Delta RMC101 - Manual (Page 429)

Modbus Plus 5.3
Communications 5-179
and Modbus/RTU). The addresses documented in RMC Register Map (PROFIBUS-DP Message
Mode) happen to already have one subtracted, so you may prefer to use that register map even
though it describes PROFIBUS-DP.
• The RMC handles incoming packets on a first-in first-out (FIFO) basis, making it possible to send
multiple requests and then wait for the replies. However, the RMC cannot hold a large number of
requests at a time, so it is advised that only one or two requests be sent before waiting for the
replies.
• The RMC can handle up to four open TCP/IP connections at once. Typically each device uses
one connection at a time, allowing the RMC to typically be connected to four devices at once.
• Multiple-register reads and writes are limited to 100 words for writes and 125 words for reads—
per the Modbus/TCP specification—even though the length field in the Modbus/TCP header
makes it look like larger packets would be supported.
5.3 Modbus Plus
5.3.1 Using the Modicon Modbus Plus Communication
Module
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.