Whereas RS-232 and RS-422 enable point-to-point serial links, the RS-485 standard enables multiple- node networks. Like RS-422, RS-485 provides differential signaling to enable communications across spans of twisted-pair wire exceeding 1.2 km. Unlike RS-422, the RS-485 standard allows up to 32 transmit/receive nodes on a single twisted pair that is terminated at each end. Modern low-load receivers that draw very little current from the RS-485 bus can be used to increase the number of nodes on an RS-485 network well beyond the original 32-node limit to 256 nodes or more. A single pair of wires is used for both transmit and receive, meaning that the system is capable of half-duplex (one-way) operation rather than full-duplex operation (both directions at the same time). Half-duplex operation restricts the network to one-way exchange of information at any given time. When node A is sending a packet to node B, node B cannot simultaneously send a packet to node A. RS-485 directly supports the implementation of bus networks. Bus topologies are easy to work with, because nodes can directly communicate with each other without having to pass through other nodes or semi-intelligent hubs. However, a bus network requires provisions for sharing access to be built into the network protocol. In a centralized arbitration scheme, a master node gives permission for any other node to transmit data. This permission can be a request-reply scheme whereby slave nodes do not respond unless a request for data is issued. Alternatively, slave nodes can be periodically queried by the master for transmit requests, and the master can grant permissions on an individual- node basis. There are many centralized arbitration schemes that have been worked out over the years.
A common distributed arbitration scheme on a bus network is collision detection with random
back-off. When a node wants to transmit data, it first waits until the bus becomes idle. Once idle, the node begins transmitting data. However, when the node begins transmitting, there is a chance that one or more nodes have been waiting for an opportunity to begin transmitting and that they will begin transmitting at the same time. Collision detection circuits at each node determine that more than one node is transmitting, and this causes all active transmitters to stop.
A common distributed arbitration scheme on a bus network is collision detection with random
back-off. When a node wants to transmit data, it first waits until the bus becomes idle. Once idle, the node begins transmitting data. However, when the node begins transmitting, there is a chance that one or more nodes have been waiting for an opportunity to begin transmitting and that they will begin transmitting at the same time. Collision detection circuits at each node determine that more than one node is transmitting, and this causes all active transmitters to stop.
