GE GFK-1541B manual Read Data from a Modbus/TCP Device

4

Read Data from a Modbus/TCP Device (3003)

The Read Data from a Modbus/TCP Device COMMREQ requests a data transfer from a Modbus/TCP device to the PLC. The Read Data COMMREQ must reference an active Modbus/TCP channel previously established with the Open Modbus/TCP Client Connection

COMMREQ.

Registers, Coils or Exception Status data may be read from the remote Modbus/TCP device. The Modbus Function Code specifies the data type. Valid Function Codes for the Read Data COMMREQ are presented in the following table:

Function	Description	Base Address	Data Unit Size	Maximum Data
Code				Units
1	Read Coils	00001	Bit	2000
2	Read Input Discretes	10001	Bit	2000
3	Read Multiple	40001	Register	125
	Registers		(16-bit Word)
4	Read Input Registers	30001	Register	125
			(16-bit Word)
7	Read Exception	Not Applicable	Byte	Not Applicable
	Status

An Address and Length specify the location of the data in the remote device and the number of data units to transfer. The Length is the number of Registers or Coils to transfer. Modbus Function Code 7, Read Exception Status does not require the address as the remove device retrieves the exception status from an internal location.

Coil data is always padded with zeros when less than an even multiple of 16 bits of coil data is returned to the PLC.

Data returned from the remote device is stored in the PLC data area specified in the Read Modbus/TCP Device COMMREQ. Data can be stored in any of the PLC data areas. Refer to page 4-14 for the list of data areas and identification codes for the PLC. Note that the first item referred to in each data area is item 1 not item 0.

The COMMREQ Status Word (CSR) indicates the success or failure of the Read Data COMMREQ. If the COMMREQ requests an invalid channel number or any other field is invalid 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.

GFK-1541B Chapter 4 Programming Modbus/TCP Channel Commands 4-11