422
Q5: What’s the use of execution instruction LDD/OUTD etc?
A5:
When PLC executes program, state of input point state will map to image register. From then
on, PLC will refresh input state at the beginning of every scan cycle; if we use LDD
instruction, then the state of input point will not need map to image register; the same with
output point (OUTD).
LDD/OUTD instruction usually apply to the occasion that I/O need refresh immediately,
which makes the state of input and output avoid the influence of the scan cycle.
Input point X0 sequence chart of LDD and LD
Q6: Why the output LED keeps flashing when using ALT instruction?
A6:
For ALT and many calculation instructions, these instructions will execute every scanning
period when the condition is fulfilled (for example, the condition is normal ON coil). We
recommend that the condition is rising edge or falling edge.
Q7: Why the M and Y cannot output sometime?
A7:
Output mainly has two ways: 1. OUT instruction; 2. SET instruction. The coil will keep
outputting if there is no RST instruction.
Usually in the program, one coil M or Y should use the same output way. Otherwise, the coil
cannot output.
For example:
M0 is ON,M1 is OFF, Y0 cannot output
M0 is OFF, M0 is ON, Y0 will output
Reason: two different coils drive the same
output coil
Y0 will be ON for one scanning period
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