To help understand the process of reading or writing to the RMC’s registers over Modbus Plus, imagine the following diagram represents a Modbus Plus network with a Quantum PLC as the master and two RMC modules as slaves (the numbers indicate node addresses):
When the master wishes to read from or write to a node it must take the following steps:
Wait for the token from the other nodes.
Send the request to the slave over the network to read or write data.
Give up the token.
Wait for the slave to receive the token and respond over the network.
Data Paths
This operation can take several milliseconds to complete, especially if many nodes are on the network, which increases the token loop time. For this reason, Modicon allows several read and write transactions to be in progress at a time. Each device has multiple data ”r;paths”. The RMC has eight slave paths and therefore can handle eight simultaneous transactions. Most Modicon PLCs have four master paths. The master (PLC) must specify the slave data path when reading or writing data. This is described in greater detail in Using the MSTR Modicon Ladder Logic Block.
Another method of speeding up the communication is to utilize Global Data. For details on this feature, see Using Modbus Plus Global Data.
For details on reading and writing using a Modicon PLC, see Using the MSTR Modicon Ladder Logic Block.
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