can

Connecting CAN modules together

A single CAN module will not work by itself. It needs another CAN device to send back an ACK before it will work properly.

The CANH and CANL wires go to all modules. They are polarity sensitive so CANH must go to CANH and CANL to CANL.

These wires should ideally be a twisted pair although screening is not necessary. While it would be usual to wire the bus sequentially round the various modules, it is not essential and individual nodes can be star connected if this is more convenient. The CBUS wiring should be kept separated from any DCC supply wiring to prevent possible interference.

The CAN bus requires termination resistors at some point in the network. If the bus is wired sequentially round the modules, then a resistor of 120 ohms should be fitted across the bus at each end. For small layouts, it is sufficient to have a resistor across the bus at one point. The value is not critical and a 68 ohm resistor will suffice.

There is a limit of 110 modules on any bus segment. For larger numbers of modules, a repeater or hub module will need to be designed to join bus segments together.

The resistor on the MCP2551 pin 8 controls the transmitter edge speed, a wire link gives about 25V/uS, 100k gives about 4V/uS. Either seems to work, 100k should radiate less interferance but more testing is needed.