Posted By
TLC
on 2022-09-06
09:19:37
|  Re: External User Port for the C16: Let's make it!
I have read a lot of points already said in the thread, I'm just gonna add some 2 cents.
- As you're already well aware of that, the guys had pretty much messed things up by misusing one port bit of the 6529B SPI for cas sense in the Plus/4, and the equivalent address space in the C16/C116. This is clearly a (dirty) hack. (But this detail makes things no easier...)
- CTS' was disconnected and rerouted in the original Plus/4 design for a reason. The CTS functionality of the 6551 is buggy. Dropping this control signal causes the 6551 to stop transmitting immediately, I mean it'd asynchronously disable the transmitter in the ACIA. That is, whenever the opposite party says "please stop transmitting more bytes", the byte currently being transmitted would be immediately broken. Reverting this part, i.e. CTS to be fed from the 6529B rather than the 6551, appears IMHO a good idea.
- That'd probably also mean that working around the $FD10 thing is unavoidable, if full compatibility is desired someday.
- About inverted / non-inverted control lines: the Plus/4's user port layout mimics the earlier Commodore machines' user ports, which implement inverted TTL RS-232 control lines. (Those machines have no physical ACIA onboard, they "emulate" this functionality from Kernal routines and bit-banging.) If you check either the VIC-1011A as you already have, or, check some popular replacement say GGLabs' GLINK-LT https://gglabs.us/node/2044 you'll see that both of them uses inverters to invert the control lines. (About the same way as you did in your design BTW.)
- One particular thing I'd be suspicious about in a hybrid design (i.e. a user port + permanent rs-232 line transceiver combo), is user port backwards compatibility. That is, to make sure, that the rs-232 line transceiver part never interferes with "standard" user port functionality as long as the rs-232 port is free. (Considering that both the '04 inverters and the Max chip have Totem Pole outputs, currently I'm not sure if/whether that part suffices).
- I think the limit here would ultimately be current consumption, especially if 9VAC capability was to be added someday. The power supply part of the C16 / C116 is particularly... errr... "interesting", and weak.
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