CANSim message format

On hardware side of CANSim, the communication is handled by RJ45 sockets and plugs. Its time to discuss the message exchange protocol that I use.

I use 1Mbps CAN with extended IDs to handle communication between gauges in the cockpit.

Every message is identified by it’s ID (29 bits) and payload (8 bytes).

CAMSim message format

CAN id (high bit -> low bit, total 29 bits, only extended format supported, data is little-endian):

• 4 bits: (25 .. 28) priority (0 .. 15, less is more priority)
• 5 bits: (20 .. 24) port
• 10 bits (10 .. 19): dst address (0 .. 1023)
• 10 bits (0 .. 9): src address (0 .. 1023)
• CAN payload (0 - 8 bytes)

Every instrument has local address in CAN network and also one or more “ports” which work as input/output data channels.

Each address has max 10 bits allowing to maximum 1024 devices in the network.

Address reservation:

• 0 - unknown
• 1 - 15 master (connected to simulation PC)
• 16 - 1023 client devices

Payload is full defined by the device and port. Every devices has to specify how many ports it has and data input/output for each port.

Example

So let’s assume we have an indicated airspeed gauge. It has the following properties labeled somewhere on the enclose: address 25, 1 input-only port, accepts 4 byte IEEE floating point value. We’re going to send a speed update to 45 kn from a sim master with address 1.

The message will look like this:

• priority 0 (default)
• port 0
• dst address 25
• src address 1
• payload 4 bytes, 45