Bcc Calculation - Honeywell CIU 888 Protocol Manual

Part No.: 4417589_Rev13
Appendix A - GPU Communication Protocol
Item Position
STX 1
N 2
nn 3, 4
TOI 5
TOR 6
data 7, 8
ETX n-1
BCC n
A.5 BCC calculation
The Block Check Character (BCC) is calculated by taking the binary
sum without carrying over the seven individual data bits (exclusive or) of
all transmitted bytes, according to ANSI standard X3.28 - 1976. STX is
excluded; ETX is included. The BCC is also called the 'horizontal parity
check'.
Since the BCC summation can produce any one of the 128 code
combinations, be sure to interpret it as the BCC, not as something else.
For example, if the value of the BCC is 02 hexadecimal do not read this
as an STX character.
Here is the answer record from the CIU to the computer system:
STX 1 R
Protocol Manual - CIU Emulation
CIU 888
Description
Start of Text Character (Start of transmission)
CIU address
Instrument address
Type of Instrument:

B = 811, 854, 873 instrument with GPU protocol
and instruments with Instrument type B

C = Instruments with Gauge type C

E = 811 instrument with external alarms and
instruments with Gauge type E
Type of Record
See Protocol Manual of the related field instrument for
more information.
For the data in answer record, see Protocol Manual
End of Text Character (End of transmission)
Block Check Character
X 8 8 8 R 1 0 0 ETX BCC
A - 5