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4-16 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual
–
May 2002 GFK-1541B
4
Write Data to a Modbus/TCP Device (3004)
The Write Data to a Modbus/TCP Device COMMREQ requests a data transfer from the PLC to a
Modbus/TCP server. The Write Data COMMREQ must reference an active Modbus/TCP channel
previously established with the Open Modbus/TCP Client Connection COMMREQ.
Registers or Coils may be written to the remote Modbus/TCP device. The Modbus Function Code
specifies the data type. Valid Function Codes for the Write Data COMMREQ are presented in the
following table:
Function
Code
Description Base
Address
Data Unit Size Maximum Data
Units
5 Write Coil 00001 Bit 1
6 Write Single Registers 40001 Register 1
16 Preset Multiple Registers 40001 Register 100
An Address Offset and Length specify the location in the Modbus/TCP device and the number of
data units to transfer. The Address Offset is the offset from the Base Address. The Length is the
number of Registers or Coils to transfer.
A PLC data area is the source for the data written to the Modbus/TCP device. The source of data
can be any of the PLC data areas (see page 4-14).
Function Code 5, Force Single Coil, is the only coil operation supported by the Modbus/TCP
Client. This function forces a Coil On or Off. To force a coil off, the value zero (0) is used as the
COMMREQ data value. To force a coil on, the value one (1) is used as the COMMREQ data
value.
The COMMREQ Status Word (CSR) indicates the success or failure of the Write Data
COMMREQ. If the COMMREQ specifies an invalid channel number or any other invalid field
the COMMREQ fails and the CSR is set to a non-zero value to identify the failure. See the section
“Status Data” later in this document for detailed CSR failure codes.