Why stays the output low even if the condition is true and a value should be transmitted?

FAQ #100413

 

Problem
The output of a logic operation stays low, even if the condition is true
or
The output to a counter/timer stays 0 even if a value should be transmitted

Reason
If the output accesses a physical address p.ex. O 48, and there is no Digital Output card plugged in this slot, the output value in the program will always stay low.
If the output value for a Timer or Counter is negative p.ex. -1 which is an invalid value for a Timer or Counter the content of the timer or Counter will always stay 0

Solution
Plug a card or map the output to flag instead of a physical address
Write a valid value to the Timer/Counter