Emerson PACSystems RX3i - Page 177

PACSystems™ RX3i and RSTi-EP CPU Reference Manual Section 6
GFK-2222AK October 2019
Serial I/O, SNP & RTU Protocols 169
6.3.2.4 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.
6.3.2.5 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
76
2
Read Input Table
6
3 + 3rd byte
76
3
Read Registers
6
3 + 3rd byte
76
4
Read Analog Input
6
3 + 3rd byte
76
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
76
6
16
Preset Multiple
Registers
7 + 7th byte
76
6
17
Report Device Type
2
8
74
The receiver processes incoming data through the same CRC algorithm as the transmitter. The example for the receiver starts at the point
after all the data bits but not the transmitted CRC have been received correctly. Therefore, the receiver CRC should be equal to the transmitted
CRC at this point. When this occurs, the output of the CRC algorithm will be zero indicating that the transmission is correct.
The transmitted message with CRC would then be:
Address
Function
CRC–16
01
07
41
E2
75
The MSB and LSB references are to the data bytes only, not to the CRC bytes. The CRC MSB and LSB order are the reverse of the data byte
order.
76
The value of this byte is the number of bytes contained in the data being transmitted.