Rockwell-automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command Manual do Utilizador descarregar pdf (Página 79)

5–7Data-link Layer Message Frames

Publication

17706.5.16 - October 1996

The full-duplex and half-duplex slave and master protocol examples

below provide you with procedures for determining the CRC-16

value.

Full-duplex and half-duplex slave protocol

data_byte = all linklayer data, ETX

CLEAR CRC_REGISTER

FOR each data_byte

GET data_byte

XOR (data_byte, right eight bits of CRC_REGISTER)

PLACE RESULT in right eight bits of CRC_REGISTER)

DO 8 times (on CRC register_

Shift bit right, shift in 0 at left

IF bit shifted = 1

XOR (CONSTANT, CRC_REGISTER)

PLACE RESULT in CRC_REGISTER

END IF

END DO

END FOR

TRANSMIT CRC_REGISTER as 2byte CRC field (low byte, high byte)

Half-duplex master protocol

data_byte = STN, STX, all linklayer data, ETX

CLEAR CRC_REGISTER

For each data_byte

GET data_byte

XOR (data_byte, right 8 bits of CRC_REGISTER)

PLACE RESULT in right 8 bits of CRC_REGISTER

DO 8 times

Shift bit right, shift in 0 at left

IF bit shifted = 1

XOR (CONSTANT, CRC_REGISTER)

PLACE RESULT in CRC_REGISTER

END IF

END DO

END FOR

TRANSMIT CRC_REGISTER as 2 byte CRC field

Below is an actual frame that you can use to validate your CRC

routine:

Use this frame to validate the CRC

53 B911 004110 02 07 00 00 00 00 00 00 00 00 00 00 00 00 10 03 6B 4C

1 2 ... 74 75 76 77 78 79 80 81 82 83 84 ... 250 251

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Rockwell-automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command Manual do Utilizador Página 79