I just received an unpleasant surprise when trying to use differential serial on the Advantech PCM-5820. The system has jumpers for the second serial port; RS232, RS422 or RS485. In either RS422 or RS485 mode, I see no activity at all on the TxD+/- pins.
My software is asserting DTR and RTS, so I don't know what else I could possibly do to enable the drivers (and they should always be enabled in RS422 mode anyway, right?) I know I'm talking to the right I/O port and IRQ because when I jumper it back to RS232 I see activity on the TxD pin. I'm giving the board both +5 and +12, so it can't be missing an input voltage. I have ritually waved a White Castle hamburger over the machine and threatened it with a steak knife, with no discernible effect.
I have jury-rigged the system to work on the port in RS232 mode by connecting Rx- on my DCE to gnd and Rx+ to TxD on the DTE, but although this proves my device is working, it is not deployable.
Any ideas? Does this feature even work? Advantech's stellar documentation doesn't tell you how to use it at all, they just give you the pinout of the 14-pin connector on the board. You have to pull off the backshell to work out which pins go where on the "RS422/485" DB9M.