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THEORY OF OPERATION
1-
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This section describes the operation of the HP 98640A A -to-D card in considerable detail. To follow
this description you should have a good working knowledge of the operation of the DIRECT-I/O
backplane. If you need to beef up your background in this area, we suggest that you read the Pascal
2.0 System Designer's Guide (part number 09826-90074).
We will break our discussion down into the following topics:
Power and timing circuitry - - brief description of the power supply and the system clock.
Registers -- description of the registers that can be accessed via the backplane.
Analog Icircuitry -- description of the circuitry that transfers signals from the analog inputs to the
analog-to-digital converter (ADC).
Digital
(~onversion
circuitry - - description of the section that controls the data conversion.
Digital llackplane circuitry - - description of the backplane interface.
Sequenc.~
of operations -- summary of the interactions of the analog and digital sections; essentially
an annotated timing diagram.
Analog »ipeline -- summary of the interrelationships of successive analog readings.
Note that in this section we use the term "analog read" to refer to a read from one of the eight analog
input channels on the A -to-D card, as contrasted with a read (or write) from (or to) the status, ID, or
pace timing register.
In our discussion we will frequently refer to integrated circuits (ICs) by their "U" numbers. These U
numbers can be found on the schematic diagrams of the card (in Section 7 of this manual) and in the
replaceablf~
parts list (in Section 6). We will make occasional references to schematic locations by grid
numbers; these grid numbers are found on the outer edges of the schematic diagrams. Also, we will
refer to two asynchronous state machines: the BUSY state machine and the Conversion state machine.
Though running asynchronously, these state machines do influence each other.
In this section we will use the following convention for signals:
"BUSY"' indicates a signal which is positive true.
"BUSY _" indicates a signal which is negative true.
Figure 3-1 shows a generalized functional block diagram of the A-to-D card.
3-1
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