Table of Contents
Advertisement
Appendix
B.6 CRC calculation
Only the internally broadcasted flow-value is checked. The expected broadcast-rate is read at
power up. If a broadcast is not received after 10 times the expected broadcast rate, the broadcast
rate is re-read from the USM (it may have been changed since power up). If the broadcast rate
was not changed (and the rate was not zero), the Run Indicator is set OFF. Reception of ten
broadcasted (flow-) values within 10 times the expected window will set the run-indicator TRUE
again.
B.6
CRC calculation
The Cyclical Redundancy Checking (CRC) field is two bytes, containing a 16–bit binary value. The
CRC value is first generated by the transmitting device, which appends the CRC to the message.
The receiving device recalculates a CRC during receipt of the message, and compares the
calculated value to the actual value it received in the CRC field. If the two values are not equal an
error results.
In the following a short text description of how the CRC is calculated is provided. This description
is then followed by a C programming example.
CRC calculation
1. Load a 16–bit register with FFFF hex (all 1's). Call this the CRC register.
2. Exclusive OR the first 8–bit byte of the message with the low–order byte of the 16–bit CRC
register, putting the result in the CRC register.
3. Shift the CRC register one bit to the right (toward the LSB), zero–filling the MSB. Extract and
examine the LSB.
4. (If the LSB was 0): Repeat Step 3 (another shift). (If the LSB was 1): Exclusive OR the CRC
register with the polynomial value 0xA001 (1010 0000 0000 0001).
5. Repeat Steps 3 and 4 until 8 shifts have been performed. When this is done, a complete 8–bit
byte will have been
6. processed.
7. Repeat Steps 2 through 5 for the next 8–bit byte of the message. Continue doing this until all
bytes have been processed.
8. The final content of the CRC register is the CRC value.
9. When the CRC is placed into the message, its upper and lower bytes must be swapped as
described below.
Placing the CRC into the Message
When the 16–bit CRC (two 8–bit bytes) is transmitted in the message, the low-order byte will be
transmitted first, followed by the high-order byte.
For example, if the CRC value is 1241 hex (0001 0010 0100 0001):
Addr
Func
68
Data count
Data n
Data n+1
Data n+2
Operating Instructions, 03/2021, A5E00753974-AB
Data n+x
CRC LO
0x41
Modbus RTU RS 485
CRC HI
0x12
Advertisement
Table of Contents
