AB PLC5 - when are timers evaluated?

Hi,

I am using an AB PLC5. I use a timer (TON) in my logic. I have two rungs that use the .DN bit of this timer as an XIC. Lets say these are rung #10 and rung #20.

I am wondering when the PLC updates the timer .DN bit. It would screw me up if the PLC changed the .DN bit while it was in between processing rungs #10 and #20. Can I be sure that for every scan, rungs #10 and #20 will see the same value for this .DN bit?

Thanks,
Paul

The TON timer DN bit is set when the timer is evaluated with input conditions true and its ACC>=PRE. It will remain set until the next time the timer is evaluated with false input logic (not ENabled). At that time the DN bit will be reset and the acc will be reset also. So if, on rung ten, the timer DN bit is set, it will remain set until rung ten is evaluated on the next scan at the very least.

If the timer instruciton is placed on a rung before rung 10 or on a rung after rung 20, you won't have a problem, it will always be the same state on rung 10 and 20. If you put the timer instruction on rungs 11-19, then the poential exists for rung 10 and 20 to be evaluated differently.

OkiePC said:

...The TON timer DN bit is set when the timer is evaluated with input conditions true and its ACC>=PRE....

Click to expand...

FYI, this not entirely true. Consider this: make a timers preset equal to 10 and make the condition true so that the timer counts up and hits the preset, setting the done bit. Now change the preset to something greater than what is in the accumulated value (say 20). Even with the rung condition true and the accumulated value less than the preset value, the timer will not continue to count nor will the done bit clear.

Apparently the done bit latches once the count hits the preset and continues to be set ONLY until the preceding rung conditions go false. Changing the preset to something higher OR the accumulated to something lower has no effect on the done bit.

Normally, this means nothing but I had run into one circumstance where the customer was changing a preset through a Panelview and wondered why the action being stopped on the timer's done bit did not continue for that cycle (ie pickup where it left off).

Yes, I understand that, I stated when the bit would be set, and that it would not be reset until the rung logic goes false. Thanks for helping clarify the details.

Paul C.