Advantech 3718HG, User Manual, Page: 43 / 68

Chapter 5 A/D Conversion 39
2.5 MHz and 71 minutes per pulse. Chapter 7 cover the details of
the Intel 8254 timer/counter.
Pacer triggering is ideal for interrupt and DMA data transfer,
normally 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 condi-
tionally, e. g., to measure a voltage when a limit switch closes. The
A/D conversion starts at the rising edge of the external trigger pulse.
5.5 A/D Data Transfer
You can perform A/D data transfer by Program Control, Interrupt
Routine or DMA.
1. Program controlled data transfer operates by polling the A/D status
register. After the A/D conversion has been triggered, your applica-
tion program checks the INT bit (data valid) of the A/D status
register. When it detects that the INT bit is on (1), it sends the A/D
data to the PC’s memory using DMA. Reset the INT bit (by writing
to register BASE+8 with any value) after you transfer the A/D data.
When you use software triggering, you can check either the INT or
EOC bits for data validity. Since you use the program to trigger the
A/D conversion, you do not need to poll the INT bit to see if the
conversion has occurred. It is easier to use the EOC bit, because
you do not need to clear it after you transfer the data.
2. With interrupt data transfer, you write an interrupt routine handler
program, which transfers data from the module’s A/D data registers
to a previously defined memory segment in the PC. At the end of
each conversion, the EOC signal generates an interrupt, and the
interrupt handler routine performs the transfer. You must specify the
interrupt control bit and the interrupt level selection bits in the
PCM-3718H/3718HG control register (BASE+9) before using the