Service manual

PN 4237242C
2.5-8
INSTRUMENT DESCRIPTION
DATA ACQUISITION
Octal Digital-to-Analog Converter (DAC)
The octal DAC contains eight identical DACs. They are attached to a common reference but
are individually latched. They provide:
r RBC aperture current voltage setting (coarse and fine)
r WBC aperture current voltage setting (coarse and fine)
r RBC counting threshold
r WBC counting threshold
r Plt lower and upper thresholds.
Aperture Signal Processing
The signal processing functions are programmed into another EPLD. This programming
actually spans two chips, U22 and U33. They are labeled BSPAD, and contain the WBC
processor and the RBC/Plt processor. These processor functions are capable of running
concurrently, allowing information from the RBC and WBC baths to be processed at the same
time, speeding up the instrument cycle.
The WBC processor counts and channelizes pulses that it receives from the WBC preamp (on
the Sensor Preamp Adapter card). As they are received by the processor, all pulses are sent to
the channelizing section, but only pulses that exceed a threshold are sent to the 17-bit
counter. This threshold is set by the system software and allows pulses representing particles
greater than 35 fL to be counted. Pulses sent to the channelizing section are first edited. The
editor issues a "good pulse" strobe based on the shape, size, and width of the pulse. If a pulse
is judged to be good, it is peak detected and applied to the input of an A/D converter. The
8-bit output of the A/D converter is then transferred directly to memory and forms the
channelized data or histogram.
The RBC/Plt processor counts and channelizes pulses that it receives from the RBC and Plt
preamp (on the Sensor Preamp Adapter card). RBC pulses received by the processor are
sorted according to thresholds established by the CPU. Pulses that exceed a threshold
representing 36 fL are sent to the 17-bit counter where they are counted as RBC cells.
Pulses that fall into a threshold range representing 2 to 20 fL are considered Plts and are peak
detected and sent to an A/D converter. The peak detector is used to hold the peak amplitude
until the A/D process is finished. The output of the A/D converter designates a memory
location to be incremented, creating the Plt histogram as a series of memory locations. All
pulses not considered Plts are sent to the RBC editor and channelizing section. The editor
issues a "good pulse" strobe based on the height, shape and width of the pulse. If a pulse is
judged to be good, it is peak detected and applied to the input of an A/D converter. The 8-bit
output of the A/D converter is then transferred directly to memory and forms the channelized
data or RBC histogram.
Data Acquisition Circuit
There is a Data Acquisition circuit on the SPAD card that is used to monitor the +5 V, +12 V
and +15 V power supplies. They are monitored using the system software DVM function. The
Data Acquisition circuit is contained on chip U39. It is an eight-channel device with the
following instrument data assigned to three of the eight channels:
r +5 V supply
r +12 V supply
r +15 V supply
r Five channels are currently not used.