ABB PM861AK01 3BSE018157R6 controller

RS485 Communications Standard RS485 is a differential twisted pair interface. A key feature of RS485 over RS422 is its ability to support configurations where multiple line-drivers share a common twisted pair, as distinct from multi-drop (i.e. multiple receivers only) which is supported by both RS422 and RS485. There are several modes of operation for the RS485/RS422 ports of the CI.

This mode is a 4 wire point to point connection. The System Configuration setting for this mode is ‘rs485fd’. The receive end of the link should always be terminated. The CI shall be AC terminated with a 120 Ω resistor and 68 nF capacitor in series across the receive pair. The Interface Adapter (8153 for the CI and 8130 for the MP) provides link selectable terminations for the receive circuit. A diagram of the topology is shown in Figure 20 below

RS485 Full Duplex Multiplexed 

This mode is a 4 wire multi-drop connection. The System Configuration setting for this mode is ‘rs485fdmux’. On circuits with multiple transmitters the line should be terminated at both ends. On circuits with one transmitter the line should be terminated at the far end (usually the last receiver in the chain). The Interface Adapter (T8153) provides links for termination on the receiver and transmitter circuit. A diagram of the topology is shown in Figure 21 below.

Full Duplex Multi-drop Topology

 The maximum number of nodes that the CI supports before hardware build C for multi-drop applications is 6. From hardware build C, the module has internal terminations changed to support the full 32 drop capacity. 6.2.3. RS485 Half Duplex Multiplexed This mode is a 2 wire multi-drop connection. The System Configuration setting for this mode is ‘rs485hdmux’. As this configuration contains multiple transmitters on the same circuit there should be terminations at both ends. The Interface Adapter (T8153) provides links for both terminations since the receive and transmit are multiplexed onto the ‘Receive Circuit’

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