Manual 104-AIO16A and 104-AIO16E
29
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. This mode can be used to provide a delayed
software trigger for initiating A/D conversions.
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.
Programming
On this card the 8254 counters occupy the following addresses (hex):
Base A 8:
Read/Write Counter 0
Base A 9:
Read/Write Counter 1
Base A A:
Read/Write Counter 2
Base A B:
Write to Counter Control register
The counters are programmed by writing a control byte into a counter control register. The control byte
specifies the counter to be programmed, the counter mode, the type of read/write operation, and the
modulus. The control byte format is as follows:
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3Bit 2Bit 1 Bit 0
SC1 SC0 RW1 RW0 M2 M1 M0 BCD
SC0-SC1:
These bits select the counter that the control byte is destined for.
SC1
SC0 Function
0
0
1
1
0
1
0
1
Program Counter 0
Program Counter 1
Program Counter 2
Read/Write Cmd.*
* See section on READING AND LOADING THE COUNTERS.
RW0-RW1:
These
bits
select the read/write mode of the selected counter.
RW1 RW0
Counter Read/Write Function
0
0
1
1
0
1
0
1
Counter Latch Command
Read/Write LS Byte
Read/Write MS Byte
Read/Write LS Byte, then MS Byte
M0-M2:
These bits set the operational mode of the selected counter.
