ABB REF601 CEI - Page 76

1MDU07205-YN rev. F
REF601 CEI
User’s manual
76
Start
3.5 Char
Address
8 bits
Function
8 bits
Data
N x 8 bits
CRC Check
16 bits
End
3.5 Char
The entire message frame must be transmitted as a continuous stream of
characters. If a silent interval of more than 1.5 character times occurs between two
characters, the message frame is declared incomplete and should be discarded by
the receiver.
Frame 1
3.5
character
3.5
character
4.5
character
Frame 2 Frame 3
Fig. 44 – RTU Message Frame
7.3.2 CRC Checking:
The RTU mode includes an error–checking fi eld that is based on a cyclical redundancy
checking (CRC) method performed on the message contents. The CRC fi eld checks
the contents of the entire message. It is applied regardless of any parity checking
method used for the individual characters of the message.
The CRC fi eld contains a 16–bit value implemented as two 8–bit bytes. The CRC fi eld
is appended to the message as the last fi eld in the message. When this is done, the
low–order byte of the fi eld is appended fi rst, followed by the high–order byte. The
CRC high–order byte is the last byte to be sent in the message.
Serial Communication