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414
mikoC PRO for PIC32
MikroElektronika
Message format and CRC calculations
Q: How is CRC checksum calculated on RS485 master side?
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START_BYTE = 0x96; // 10010110
STOP_BYTE = 0xA9; // 10101001
PACKAGE:
--------
START_BYTE 0x96
ADDRESS
DATALEN
[DATA1] // if exists
[DATA2] // if exists
[DATA3] // if exists
CRC
STOP_BYTE 0xA9
DATALEN bits
------------
bit7 = 1 MASTER SENDS
0 SLAVE SENDS
bit6 = 1 ADDRESS WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE
0 ADDRESS UNCHANGED
bit5 = 0 FIXED
bit4 = 1 DATA3 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE
0 DATA3 (if exists) UNCHANGED
bit3 = 1 DATA2 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE
0 DATA2 (if exists) UNCHANGED
bit2 = 1 DATA1 (if exists) WAS XORed with 1, IT WAS EQUAL TO START_BYTE or STOP_BYTE
0 DATA1 (if exists) UNCHANGED
bit1bit0 = 0 to 3 NUMBER OF DATA BYTES SEND
CRC generation :
----------------
crc_send = datalen ^ address;
crc_send ^= data[0]; // if exists
crc_send ^= data[1]; // if exists
crc_send ^= data[2]; // if exists
crc_send = ~crc_send;
if ((crc_send == START_BYTE) || (crc_send == STOP_BYTE))
crc_send++;
NOTE: DATALEN<4..0> can not take the START_BYTE<4..0> or STOP_BYTE<4..0> values.