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GFK-1541B Chapter 4 Programming Modbus/TCP Channel Commands 4-15
4
Command 3003, Example 3 – Read Exception Status
Read the Exception Status from the remote Modbus/TCP server. Store the ExceptionData at
location %Q4(bit mode). Return the COMMREQ Status word to %R10.
Dec (Hex)
Word 1
00008 (0008) Length of Channel command Data Block
Word 2 00000 (0000) Always 0 (no-wait mode request)
Word 3 00008 (0008) Memory type of CRS word (%R)
Word 4 00009 (0009) CRS word address minus 1 (%R10) *
Word 5 00000 (0000) Reserved
Word 6 00000 (0000) Reserved
Word 7 03003 (0BBB) Read from a Modbus/TCP Device
Word 8 00006 (0006) Channel number (6)
Word 9 00007 (0007) Modbus Function Code
Word 10 00072 (0048) Local PLC Memory Type
Word 11 00004 (0004) Local PLC Starting Address
Word 12 00000 (0000) Reserved
Word 13 00001 (0001) Data Size
Word 14 00001 (0001) Unit Identifier
* Word 4 (CRS word address) is the only zero-based address in the Command Block. Only this
value requires subtracting 1 from the intended address.
(Word 7) Channel Command Number:
Word 7 identifies the COMMREQ as a Read Exception
Status from the Modbus/TCP device.
(Word 8) Channel Number:
Word 8 identifies the channel number previously allocated for
communication with the remote Modbus/TCP server.
(Word 9) Modbus Function Code:
Word 9 specifies Modbus Function Code 7, Read Exception
Status.
(Word 10) Local PLC Memory Type:
Words 10-11 specify the location in the local PLC where
the Ethernet Interface will store data received from the remote device . Valid values for Word 10
are listed on page 4-14.
(Word 11) Local PLC Memory Address:
Word 11 determines the starting address in the local
PLC in which the data from the remote device is to be stored. The value entered is the offset (1-
based) from the beginning of PLC memory for the memory type and mode specified in Word 10.
This offset will be either in bits, bytes, or words depending on the mode specified. Valid ranges
of values depend on the PLC’s memory ranges. The user is responsible for assuring that this area
is large enough to contain the requested data without overwriting other application data.
(Word 12) Reserved:
Word 12 is reserved and must be set to zero.
(Word 13) Data Size:
Word 13 is the data sizeand must be set to 1.
(Word 14) Unit Identifier:
Default is 1. This field is typically used by Ethernet to Serial bridges to
specify the address of a Modbus Slave on a multidrop link. The Modbus/TCP Unit Identifier is a
special control code used in a Modbus/TCP message block. This value is 1 for most Modbus/TCP
messages.