I don't know what you mean by "on board units", but it doesn't sound relevant anyway.
What you are apparently asking about is the protocol. RS-485 is a electrical standard only. It's a multi-drop differential single-signal bus. How you send bits over it, how they are delimited into bytes, and what the bytes mean is up to the implementation. That said, most implementations use UART-like signalling. That means there is a start bit, 8 (usually) data bits, and one (usually) stop bit. It's very likely your protocol uses that or something close that can be sent and received with a hardware UART and the appropriate bus transceiver.
However, that's where common practise ends. There are some officially published protocols above that, but its very common for individual implementations to roll their own. You get this information from the manufacturer.
By the way, I wouldn't use RS-485 for new designs. CAN is electrically similar in that it is a differential multi-drop bus, but the standard goes as far as defining whole packets of up to 8 data bytes each, with checksum, ID, and other out of band signalling. Another big advantage of CAN is that the hardware to send and receive whole packets is available in small micrcontrollers.