So I designed a PCB to fit a 6551 internally on the 128. This gives it a nice high speed hardware serial port. Instead of putting a line driver and real RS232 socket I put an ESP WiFi modem on it, so it's an internal WiFi modem that doesn't take up the cartridge port OR the user port.
Ok, So I got a 1581 and it wouldn't work. Turns out the wifi modem I had plugged in was shorting some userport pins to get 9600 and that broke burst mode loading on the 1581 (1571?).
So I had the choice of unplugging one or the other. As the first disk image I wrote to test the 1581 was DesTerm3.0, unplugging the modem didn't really help. So my solution was a swiftlink cart. Sadly these are uncommon, expensive and have a REAL RS232 port not a wifi modem.
So I have designed a PCB to go inside the 128. It's a riser card that fits under one of the CIA chips (6526) and has the 6551 UART and an ESP-01 module onboard. It requires a single jumper wire to the decoded address line (io1/2 on the cart port) and should then just appear as if it was a swiftlink cart to the 128. However, it will have the WiFi modem onboard. Sadly that ESP has no hardware flow control so I had to loop those on the 6551 but I did pass DCD and DSR to the ESP via it's 2 spare GPIO, I plan to re-write the modem firmware to make use of these. It should in theory allow a BBS program to detect a disconnect by dropping DCD etc. I did pass RTS/CTS to a header if a better than the ESP solution is found or for using a TTL to USB Serial cable instead of the ESP.
I have ordered PCBs and will update when they arrive and I can test it.
I use the 5v in the AV din for my adapter. A single cable brings all the pins from teh AV din to my board which takes 5v and splits the S-Video out to a real S-Video socket and takes audio to a 3.5mm mono jack socket. The 5v is passed through to the 15pin output so you can easily make a 15pin to SCART with auto RGB switching. You can see it here (no audio lead connected)
This gives quite a minimal mess install without loads of wires all over the place! I'm actually using a 9pin extension at the moment rather than plugging it directly into the 128 but thats because I am testing every board I assemble on my 128 and didn't want to keep plugging them into the 128's socket.