesigncenter.com/wdc/documentation/w65c51s.pdf see Effect of CTSB on Transmitter . This datasheet clearly states that the transmitter is not stopped until the last byte (which is being transferred) has been sent.
OTOH, your other candidate i.e. the W65C51N https://www.westerndesigncenter.com/wdc/documentation/w65c51n.pdf is probably still buggy (as far as CTS is concerned). At least, the datasheet says that "The CTSB input pin controls the transmitter operation. The enable state is with CTSB low. The transmitter is automatically disabled if CTSB is high." and nothing more. Pretty much the same what the original 6551 datasheet says.
Sure, the jumper option should be fine.
(Footnote: IMHO it was pretty easy for the guys not to get this correctly in the original ACIA design. AFAIK CTS/RTS handshaking wasn't common until the PC's came around i.e. by the mid '80s. In previous / original practice, when CTS dropped it pretty much meant that the opposite party was gone for good... that is, some corrupted last byte really made no difference. On a related note, if you take a closer look at the VIC-1011A schematics, you might notice that it looks ancient, compared to any RS-232 setups we know from PC related experiences. This is all pretty much from a modems / terminals / dialup TTY accesses etc. (pre-PC) era. ...But at least the guys already got the CTS thing right, as far as the Plus/4's design is/was concerned  .)
In my design the MAX23x is connected directly to the ACIA, *before* signals get to the userport, so they shall just be connected straight. Does that make sense?
Oh yes, sure. (Sorry, I always get lost after a couple of signal inversions :-/ ). BTW, there used to be a C64 6551 cartridge, the CMD SwiftLink, which was once "cloned" to an open source design called "DataPump". It's basically also a 6551 + MAX*** on a cartridge... albeit for the C64. http://www.zimmers.net/anonftp/pub/cbm/documents/projects/rs232/datapump2.zip . And it's practically doing what you suggested, i.e. no inverters are used.
Has anyone ever heard of such a peripheral?
The SP337E https://docs.rs-online.com/94ca/0900766b80f6c005.pdf appears to have output enable capability for the RS232 receivers... and I vaguely remember some (albeit rare..) other examples from the past... But, yes... things are getting a bit esoteric from that point on.
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