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I've heard of On Board Units (OBU) can communicate via RS485. My question is, how?

For example, I would like to ask the state of some runtime variables from the OBU (what doors are open?, etc.) Or another example, I would like to send some data to the OBU (outside temperature).

What protocol must I implement to do these? Are the any sample implementations available somewhere?

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Are you maybe asking about the on-board units associated with the on-board diagnostics (OBD) port in most ground vehicles? Or maybe the units that do on-board data handling on satellites? Or the on-board unit used for electronic toll collection? Or maybe something entirely different? –  davidcary Jan 4 '13 at 17:26

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There are a number of protocols that you can support over RS-485. One of the more common ones is Modbus. From a hardware stand point you will need an RS-485 level translator attached to the UART module of the microprocessor.

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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.

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If you are asking about about the protocols used in cars, its not RS485, its a protocol called CAN accessed using a connector standard called ODBII

The traffic on the network as I understand it is just ASCII so easy to hack so theres a lot of ODBII reverse engineering going on, have a google!

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