The last time I got computer parts I threw in a Digilent PmodRS232. That's a MAX3232 on a db-9. A chip that converts from modern GPIO style 3.3v signals to +/-12v RS232 signals for some old-school serial port action. I didn't really need it but I used to do lots of rs232 stuff.
Last night I decided that I could burn a few hours by using it to get the Pi2's serial port out and useable under RISC OS.
Couldn't get it to work last night, so tonight I took another stab at it.... Success!!!
In no particular order:
the pmod is DCE, so it doesn't want a null-modem to talk to another computer.
The pins labeled TxD and RxD are relative to the MAX3232. TxD goes to the pin labeled RxD on GPIO, and RxD goes to TxD on GPIO!
The MAX3232 (according to digilent's datasheet) really prefers to be fed 3.3v rather than 5v.
A TRS-80 Model 100 is a very nice tool to have when rs232 misbehaves. Within its limits, it just works, with no nonsense. If it's not getting/sending, something on the other end is wrong.
!connector, !serial, and the whole "block driver" stack in RISC OS sounds scary, but really does work.
Unfortunately I can't really leave this hooked up, my case doesn't have a space for the pmod and I have too many distructo-cats to leave powered circuit boards just hangin out. ;-)
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko