I recently found this little retro gadget: the retroNET v4
It’s basically a wifi chip on a small board with a serial DB25 interface. With a terminal program it gives a telnet or ANSI BBS connection over Wifi and the Internet.
That’s already cool, but I wanted an even better look & feel of the good old times!
So I put it into an old ZyXEL 2864 modem!
The retroNET board is meant to be tampered with and has extra solder pads for additional connectivity. But the ZyXEL modem isn’t… and I wanted to use as much of the ZyXEL hardware as possible, like the serial port, PSU, LEDs and buttons.
The retroNET is supposed to be powered by a 5v USB power adapter. But I didn’t want to have an extra USB cable going into the modem, this would completely destroy the look of the whole thing. Fortunately the ZyXEL PSU has three power outputs: 5V, +12V and -12V. So I both disconnected the -12V and +12V on the modem by disconnecting those diodes. Then I traced the 5V to various locations on the board to find a good spot to tap it to power the retroNET. The modem does have an extra internal connector for an optional ISDN module and one of the pins of this connector does have the 5V. First issue solved, I got the power!
Now I needed to find the send and receive pins of the serial port to redirect them to the retroNET. I couldn’t easily connect to the pins on the port itself, as they are directly soldered to the board. So I traced them to their first ICs and tapped them at the feet.
Technically the retroNET is already working now with the connected Send (Tx), Receive (Rx) and 5V pins. But I wanted more: LEDs and reset button!
The ZyXEL has a separate board in the front with all the LEDs and it’s connected with a ribbon cable to the mainboard. I wanted to keep it like that because it’s easier to open the case without ripping out the soldered connections. But to get it to work with the retroNET I had to reverse the LEDs. Which meant to “unsolder” and “resolder” them in backwards. But this was the easy part. Now I have to find good spots to tap the lines which go to the connector. Unfortunately they were teeny tiny solder pads on very small ICs. While trying to get them on I destroyed about 5 pads…
I also replaced the ribbon cable with single connectors to get more flexibility, especially for getting one of the buttons on the front of the ZyXEL to work as reset for the retroNET.
It was a lot of try and error, and my solder joints don’t look pretty, but I got it to work in the end!