HP 8712ET User Manual page 271

NOTE
EXT TRIG
IN/OUT This rear panel female BNC connector allows
triggering of a analyzer sweep by an external signal.
When the external signal is pulled to a high TTL level,
the analyzer is triggered to do either one complete
sweep, or sweep to the next (frequency) point. (The type
of sweep that gets triggered is determined by the
softkey selection made under the Trigger Source
menu.) When the external signal is pulled to ground,
triggering is inhibited. This connector also acts as an
output: during a sweep, the analyzer pulses a low TTL
level each time a (frequency) point is measured. This is
an open-collector signal which you can drive low, but
must not drive high, since the analyzer also drives it.
LIMIT TEST
TTL IN/OUT This rear panel female BNC connector provides a
bi-directional open-collector TTL high signal. The
output goes high when the limit test passes. The output
goes low if the limit test fails. This is an open-collector
signal which you can drive low, but must not drive
high, since the analyzer also drives it.
Limit testing can be set independently on each of the two measurement
channels. If both measurement channels are being used, and the limit
test is ON for both channels, both measurement channels must pass for
the output to go high. See the following table.
ET User's Guide
Front/Rear Panel
Connectors
7-5