142 SI-Ethernet User Guide
Issue: 3
7.2.10 FC23 – Read/Write multiple registers
This function code allows a contiguous series of registers to be written and another contiguous
series of registers to be read. The maximum number of registers that can be read is 120 and
similarly the maximum number of registers that can be written is 120, this allows up to 120 16-bit
parameters or 60 32-bit parameters to be read and / or written in a single transaction. If this is
exceeded the server will issue an exception response code 2.
Master request data Slave response data
The normal response includes the function code, number of data bytes in the read block followed
by the register data (unless an exception occurs).
If 32-bit parameter addressing is used:
• For each parameter read or written, two 16-bit registers must be used in the request
• For each parameter written, four bytes must be specified in the request
• For each parameter read, four bytes of data will be used in the response
Byte Description Byte Description
7 Function code 0x17 7 Function code 0x17
8
Start read register address
(MSB)
8
Length of data in read block
(Bytes)
9
Start read register address
(LSB)
9
Register data
(MSB)
10
Number of registers to read
(MSB)
10
Register data
(LSB)
11
Number of registers to read
(LSB)
12
Start write register address
(MSB)
13
Start write register address
(LSB)
14
Number of registers to write
(MSB)
15
Number of registers to write
(LSB)
16
Length of register data to write
(Bytes)
17
Register data 0
(MSB)
18
Register data 0
(LSB)
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