So once installed the C128 acts as if it has a WiFi modem installed. I used the firmware from Github which works quite well. This means it does TCP connections, from what I read it can accept incoming as well as outgoing. You can connect to a BBS directly or to your PC if you have software on the PC capable of dealing with the IP connection. It's possible to simply unplug the ESP module and connect a TTL Serial to USB adapter directly giving you a USB serial cable from the 128 if you need as well, that would just appear as a serial device on the PC.
The WiFi modem is meant for comms software like Desterm and acts as a modem, so expects AT commands to connect.
You can probably rewrite the ESP fw to do what you want easily. My idea was to have a boot rom on the 128 that initialised the 6551 and sent the modem commands to connect to a port on my server. The server would accept the connection and send a file (whatever was in a shared direcory at the time called boot.rom). The 128 boot rom would accept that data and write it into ram as per the load address then jump to the start
Thats quite a bit of coding but it would mean you had a net boot 128!
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.