GE PACSystems RX7i Reference Manual page 246

Calculating the Length of Frame
To generate the CRC-16 for any message, the message length must be known. The length for all types of
messages can be determined from the table below.
RTU Message Length
Function Name
Code
0
1 Read Output Table
2 Read Input Table
3 Read Registers
4 Read Analog Input
5 Force Single Output
6 Preset Single Register
7 Read Exception Status
8 Loopback/Maintenance
9-14
15 Force Multiple Outputs
16 Preset Multiple
Registers
17 Report Device Type
18-21
22 Mask Write 4x Registers
23 Read/Write 4x Registers
24–66
67 Read Scratch Pad
68-127
128-255
86
The value of this byte is the number of bytes contained in the data being transmitted.
GFK-2222AD
Chapter 6. Serial I/O, SNP & RTU Protocols
Query or Broadcast Message
Length Less CRC Code
Not Defined
6
6
6
6
6
6
2
6
Not Defined
86
7 + 7th byte
86
7 + 7th byte
2
Not Defined
8
86
13+byte 11
Not Defined
6
Not Defined
Not Defined
April 2018
Response Message Length
Less CRC Code
Not Defined
86
3 + 3rd byte
86
3 + 3rd byte
86
3 + 3rd byte
86
3 + 3rd byte
6
6
3
6
Not Defined
6
6
8
Not Defined
8
86
5+byte 3
Not Defined
86
3 + 3rd byte
Not Defined
3
235