Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 133)

Half-Duplex Protocol
Chapter 11
11Ć6
Block Check
The block check character (BCC) is a means of checking the accuracy of
each packet transmission. It is the 2’s complement of the 8–bit sum
(modulo–256 arithmetic sum) of the slave station number (STN) and all
the data bytes in the packet. For polling packets, the BCC is simply the
2’s complement of STN. The BCC does not include any other message
packets codes or response codes.
For example, if the master station wanted to send the data codes 8, 9, 6, 0,
2, 4, and 3 to slave station 20 hex (40 octal), the master message codes
would be (in hex):
10 01 20 10 01
DLE SOH STN DLE STX
08 09 06 00 02 04 03 10 03 A0
Data
DLE ETX BCC
The sum of the STN and data bytes in this message packet is 40 hex. The
BCC is the 2’s complement of this sum, or C0 hex. This is shown in the
following binary calculation:
0100 0000 40 hex
1011 1011 ls complement
+1
1010 0000 2s complement (E0 hex)
To transmit the STN or data value 10 hex, you must use the data code
DLE DLE. However, only one of these DLE text characters is included in
the BCC sum. For example, to transmit the values 8, 9, 6, 0, 10, 4, and 3
hex, a slave station would use the following message codes:
Represents single text value of 10
10 02 08 09 06 00 10 10 04 03 10 03 D2
DLE STX Data DLE ETX BCC
In this case, the sum of the data bytes is 2E hex because only one DLE
text code is included in the BCC. So the BCC is D2 hex.