GE PACSystems RX7i Reference Manual page 260

Message (23): Read Write 4x Memory
Performs a combination of one read and one write operation in a single Modbus transaction. The function
can write new contents to a group of 4x registers, and then return the contents of another group of 4x
registers. Broadcast is not supported.
Query:
The query specifies the starting address and quantity of registers of the group to be read. It also specifies
the starting address, quantity of registers, and data for the group to be written. The Byte Count field
specifies the quantity of bytes to follow in the Write Data field.
Here is an example of a query to read six registers starting at register 5, and to write three registers
starting at register 16, in slave device 17:
GFK-2222AD
Field Name
Slave address
Function
Read Reference Address Hi
Read Reference Address Lo
Quantity to Read Hi
Quantity to Read Lo
Write Reference Address Hi
Write Reference Address Lo
Quantity to Write Hi
Quantity to Write Lo
Byte Count
Write Data 1 Hi
Write Data 1 Lo
Write Data 2 Hi
Write Data 2 Lo
Write Data 3 Hi
Write Data 3 Lo
Error Check (LRC or CRC)
April 2018
Chapter 6. Serial I/O, SNP & RTU Protocols
Example
(Hex)
11
17
00
04
00
06
00
0F
00
03
06
00
FF
00
FF
00
FF
--
249