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2 Simple Application Controllers
The Command String 7
7 - 13
7.3.10 Sum Check
The Sum Check is the four digit Hexadecimal total of the numbers between the Format No. and
the ETX command. The Sum Check numbers are Hexadecimal numbers with the Lower byte
written directly after the ETX command and the Higher byte as the last two digits in the
Command String. The Sum check is used to verify the validity of the message.
Format B - READ message
41H+00H+00H
Computer
α
2 Controller
+01H+42H+01H+00H
Sum Check = 00 85 H
S
T
X
E
T
X
No. comm
bytes
07H
Format No.
41H
Station
No.
00H
Command
00H
No. of
devices
01H
Device
code
42H
Device No.
(LB)
01H
Device No.
(HB)
00H
Sum check
(LB)
85H
Sum check
(HB)
00H
Lower byte
Higher byte
or
STX
STX
ETX
No. comm
bytes
03H
Format No.
40H
Station
No.
00H
Complete
flag
21H
No. comm
bytes
05H
Format No.
41H
Station
No.
00H
Command
00H
No. of
devices
01H
Device
status
00H
Sum check
(LB)
42H
Sum check
(HB)
00H
41H+00H+00H+01H+00H
STX
NAK
Format No.
Station No.
STX
Error Code
No. comm
bytes
04H
Sum Check = 00 42 H
Lower byte
Higher byte