Allen-Bradley 1404-M4, User Manual

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Publication 1404-UM001D-EN-E - October 2004
4-26 Communications
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 the user is familiar with Modbus communications. 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
http://www.modbus.org )
Modbus is a half-duplex, master-slave communications 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
will never transmit data without receiving a request from the master
node. The slave nodes will never communicate with each other. The
master node initiates only one Modbus transaction at a time.
The Powermonitor 3000 supports Modbus RTU, the version of
Modbus applied to serial communications in which each byte of data
consists of 2 hexadecimal values. Modbus ASCII, Modbus Plus and
Modbus TCP are not supported.
The Powermonitor 3000 does not initiate Modbus commands but
responds to commands sent by the Modbus master. The following
Modbus function codes are supported:
•
03 Read Holding Registers
•
04 Read Input Registers
•
16 Write Multiple Holding Registers
IMPORTANT
Because the floating-point word order in the
ControlLogix controller is reversed from that in the
Powermonitor 3000, your ladder logic will need to
reverse the word order so the data may be
interpreted correctly. The swap byte (SWPB)
instruction performs this function.