It is an improved version of the 5T1. The job of the 5T1A is to produce three signals: the sampling pulse, the horizontal sweep, and the blanking signal. The input to the 5T1A is a trigger signal. The trigger can arrive from any of four sources:
- internally from the trigger pick-off in a 4S1 or 4S2A plug-in
- externally via a GR-874 connector on the front panel of the 5T1A
- from the calibration oscillator in the 661, via the multi-pin plug-in connector
Each of these modes is suited to some measurement scenarios. In the case of internal triggering, the signal goes from the trigger pick-off in the 4S1 to a coaxial interconnect that passes through the 661, connecting the sampling unit to the timing unit.