Conversion Clock Sources - Keithley DAS-800 series User Manual

Conversion Clock Sources

2-8
You must specify a single-ended input configuration for all
Notes:
on-board channels associated with channels on EXP-16, EXP-16/A, or
EXP-GP expansion boards or MB-02 backplanes.
If you are using EXP-16, EXP-16/A, or EXP-GP expansion boards or MB
Series backplanes, the digital output lines of the DAS-800 Series board
select the particular channel on the expansion board or backplane to read.
Refer to the appropriate expansion board documentation for more
information about the EXP-16, EXP-16/A, and EXP-GP expansion
boards. Refer to the MB Series User's Guide for more information about
the MB-02 backplane.
The conversion clock source determines when each analog-to-digital
(A/D) conversion is initiated. DAS-800 Series boards provide the
following software-selectable conversion clock sources:
Software - When using a software conversion clock, the host
computer issues a command to initiate a conversion. The host polls
the board to determine if the conversion is complete. When the
conversion is complete, the host reads the data from the ADC and
returns the value. If the host reads data before the conversion is
complete, the data will be invalid.
Software-initiated conversions are suitable for measuring DC
voltages; however, in applications where you must accurately control
the sampling rate (as when measuring time-varying signals), it is
recommended that you use either an internal or an external hardware
conversion clock source.
At power-up or system reset, the board assumes that conversions will
be initiated through software.
Hardware (internal clock source) - The internal clock source uses
the on-board 8254 counter/timer circuitry and a crystal-controlled
1 MHz time base. The 8254 counter/timer circuitry is normally in an
idle state. When you start an analog input operation, a conversion is
initiated immediately. The 8254 is loaded with its initial count value
Functional Description