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APPENDIX
Appendix 3 Precautions For Reading A Digital Output Value
(For Hardware Version "C" or Earlier)
Example 1) When a digital output value is read from the PLC CPU during A/D conversion processing
(during carry occurrence) of the A1S66ADA
A/D conversion processing
Digital output value
255(00FF
256(0100
H
Example 2) When a digital output value is read from the PLC CPU during A/D conversion processing
(during borrow occurrence) of the A1S66ADA
A/D conversion processing
Digital output value
256(0100
255(00FF
App - 4
If a digital output value is read from the PLC CPU during A/D conversion processing of
the A1S66ADA (For Hardware Version "C" or Earlier), data may be read to the lower
order byte (Xn0 to Xn7) and higher order byte (Xn8 to XnB) at different timings.
In this case, old A/D conversion data are stored into the lower order byte, and new A/D
conversion data into the higher order byte as the read data from the PLC CPU,
resulting in data mismatch. (When a carry or borrow occurs, a difference of 256 occurs
in the digital output value.)
Read processing from PLC CPU
Lower order byte (Xn0 to Xn7) read (FFH)
)
H
Higher order byte (Xn8 to XnB) read (01H)
)
Read processing from PLC CPU
Lower order byte (Xn0 to Xn7) read (00H)
)
H
Higher order byte (Xn8 to XnB) read (00H)
)
H
The read digital output value is compared every scan with the digital output value read
at the previous scan to update only normal data.
An example of a program to read a digital output value converted from analog on
Channel 3 is shown below.
POINT
Since data inconsistency does not occur in modules of hardware version "D" or
later, measures for updating only normal data are not needed.
For checking the hardware version, refer to the next page.
MELSEC-A
Read data
511(01FF
)
H
Read data
0(0000
)
H
App - 4
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