4-3
Section
CPM2A/CPM2C Communications Functions
239
When dividing commands such as WR, WL, WC, or WD that execute write op-
erations, be careful not to divide into separate frames data that is to be written
into a single word. As shown in the illustration below, be sure to divide frames so
that they coincide with the divisions between words.
@
FCS
↵
0
0
W
D
FCS
*
↵
Frame 1
Node
No.
Header
code
Data
One word of data
Data from the same word is not divided.
Frame 2 (128 characters maximum)
Terminator
Data
Delimiter
One word of data
Data from the same word is not divided.
(131 characters maximum)
x10
1
x10
0
FCS (Frame Check Sequence)
When a frame is transmitted, an FCS is placed just before the delimiter or termi-
nator in order to check whether any data error has been generated. The FCS is
8-bit data converted into two ASCII characters. The 8-bit data is the result of an
EXCLUSIVE OR performed on the data from the beginning of the frame until the
end of the text in that frame (i.e., just before the FCS). Calculating the FCS each
time a frame is received and checking the result against the FCS that is included
in the frame makes it possible to check for data errors in the frame.
@
→
40
→
0100
0000
XOR
1
→
31
→
0011
0001
XOR
0
→
30
→
0011
0000
XOR
R
→
52
→
0101
0010
XOR
0
→
30
→
0011
0000
XOR
0
→
30
→
0011
0000
XOR
to
0
→
30
→
0011
0000
XOR
1
→
31
→
0011
0001
0100
0010
↓
↓
4
2
FCS
*
0
1
R
R
0
@
0
0
1
4
2
Text
Node No. Header code
FCS calculation range
Terminator
↵
Converted to hexadecimal.
Handled as ASCII characters.
Calculation results
ASCII code
Leftmost
Rightmost
Precautions for Long
Transmissions
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