Fig. 6 illustrates the overall architecture of the railway controller network. There are 3 types of boards in the network, the command node, the control nodes and the bridges. Only the command node MUST be present in all railway controller networks, all other nodes are OPTIONAL although it should be noted, a network with only the command node wouldn’t be very useful.
The I2C bus MUST support arbitration using SDA to allow for multiple controllers to be on the bus at once.