Specifications

Troubleshooting
60
COUNTING
There are several reasons why a counter may not
seem to be counting correctly.
1) Counting is not enabled. The COUNT led
does not turn on when the START key is
pressed. This is because the counters need to
be reset. Use the STOP key to reset the
counters. Verify that the preset count period is
long enough so that a nonzero count can
occur.
2) The counter input is set to the wrong source or
input. Check the counter input menu line and
the counter discriminator output for the correct
signal. If the preset counter has no input, then
the count period will never terminate.
3) The counter is gated. If the count led is on and
the source is correct but no counting occurs,
the counter gate may be disabling the counter.
Check that the gate is in CW mode if no
gating is needed or, if gating is required, that
the trigger is connected and discriminating
correctly. The gate may be viewed through the
GATE output. The output is low when the gate
enables the counter.
4) Incorrect gate position. When nanosecond gate
delays or widths are used, timing accuracy is
critical. Gate timing should be verified by
viewing the DISC and GATE outputs on an
oscilloscope. Be sure to use equal length
cables so as not to introduce timing skew. The
timing accuracy of the outputs from the SR400
is 2 ns.
5) Bad gate triggering. The trigger input may not
be discriminated correctly. Check the trigger
slope and level. The discriminated trigger
pulses may be viewed through the T DISC
output when the input to counter T is TRIG.
Double triggering or the wrong slope can
cause erroneous gates.
6) Incorrect discriminator levels or slopes. Verify
that the discriminator settings are correct. Use
the DISC output to check the discriminator
output pulses on an oscilloscope.
7) Signal inputs are overloaded. Check that the
inputs are <±300 mV or, if the signal exceeds
300 mV, that the overloads do not last longer
than 10 µs. If the overload lasts longer than 10
µs, the input amplifiers will not recover for
about 200 ns.
8) Discriminators are inhibited. Check that the
INHIBIT input on the rear panel is not held at
TTL high. Leave the INHIBIT input
unconnected when not in use.
9) EXT STOP is resetting the counters. If the
DWELL TIME is programmed (i.e. not
EXTERNAL) an EXT STOP will reset the
counters. In general, EXT STOP should be
disconnected when the DWELL TIME is
NOT EXTERNAL.
10) The DWELL TIME is EXTERNAL and EXT
START and STOP are disconnected or
connected to the wrong signal sources.