Communications between the High Speed Counter
module and the Central Processor Unit (CPU) is accomplished via 32 consecutive input and output bits. (I/O address selection is discussed under Installation.) Command
and preset data is transferred from the CPU to the counter
module, while board status, output status, accumulated
count, or counts/time-base is returned from the counter
to the CPU. A two way data transfer will occur with every
CPU I/O scan.
Since data is only transferred between the CPU and the
counter in the I/O scan, the user needs to consider carefully the use of the Suspend I/O instruction. The counter will
continue operation without communication to the CPU
when the I/O is suspended, but it will not be able to apprise the CPU of its status. In similar context, use of the
DO I/O instruction, which could poll the counter too
frequently, would tend to cause the module to respond
slowly to the degree that it may miss a pulse. To prevent
this, a minimum of 6 msec should elapse between I/O
scans that use the I/O addresses of the counter.