The P635 is a single-width, 3U, PXIbus module with eight frequency measurement channels. This counter module can be used to monitor a variety of pulse sources. Moreover, its unique circuitry allows the monitoring of a wide range of frequencies without changing any module settings. TTL inputs are provided as well as differential input circuits, with filtering and hysteresis to provide high noise immunity. The switching threshold is programmable. The input voltage should be at least twice the threshold voltage for noise immunity. AC or DC coupling of the differential inputs is programmable on a per-channel basis.

The P635 provides for two types of signal sources: analog and TTL. The analog input paths are differential for high noise immunity. These inputs can be unterminated (high impedance) or terminated with 100O, selected by an on-board switch. The analog input coupling can be DC (the normal setting for most applications) or AC (with a series capacitor in each leg of the input path). Each analog path also includes programmable hysteresis control and a programmable high-frequency noise filter. The hysteresis control sets the input pulse detection threshold, while the filter provides a 3 dB rolloff at 50 kHz. The selection of analog or TTL input is programmable on a per-channel basis.

The internal clock for the P635 is driven from a Master 10 MHz clock having a 1 ppm accuracy from 10 °C to 50 °C. The internal clock can be set to 1 MHz or 10 MHz under program control. The 10 MHz clock provides an order of magnitude increase in counting resolution over the 1 MHz clock. The 1 MHz internal clock, however, allows detection of slower frequencies: 0.06 Hz for the 1 MHz clock, as compared to 0.6Hz for the 10 MHz clock.

All input channels on the module are monitored over a single user-selectable period, called the observation window.