The RMC SERIAL supports two network topologies: point-to-point and multi-drop. Which topologies are available depend on the line driver (RS-232, RS-422, or RS-485) used. See Line Drivers: RS-232/422/485 for details on choosing the appropriate line driver.
Point-to-Point
Point-to-point means that exactly two devices are wired together. For the RMC, this means that there will be one RMC wired to one host. All three line drivers support point-to-point, as shown below:
Note: The above four-wire RS-422/485 diagram shows biasing internal to the RMC on the Tx wire pair. This is not available on RMC SERIAL hardware revision 1. Biasing will have to be provided externally or in the host.
The RS-422 and RS-485 diagrams above show biasing and termination included. Termination and biasing can be left out of networks at the expense of maximum cable distance and noise immunity. See RS-422/485 Termination and Biasing for details.
Multi-Drop
Only RS-485 supports multi-drop. Multi-drop is the connecting of multiple slaves with a single master. Slaves should be chained together. Neither a star topology nor a chain with long stubs (wires from the main chain to the device) should be used. These topologies will cause excessive ringing on the network and unreliable data transmission.
The following diagram shows a typical multi-drop chain. Notice that termination should only be located at the extreme ends of the network:
The number of devices that can be connected to the network is dictated by the number of unit loads that each represents. According to the TIA/EIA-485-A specification, there can be a maximum of 32 unit loads connected to a single network. Each RMC represents ¼ unit load for a total of 124 RMCs on the network, assuming the host is a unit load.
The following diagram shows one host with two RMC controllers in a daisy-chained two-wire RS-485 configuration:
The following four-wire RS-485 network diagram is also supported by the RMC and allows full-duplex communications to the host from the RMC. Most multi-drop protocols do not support full-duplex communications between devices and so the actual utility must be carefully weighed against the extra cost of the cabling required for implementation.
Note: The above 4-wire multi-drop RS-485 network diagram shows internal termination and biasing on the RMC for the Tx wire pair. This is not available on RMC SERIAL hardware revision 1. Termination will have to be provided externally. Biasing will have to be provided externally or in the host.
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