Can you provide some more details to your question? Digi sells part 20-101-1183, a USB programming cable that includes an FT232RL IC. I see the BAT54 diodes in the schematic:
My guess is that the RCM32xx includes pullup resistors on the RXA and /RESET pins, and the diodes ensure that the FT232RL will only pull those pins to ground and won't drive them with a voltage possibly higher than the board's Vcc.
If you're trying to build an alternate programming cable using an FTDI TTL-level USB serial cable, then you'll need to understand the Digi programming cable schematic and the wiring between the programming port and the rest of the RCM32xx. If you're trying to build a cable to power the RCM32xx and program it when not installed, then the BAT54 diodes might not be necessary.