GE PACSystems RX7i - Page 246

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