Service manual

PN 4237242C
2.4-4
INSTRUMENT DESCRIPTION
SYSTEM CONTROL
Diagnostic Data Acquisition Circuit
The Diagnostic Data Acquisition circuit receives voltage data from the Vacuum Sensor, Hgb
Preamp and Sensor Preamp Adapter cards and converts this for use by the instrument
software. The supplies are input to an HI508a analog multiplexer. The CPU selects which
input to the multiplexer is output to an AD7572 A/D converter. The A/D converter outputs
digital data for the CPU, producing a digital voltage representation of medium resolution.
There are eight channels used to represent the following signals:
r HGB VOLTAGE
r VACUUM READING
r WBC 26-PERCENTILE VOLTAGE
r RBC 26-PERCENTILE VOLTAGE
r WBC APERTURE CURRENT READING
r RBC APERTURE CURRENT READING
r WBC APERTURE VOLTAGE (not currently implemented)
r RBC APERTURE VOLTAGE (not currently implemented).
Utility Timer Circuit
An 8254 programmable timer device is provided as utility timer hardware (Figure 2.4-2) for
use by the system software. This device has three timers that can all produce an interrupt. An
interrupt control latch and buffer forms an 8-bit read/write port with the timer outputs each
assigned a bit. The GAL provides the logic that controls and synchronizes the interrupts,
allowing the timers to be enabled or disabled and to be used individually or chained. The
GAL produces an IRQ15 system interrupt request.
Support Circuits
The support circuitry for the URA card consists of Power Supply Conditioning, a Bus
Interface, an Internal Bus Controller and an Oscillator circuit.
The URA receives +5 V, -5 V and +12 V from the motherboard bus connector. It also receives
+15 V and -15 V through connector J1. The +5 V, +15 V, and -15 V power supplies are fitted
with a spike supressor and a large filter capacitor when they enter the card. A bypass capacitor
is provided for the -5 V and +12 V power supplies.
The Bus Interface circuitry allows the URA card to communicate with the AT motherboard
and follows AT convention. There is a bidirectional data bus buffer, address buffer and a
command and interrupt buffer.
The URA card has its own internal data bus for the onboard logic devices. An Internal Bus
Controller is responsible for the operation of this local bus.
The AT bus provides an 8-MHz clock signal, but the URA card has its own 1-MHz Oscillator
circuit. The 1-MHz Oscillator circuit is used by the 8254 timer, the keypad controller chip
and the A/D converter chip.
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