100 ohms *should* be enough to avoid frying the LEDs if you mess up the programming and drive both the switch and LED rows simultaneously.
Sure it's ok for a baud rate generator to be "all over the place" (not!) I wonder what genius had that idea? The hurried progress of the BCM series, and the corresponding rush from Pi 2 to Pi 3, were always going to be grounds for wondering what mistakes had slipped through the cracks.
The really easy way to get serial without modding.
ExecStart=-/sbin/agetty ttyUSB0 9600 vt100
A Pi running with no networking (no Ethernet, no WiFi) has power consumption well within what a standard USB port should be able to deliver. I'm not certain what the front panel consumes
It's even possible that you could take the data connections and ground from the back to back USB serial solution, and route it out through a carefully wired Y-adapter which also connects to the power port. This would give you a PiDP8 which has exactly one USB connection that you plug into your computer.
It doesn't get much cleaner than that.
will this approach work for (say) a keyspan usb using your standard pipaos image?
I followed your notes below but I assume I do nothing here for pipaos right?
- '/etc/systemd/system/xyz' - where xyz is a 'wants' subdirectory whose exact name I forgot, but it'll be obvious.
And of course I still have all the attach ttix /dev/ttyusb0 stuff as well?
Well this is a "Back to the Future" moment .....
the RCA terminal has MANY options (flexble, but complicated if unfamiliar with that era).
1. Your RCA appears to be 7-bit only (no option for 8-bit).
2. You can change the RCA's PARITY to EVEN, ODD, NO.
3. The Serial option is either simple 3-wire (TxD, RxD, GND) or FULL RS-232 (CTS/RTS, hardware flow controls).
START with the 3-wire option -- easier and with USB/RS-232 conversions you are performing -- less headaches.