I think it's pretty much the "default" DIP settings I'm using now:
You can see what the ap2000 actually receives in its buffer by doing this in the debugger:
focus 1 (this selects the ap2000 upd7810 cpu)
8000 (the internal ap2000 buffer is at $8000)
Here you can see the ap2000 getting 9E and 1D as the first characters when I've got the centronics firmware selected on the PIC.
quick and dirty: apple 2 ctrl+reset PR#1 then just type a bunch of keys and the printer will print junk:
or another idea (since the control panel keys are working now:)
when you start up mame, hold down the keypad 7 and 9 keys (LF and FF) and that will put the printer into Data Dump Mode where it displays the hex sent. It won't print anything until it gets a line of text, but you can override that by toggling keypad 0 (online/offline) and that will print the last characters sent.