There were a few breakthroughs which helped me get to this point. On the off chance they might answer, I asked the manufacturer for the pinout information for the serial port on the PD-22. Not only did they send me a pin diagram, they also included a schematic for the cable.

Pins 1-8 are straightforward and what you’d expect to find on RS232. I was originally thrown off because 9 looked like signal ground. Pins 9 and 10 are tied to each other, and go to signal ground internally. I think this is used to indicate that the cable is present, as the printer defaults to IrDA otherwise. I’ve updated the Eagle schematic to tie 9 and 10 together and removed the traces which aren’t connected to anything.

I didn’t have a chance to etch the new board yet, but really wanted to try things out so I made do with the old one by just putting a blob of solder between pins 9 and 10. It’s not very pretty, but it works!

The next step is getting this contraption working with the Arduino instead of running off a USB serial port on my laptop. I have to read up on the SoftwareSerial library, as I haven’t really done anything with it before. I also need to read the command sheet more carefully, I’ve figured out how to print, and turn the thing off via serial, but haven’t got it to feed the paper after printing. Right now I just press the feed button.

But it works!