Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 238)

Chapter 17
Timer and Counter Instructions
17–6
Status Bits
•
The done bit (DN) is set when the accumulated value is equal to the
preset value. However, it is not reset when rung conditions become false;
it is reset only when the appropriate RES instruction is enabled.
•
The timing bit (TT) is set when rung conditions are true and the
accumulated value is less than the preset value. It is reset when the rung
conditions go false or when the done bit is set.
•
The enable bit (EN) is set when rung conditions are true; it is reset when
rung conditions become false.
The accumulated value must be reset by the RES instruction. When the RES
instruction having the same address as the RTO is enabled, the accumulated
value and the control bits are reset.
Effects of processor mode changes: When the processor changes from the
Run or Test mode to the Program or Fault mode, or user power is lost while
the timer is timing but not yet at the preset value, the following occurs:
•
The timer enable and timing bits remain set.
•
The accumulated value remains the same.
When you return to the Run or Test mode or power is restored, the following
can happen:
•
If the rung is true, the accumulated value remains the same and continues
incrementing from where it stopped. The enable and timing bits remain
set.
•
If the rung is false, the accumulated value remains the same, the timing
and enable bits are reset, and the done bit remains in its last state.