Mikroelektronika MIKROE-350 Fiche De Données

Q: How is CRC checksum calculated on RS485 master side?

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.

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