canobd
I have a CAN bus network that I want to test. I want to send CAN messages over my computer and see the responses. I have a Microchip kit with CAN that sends the responses over USB Mini and can be read by their application. I don't know in what format they come.
I also have an OBD II-to-USB converter with the ELM327 chip.
Which bus terminal/monitor can I use?
You will definitely want to choose a standard network layer. I've seen some cases where people have rolled their own, and frequently the protocol is inefficient, and in the end it's a waste of time. A proven network layer provides basic functionality to solve most of the problems you'll likely face in networking with an embedded device, and that will save you precious time and money in the end.
I have familiarity with CANopen, so I'll put a bit of an explanation in here and it might help you decide if that's the way you want to go. If it doesn't fit what you need to do, I'd look at something else.
CANopen is very data centric. All CANopen slave nodes (things you control from a master node, like sensors or motor drivers) provide a dictionary that contains all configuration, control and data-acquisition data you need to read/write to get your job done. You can then read/write these objects individually via the SDO (service data object) mechanism, or read/write these objects periodically in timed CAN message transmissions via the PDO (producer data object) mechanism. Startup, shutdown, and reset, and heartbeat/node-malfunction detection are provided from NMT services, as well as a node ID assignment service (if you wish to use it). To implement a device using an embedded CANOpen library, you basically setup an object dictionary, setup PDOs to periodically send/receive data updates, and write user code that behaves based on those data entries. There are other things CANopen provides, but that is the bulk of it. Read this book for more info.
You can do that, but what you'll get on your CAN bus will be UART using CAN voltage levels. You have to supply the MCP2551 with CAN protocol messages if you want to communicate with CAN devices on your bus. Same for listening: CAN messages are so different from the UART format that the UART won't know what to do with them. You'll have at least frame errors all the time, and you won't get at the message's content.
This image shows the structure of a CAN message:
There's plenty microcontrollers around that have a CAN interface, sans the transceiver. It's for these that the MCP2551 was designed. In the past we've used the NXP LPC2294, which has 4 CAN interfaces. Each of them needs an MCP2551 to connect to a CAN bus. More recent controllers from NXP include the LPC1800 family, of which all members support CAN.
Best Answer
It appears that your question is not about CAN or even OBD2, but whatever protocol the ELM327 chip presents over the USB. If I understand it right (and I very well may not), the ELM chip tries to make the OBD information more uniform despite differences between manufacturers. If so, they surely must document how to talk to their chip. Did you look around for the protocol.
It also seems like you have access to the raw CAN bus via the Microchip converter. In that case you need to know the OBD2 standard, and perhaps how your particular car handles some of the proprietary aspects. The official OBD2 standard may be expensive, but I expect there is a lot about it on the net anyway.