Access PCI-DIO-24D(H) 用户手册

  
These cards have the option of one, two, or three 82C54 counter(s) that each include three 16-bit 
counter/timers factory configured in an optimal module for use as event counters, frequency output, pulse 
width, and frequency measurement (See Block Diagram). Each counter can be programmed to any count 
as low as 1 or 2, and up to 65,535, depending on the mode chosen. For those interested in more detailed 
information, a full description can be found in the Intel (or equivalent manufacturer's) data sheet.  
 

  
The 8254 modes of operation are described in the following paragraphs to familiarize you with the 
versatility and power of this device. For those interested in more detailed information, a full description of 
the 8254 programmable interval timer can be found in the Intel (or equivalent manufacturers') data 
sheets. The following conventions apply for use in describing operation of the 8254 :  
  
 

Clock:   

 

A positive pulse into the counter's clock input 

 

Trigger:  

 

A rising edge input to the counter's gate input 

 

Counter Loading: 

Programming a binary count into the counter 

  

 

After the counter is loaded, the output is set low and will remain low until the counter decrements to zero. 
The output then goes high and remains high until a new count is loaded into the counter. A trigger 
enables the counter to start decrementing.  
  
Mode 1: Retriggerable One-Shot

 

The output goes low on the clock pulse following a trigger to begin the one-shot pulse and goes high 
when the counter reaches zero. Additional triggers result in reloading the count and starting the cycle 
over. If a trigger occurs before the counter decrements to zero, a new count is loaded. This forms a 
retriggerable one-shot. In mode 1, a low output pulse is provided with a period equal to the counter count-
down time. 
  
Mode 2: Rate Generator

 

This mode provides a divide-by-N capability where N is the count loaded into the counter. When 
triggered, the counter output goes low for one clock period after N counts, reloads the initial count, and 
the cycle starts over. This mode is periodic, the same sequence is repeated indefinitely until the gate 
input is brought low. This mode also works well as an alternative to mode 0 for event counting. 
  
Mode 3: Square Wave Generator

 

This mode operates like mode 2. The output is high for half of the count and low for the other half. If the 
count is even, then the output is a symmetrical square wave. If the count is odd, then the output is high 
for (N+1)/2 counts and low for (N-1)/2 counts. Periodic triggering or frequency synthesis are two possible 
applications for this mode. Note that in this mode, to achieve the square wave, the counter decrements by 
two for the total loaded count, then reloads and decrements by two for the second part of the wave form.  
  
Mode 4: Software Triggered Strobe

 

This mode sets the output high and, when the count is loaded, the counter begins to count down. When 
the counter reaches zero, the output will go low for one input period. The counter must be reloaded to 
repeat the cycle. A low gate input will inhibit the counter. 
  
Mode 5: Hardware Triggered Strobe

 

In this mode, the counter will start counting after the rising edge of the trigger input and will go low for one 
clock period when the terminal count is reached. The counter is retriggerable. The output will not go low 
until the full count after the rising edge of the trigger. 

Manual PCI-DIO-24DH 

21