Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 109)

Full-Duplex Protocol
Chapter 10
10Ć5
Block Check
The block check character (BCC) is a means of checking the accuracy of
each message packet transmission. It is the 2’s complement of the 8–bit
sum (modulo–256 arithmetic sum) of all data bytes between the DLE
STX and the DLE ETX BCC. It does not include any other message
packet codes or response codes.
For example,if message packet contained the data codes 8, 9, 6, 0, 2, 4,
and 3, the message packet codes would be (in hex):
10 02 08 09 06 00 02 04 03 10 03 EO
DLE STX Data DLE ETX BCC
The sum of the data bytes in this message packet is 20 hex. The BCC is the 2's
complement of this sum, or EO hex. This is shown in the following binary
calculation:
0010 0000 20 hex
11011011 1s compliment
+1
1110 0000 2s compliment (E0 hex)
To transmit the data value 10 hex, you must use the data code DLE DLE.
However, only one of these DLE data bytes is included in the BCC sum.
For example, to transmit the values 8, 9, 6, 0, 10, 4, and 3 hex, you would
use the following message codes:
Represents single data byte 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.