Overview
Modbus Plus is a local area network system which supports up to 64 devices (nodes) and transfers data at 1 million bits per second (1Mbaud). Each node on the network must have a unique node address. For details on changing the node address of the RMC, see Changing the Modbus Plus Node Address.
The Modbus Plus network uses the Modbus Protocol, which is a master-slave protocol. A token is passed from node to node on the network. The node holding the token is the master, and therefore controls the network and may request data from or send data to any other node.
The RMC acts only as a slave, which means it will respond to master requests, but will not initiate requests. Some examples of Modbus Plus masters are the Quantum PLC, SA-85 PC-based card, 984-series Modicon PLCs, and Modicon Compact Series PLCs.
Communicating with the RMC
The master may request to read or write 16-bit registers in the RMC. The following information is stored in these registers: status, command and parameter fields of all axes, Event Step table, Input to Event table, and all graphs. For details on the registers in the RMC, see RMC Register Map (Modbus Plus). For details on reading and writing these registers, see Reading and Writing Modbus Plus Registers.
Speeding up Reads from the RMC
In addition to the standard method of reading and writing these Modbus Plus registers, the RMC allows the Global Data feature of Modbus Plus to greatly speed up accessing frequently used data. For details on using this feature, see Using Modbus Plus Global Data.
Programming from a Modicon PLC
When using a Modicon PLC as the master, the user uses a special function block called MSTR. For details on using this function block, see Using the MSTR Modicon Ladder Logic Block.
Understanding the Active LED
The Modbus Plus communication module has a single green LED labeled ”r;Active”. This LED blinks according to the Modbus Plus standard:
Fast flash (once per 160msec) – This node is working correctly.
Slow flash (once per second) – This node is monitoring the network prior to going online. The module will be in this state for five seconds during power up.
2 flashes, off 2 seconds – This node never receives the token, but hears other nodes passing the token. This node may have a faulty transmitter.
3 flashes, off 1.7 seconds – No other nodes are heard on the network. Either this node is not connected to any other nodes, or the receiver is faulty. Check the cabling and all connections.
4 flashes, off 1.4 seconds – This node detected another node using the same address. You will need to change the address of this or the like-addressed node.
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