Rockwell Automation 1404-M4_M5_M6_M8 Powermonitor 3000 User Manual, Firmware rev. 3.0 or LATER User Manual
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Publication 1404-UM001F-EN-P - November 2009
Chapter 4 Communication
Because of the DF1 protocol’s inherent handshaking, the completion
of each message may be used to activate the next message, without
any additional programmed delay.
Modbus RTU slave protocol
We assume that you are familiar with Modbus communication. The
information provided in this section is general, rather than specific.
Refer to glossary at the end of this publication for definitions of
unfamiliar terms.
For more information about the Modbus RTU Slave protocol, see the
Modbus Protocol Specification (available from
).
Modbus is a half-duplex, master-slave communication protocol. The
network master reads and writes coils and registers and obtains
diagnostic information of the multiple slaves. The Modbus protocol
allows a single master to communicate with a maximum of 247 slave
devices (however no more than the physical limitations of the RS-485
or RS-232 ports permit). The master device on a Modbus network is
not assigned an address.
Modbus messages are always initiated by the master. The slave nodes
never transmit data without receiving a request from the master node.
The slave nodes never communicate with each other. The master
node initiates only one Modbus transaction at a time.
The power monitor supports Modbus RTU, the version of Modbus
applied to serial communication in which each byte of data consists of
two hexadecimal values. Modbus ASCII, Modbus Plus and Modbus
TCP are not supported.
IMPORTANT
Because the floating-point word order in the ControlLogix controller is
reversed from that in the power monitor, your ladder logic needs to
reverse the word order so the data may be interpreted correctly. The
swap byte (SWPB) instruction performs this function.