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2-18
Functional Description
Triggers
A trigger starts or stops an interrupt-mode or DMA-mode analog input
operation. An operation can use either one or two triggers. Every
operation must have a
start trigger
that marks the beginning of an
operation. You can use an optional second trigger, the
about trigger
, to
define when an operation stops. Use one of the following trigger sources
to start an analog input operation:
●
Internal
— When you enable the analog input operation, conversions
begin immediately.
●
External Analog
— While an analog trigger is not a hardware
function of the DAS-1800ST/HR Series boards, you can program an
analog trigger using one of the analog input channels as the trigger
channel. The DAS-1800ST/HR Series DriverLINX software provides
functions for an analog trigger; refer to “DriverLINX Configuration
Notes” on page C-1 and the DriverLINX on-line documentation
provided with your DAS-1800ST/HR Series board.
●
External Digital
— Connect the digital trigger to TGIN (pin 46 of
the main I/O connector or pin 42 of STA-1800U connectors J1 and
J2). Trigger types are as follows:
–
Positive-edge trigger
- Triggering occurs on the rising edge of the
trigger signal.
–
Negative-edge trigger
- Triggering occurs on the falling edge of
the trigger signal.
The actual points at which conversions begin depend on whether the
clock source is internal or external, as follows:
●
Internal clock source
— The 82C54 counter/timer is idle until the
trigger occurs. Within 400ns, the first conversion begins. Subsequent
conversions are synchronized to the internal clock.
●
External clock source
— Conversions are armed when the trigger
occurs; they begin with the next active edge of the external clock
source and continue with subsequent active edges.
The polarity of external triggers in DAS-1800ST/HR Series boards is
software-selectable. Figure 2-5 illustrates the enabling of conversions
with a software trigger and with internal and external clock sources. In the
diagram, the software enabling of the conversion process represents the
point at which the computer issues a write to allow conversions. The
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