Key Digital ES381 - Manual (Page 17)

2.5 Counter Modes
2.5.1 General
In RUN mode, the counter continually display readings (assuming
these is an input signal of course).
Clicking ONCE, takes a single reading and then places the counter
into STOP mode, so that further readings cease.
During measurements, the indicator in the reading panel changes
colour depending on the state of the input signal or internal gate:
Yellow means the first stage of integration has started (could be the
first external signal edge or the start of the internal gate period).
Elan Digital Systems Ltd. 17 USBcount50 USER’S GUIDE Iss1