Mikroelektronika MIKROE-442 데이터 시트
mikroBasic PRO for dsPIC30/33 and PIC24
MikroElektronika
449
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 xor address
crc_send = crc_send xor data[0] ‘ if exists
crc_send = crc_send xor data[1] ‘ if exists
crc_send = crc_send xor data[2] ‘ if exists
crc_send = crc_send not crc_send
if ((crc_send = START_BYTE) or (crc_send = STOP_BYTE)) then
crc_send = crc_send + 1
end if
NOTE: DATALEN<4..0> can not take the START_BYTE<4..0> or STOP_BYTE<4..0> values.