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C4-6
IM 34M6H41-01E
3rd Edition : Aug. 1, 2005-00
(7)
Checksum
Checksum can be added for data checking. You can select whether to add the
checksum by using the Data Format Setting switch (Refer to Figures C2.4 and C3.4)
at the back of this module. If the checksum is enabled, a ‘checksum’ must be ap-
pended to a command when it is transmitted from the higher level computer (or dis-
play) to the FA-M3. When a command is transmitted with a checksum appended, then
a checksum is automatically appended to the response. If the checksum is disabled
using the setup switch, this element is not required.
How the checksum is calculated is shown below.
Add the ASCII codes of the characters following STX and preceding the checksum.
Extract the low order byte of the sum and express its hexadecimal value as a
character string (2 characters, 2 bytes) to obtain the checksum.
(Example) Reading 16 bits from X00201 from CPU number 1 of station number 01
with a 100 ms response wait time (when the ending character is CR)
6
36
B
42
STX
0
30
1
31
0
30
1
31
A
41
R
52
B
42
D
44
X
58
0
30
0
30
2
32
0
30
1
31
,
2C
1
31
9
39
02
ETX
03
CR
0D
Transmission text
(character string)
ASCII code
(Hexadecimal)
Range of checksum calculation
Check-
sum
Ending
character
FC040101_2.EPS
30+31+30+31+41+42+52+44+58+30+30+32+30+31+2C+31+36=3B9 (in hexadecimal)
The lower order byte ($B9) of the addition result ($3B9), when represented as a character string, “B9” becomes the
checksum.
(8)
ETX (End of Text)
This character terminates a block of text that is started with STX. The corresponding
character code is CHR$(3).
(9)
CR (Carriage Return)
This is a control code indicating termination. The corresponding character code is
CHR$(13) [0D in hexadecimal]. This control code is required only when the ending
character is enabled (See Figures C2.4 and C3.4) at the back of this module.
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