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98640A Analog Input Interface
7.5
BUWR- goes low. This programs the mode of operation of the ADC.
15.0
PACEN goes high. This allows the counter to be stopped by either the external pace input
or the internal pace timer.
If the external pace control input (EPCON) stops the counter, the counter remains stopped
until that input is released and EXTPAC- goes low. If the internal pace timer stops the
counter, the counter remains stopped until the timer reaches FFFF and pulses ENDCT low.
On either occurrence, the counter starts again and the conversion cycle continues.
16.5
BUC/D- goes low.
17.0
LACH- goes high. This sets up the BUSY state machine to go through its cycle when the
card receives the next analog read.
1 7.5
BUSAMP goes low. This puts the sample and hold circuit into the hold mode.
BULACH goes low. This clocks the polarity and overrange bits into their various flip-flops
and and on to the internal da ta bus.
BUWR - goes low. This starts the A -to-O conversion on the voltage held by the sample
and hold circuit.
29.5
E:USTAR T
go~s
high.
30.0
As a result, the counter resets to O. This puts WAIT high and starts the conversion cycle
over agaIn.
THE ANALOG PIPELINE
The analog pipeline is implicit in the foregoing theory of operation; now we will make it explicit. If
you follow the sequence of operations of the A-to-D card closely, you will see that the result you get
back when you make an analog read does not come from the address that you specified in tlle read; it
comes from the address that you specified two readings earlier. This is because the operations on the
card are not all serial; several operations run in parallel. This gives the effect of a pipeline: You push
addresses in at one end of the pipeline and results come out two readings later.
At any given time, the analog pipeline is working on three different analog readings. The following
table shows the events that occur in an analog read cycle and which reading those events pertain to,
assuming that you have just made the nth analog read.
EVENT
Address supplied from backplane
Address latched onto card
Address used to decode channel and gain
Voltage sampled at specified gain
Voltage converted to digital form
Digital value read onto backplane
3-25
READING
NUMBER
n
n
n-1
n-1
n-1
n-2
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