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The 1S1 is a sampling plug-in for 500-series scopes. It is essentially one channel of a sampling unit and a timing unit from a 661. It has its own trigger and timebase. The oscilloscope's timebase is usually set to a relatively slow sweep rate when using a 1S1. Alternatively, the sweep on the oscilloscope can be free-run, and the horizontal voltage from the oscilloscope can be used to control the 1S1, as described below. The 1S1 has a 50-Ohm GR-874 connector for signal input. The plug-in supplies power (+100V and -12.6V) to active probes such as the P6032 cathode follower probe.

The 1S1 can operate in normal self-swept mode or an external sweep signal can be applied to the 1S1. With the 1S1's internal sweep disabled, the horizontal-in and vertical-out connections can be used so the the 1S1 acts as a lookup table, a mapping of x to y, a function. The 661 also has this capability, in its "A vert/B horiz" mode, which is like X-Y mode for a sampler. In this mode, the horizontal-in voltage controls to the time after the trigger event when the sample should be taken, and the vertical-out voltage corresponds to the voltage measured at that instant. This allows a waveform to be digitized using an arbitrarily slow DAC to generate the horizontal voltage and ADC to read the sampled output. But perhaps more importantly, by setting a constant horizontal-in voltage, it allows the output signal at one equivalent time instant to be processed in the time domain. For example, this allows the voltage to be low-pass filtered, so that it can be more accurately measured. The other reason why one might want a signal the consists of multiple sequential measurements of the same equivalent-time instant is that statistics can be calculated on these observations. For example, one might want to know, when testing a logic gate, what is the propagation delay such that 99.9% of the transitions happen faster than this. The horizontal-in voltage can also be produced, in "manual" mode, by setting a knob on the 1S1.

The sampler in the 1S1 is quite different from the sampler in the 1S2. The 1S1 uses a four-diode sampling bridge, terminated within the 1S1. The 1S2 uses a two-diode feed-through sampler, typical for TDR instruments.

Type 1S1 was sold from 1965 through 1973.