GE RX3i

Chapter 9. Modbus/TCP Client
GFK-2224Q January 2017 153
Command 3003, Example 4 – Read FIFO Queue
Read the FIFO Queue from the remote Modbus/TCP server. Store the FIFO Queue Data at location %W1. 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
13
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 00024 (0018) Modbus Function Code (Read FIFO Queue)
Word 10 00196 (00C4) Local PLC Memory Type
Word 11 00001 (0001) Local PLC Starting Address
Word 12 00048 (0030) FIFO Pointer Address
Word 13 00001 (0001) Data Size (Unused)
Word 14 00001 (0001) Unit Identifier
(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 24, Read FIFO Queue.
(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 150.
(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. Be sure this
area is large enough to contain the requested data without overwriting other application data.
(Word 12) FIFO Pointer Address: Word 12 is the FIFO pointer address in the Remote Device.
(Word 13) Data Size: Word 13 is unused because the return data size is dependent on the number of items in
the server’s FIFO queue when the command is received. Zero (0) through 32 registers can be returned as a
result of this function code.
(Word 14) Unit Identifier: This field is typically used by Ethernet to Serial bridges to specify the address of a
Modbus Slave on a multi-drop link. The Modbus/TCP Unit Identifier is a special control code used in a
Modbus/TCP message block.