Allied Motion Spectrum Controls 2706-P22-SC User Manual page 75

Simulated Query and Response
End Of File Field
This field is used to signify the end of file for the transmission. It uses a Carriage
Return (0×0d).
Line Feed Field
This is the ready to respond field file (LF) and uses a line feed (0×0a).
Longitudinal Redundancy Check (LRC) Error Detection and
Calculation
Some sort of error detection is needed, because communication errors can occur
in an industrial environment due to machinery noise and electromagnetic
interference. The method for error detection used with Modbus ASCII is
Longitudinal Redundancy Check (LRC). The LRC is an 8-bit binary number
represented and transmitted as two ASCII Hexadecimal characters. The LRC is
produced by adding the message characters (ignoring the carry bit) and taking the
two's compliment of the result. The error check byte is done from the address up
to the data field.
The following is an example of how to calculate the LRC in a Modbus ASCII
transmission.
Example of LRC Calculations
Beg of
ADDR
Frame
05
Modbus ASCII Transmission
Message
Address
Function
Start Add H.O.
Start Add L.O.
Data H.O.
Data L.O.
Sum
1's Complement
+1
2's Complement
Transmitted as
H.O
FUNC
ADDR ADDR
06 00
Transmitted Bytes
Chapter 4: InView Protocol
L.O LRC
DATA
Error
1F 0265
05
06
00
1F
02
64
90
70
User's Manual 0300277-03 Rev. A
Ready to
EOF
response
70 CR
Binary
0000 0101
0000 0110
0000 0000
0001 1111
0000 0010
0110 0100
1001 0000
0110 1111
+0000 0001
0111 0000
4-13
rec.
LF