2-10
Functional Description
Clock
The input to the counter/timers used for A/D timing varies. The
DAS-8/LT uses the PC bus clock while the DAS-8/PGA(G2) and
DAS-8/AO have an on-board 1MHz crystal oscillator. The frequency of
the PC bus may be determined automatically as described below.
Custom Clock Frequencies
For timing, the DAS-8 Series uses the clock signal on the PC I/O bus or
an on-board crystal oscillator. While the original IBM PC XT and
compatibles always had a 2.384MHz bus clock, ISA Nd EISA bus
machines have no accepted standard for PC bus clock. On many
machines, the clock frequency is one-half the CPU clock frequency, while
on others it is adjustable using motherboard jumpers or advanced BIOS
setup options. Few manufacturers provide this information in their
documentation. This frequency, however, can by measured by
DriverLINX using the 82C54 timer/counters on the DAS-8.
To measure the PC bus clock frequency, DriverLINX requires that
Counters 1 and 2 be chained together using an external jumper. To chain
the counters, connect Counter 2 Out [6] to Counter 1 IN [4] on the DAS-8
external connector.
Then, using DriverLINX, select the Counter/Timer Section in the
Configuration Dialog Box. Choosing the item “Custom clock” in the
drop-down list of the Resolution combo-box displays the Custom Clock
Frequency Dialog Box. Selecting Measure will start a 30-second
measuring interval for determination of the PC bus clock frequency. The
frequency box will update every second with the currently measured
frequency. The measuring interval may be stopped at any time using the
Stop pushbutton. The displayed frequency is added to the Resolution list
box by selecting OK, or the measurement can be abandoned by selecting
Cancel. The displayed frequency may be edited manually by selecting the
numbers in the edit box and typing a new value for the selected digits.
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