PCM-3718 Series User Manual
46
5.4 Trigger Mode
You can trigger an A/D conversion from software, from the module’s on-
board pacer or from an external signal. Bits 1 and 0 of register BASE+9
select the trigger source.
1.
If you select software triggering, write to register BASE+0 with any
value to trigger an A/D conversion. High-speed A/D applications do
not normally use software triggering because the triggering rate is
too slow.
2.
The PCM-3718’s onboard Intel 8254 programmable interval timer/
counter can generate periodic timing signals. Counters 1 and 2 of the
Intel 8254 provide A/D converter trigger pulses with precise periods.
You can select pacer output between 2.5 MHz and 71 minutes per
pulse. Chapter 7 cover the details of the Intel 8254 timer/counter.
Pacer triggering is ideal for interrupts and DMA data transfers, nor-
mally used in A/D applications which require higher conversion
speeds.
3.
You can also trigger the A/D conversion from an external signal.
Wire the external signal to pin 1 on connector P2 and switch jumper
JP3 to EXT. You would normally use external triggering if your
application requires A/D conversions not periodically, but condition-
ally, e. g., to measure a voltage when a limit switch closes. You can
also use a function generator to create the external trigger source.
The A/D conversion starts at the rising edge of the external trigger
pulse.
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