Service manual

PN 4237242C
2.5-7
INSTRUMENT DESCRIPTION
DATA ACQUISITION
2
The SPAD’s input connections are summarized in Table 2.5-5 and their location is shown on
Figure 2.5-4. Figure 2.5-4 also shows the location and setting of the jumper. Table 2.5-5 and
Figure 2.5-4 are at the end of the description of the SPAD card. The card is comprised of the
following circuitry.
Power Supply Conditioning
This card receives +5 V from the +24 V power supply through the AT bus slot and ±15 V from
the Linear Power Supply card through connector J4. These regulated supplies are further
conditioned by the SPAD card with a Spike Suppressor circuit and a large filter capacitor. In
addition, each IC on the card has a bypass filter capacitor.
Oscillator Circuit
A 16-MHz, onboard oscillator is directed into the three programmable logic devices, U22,
U23 and U33. These devices buffer, divide, use and supply the timing signals for other
circuits on the card.
Control Functions
Card housekeeping consists of several logical functions, an AT Bus Interface, an Internal Bus
Controller, the Main Port Address Decoder, Control Port 1, and Control Port 2. These circuits
or functions do not use discrete components or specialized chips. They are programmed on
chip U23, labeled ASPAD, which is an Electrically Programmable Logical Device (EPLD).
The AT Bus Interface provides the circuitry allowing communication to and from the AT
motherboard. It is comprised of a bidirectional data buffer, an address buffer, and the
command and interrupt buffer and conforms to AT slot specifications. There is also an
Internal Bus Controller.
The SPAD card has its own data bus. The Internal Bus Controller is the controller for this
internal bus.
The Main Port Address Decoder supplies the chip select signals for all the chips on the
SPAD card.
Control Port 1 is a Read/Write port providing four control signals/commands:
r SELECT (for Red/White 26-percentile sync)
r CLEAR (the state machine)
r INTERRUPT ENABLE (DMA terminal count)
r INTERRUPT ENABLE (8-channel DAS)
Control Port 2 is a Read/Write port providing eight control signals/commands:
r APERTURE CURRENT ON/OFF
r RED APERTURE SELECT
r WHITE APERTURE SELECT
r APERTURE CLEAR
r EDITOR ENABLE/DISABLE
r PRECHARGE ON/OFF
r COUNTER CLEAR
r COUNTER ENABLE