Posted By
MMS
on 2019-10-07
17:13:49
 |  Re: C16 Expansion Port to User Port
Actually, despite the different naming of those pins, the lines used by C=serial modems (RS232 equivalent data lines) should be equivalent on the C64 user port and Plus/4 user port.
If you just check the comparison on SVS UltimateMap, it is not evident.
But if you read THIS, it becomes evident, that the Plus/4 User ort pin naming is the correct (standard), and the C64 User port pinuout naming is something really out of this world.
If you connect those modem lines to the Plus/4 same user port pins, you get the same funtion as on C64.
So, if you build a so-called C64 user port, it is the same for a modem as a Plus/4 user port.
When you have your C16 user port, based on THIS desription you may try to build your Wifi modem cheap! the linked Gearbest ready wifi circuit is just 5$.
https://1200baud.wordpress.com/2017/03/04/build-your-own-9600-baud-c64-wifi-modem-for-20/
The C64 modems ("Commodore modems") should work on Plus/4 without modification, ehm, there is a bug on the Plus/4 (in fact: on 6551 IC swapped two lines DCD and DSR) so you need to cover one pin with a insulator (flextape), otherwise the same.
Just recently I saw a link to CMD Switftlink232, also realising a fast RS232 on C64 memory expansion port.
Based on a quick view I think that circuit does not use any C64 line the Plus/4 does not have, though the memory addressing is a little more tricky on C64.
https://www.commodoreserver.com/FileDownload.asp?FID=7E31343456F4452FBF733187D255B2BD
AFAIK for a long time this was the only possibility on C64 to reach >30000 baud speed, and using the same ACIA 6551 as the Plus/4. (I have questionmarks, what speeds even Wifi modems could reach via the the C64 User Port CIA)
It is a a very detailed historical overview with a lot of detals.
Certainly you can connect (on C64) any serial modem with RS232 connector to the Switflink, and it is even faster than the C64 one. But it need to be supported by some SWs... On C64 several SW started to supports this HW protocol (DesTerm 128, Dialogue 128, NovaTerm, Terminal 1, Q-Term, Kermit). On Plus/4? Maybe the one recently linked by Luca!
https://www.commodoreserver.com/BlogEntryView.asp?EID=FA5AE758474345A9A0A7208C7F408538
There is a simplier version by JimBrain (Fast speed to expansion port), though due to simplicistic circuit diagram hard to tell, which lines it is using from the explansion port  but should be very similar to the above.
https://www.lyonlabs.org/commodore/hardware/ez232-link232.pdf
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