The network layer handles the actual communication between devices that takes place on the RS485 bus. Since the RS485 interface is primarily an electrical specification, the conversation can end there, but since it supports multipoint, it needs to be addressed in the OSI model.

There is no fixed specification for addressing at the network layer, but the RS485 bus must be properly managed by the host to avoid bus conflicts. Bus collisions occur when multiple devices try to communicate at the same time, which is very detrimental to the network. When a collision occurs, the transmitters collide at both ends and effectively create a short circuit. This causes each device to draw a lot of current, putting the transceiver into thermal shutdown.

To avoid communication conflicts, the host controls the bus and calls individual devices. This is usually accomplished by having a command set that only a specific device can recognize or by having a specific address for each device. Since the bus is shared among all devices, each device sees the command/address sent by the master, but only responds when that single device is asserted.

Figure 11: OSI model defining the network layer


 

Figure 11: OSI model defining the network layer

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