IBM 1 Series - Functional Description of the External Timer Signal Lines

Pulse or Event Counting
The ‘timer X customer clock’ input should be used, and the timer mode
controls should be set to select the customer time base. The number of
pulses recorded prior to an interrupt being posted depends on the value set
into the timer and the chosen run mode. For example, if the timer (and,
thus, the auto-load register) is set to all 0’s, and the chosen run mode is
periodic, the timer interrupts on each pulse counted. If, under these same
conditions, the timer is set to 100, it interrupts on every 101st pulse.
Pulses are recorded and interrupts generated at the leading
(passive-to-active) edge of the pulse. Also, except for power-on reset,
every pulse is counted only once even though the timers could be stopped
and started many times during the active duration of a given pulse. Note
that duty-cycle constraints on pulse inputs for the customer clock input do
not preclude pulses of very low frequency or even pulses with a random
duty cycle.
Pulse Duration Counting
The ‘timer X external gate’ input should be used for the pulse whose
duration is being measured. A standard internal time base can be used, or
a clock of arbitrary accuracy or frequency can also be used and connected
to the ‘timer X customer clock’ input. The timer mode controls should be
set to arm the external gate and to select the time base desired. The
measure of the pulse duration is a function of (1) the initial and end
values of the timer, (2) a known time base, (3) the type of interrupt
returned, and (4) the run mode used. The customer should ensure that the
‘external gate’ input is inactive before the timer is set to run state, or an
error in measuring occurs. The outputs from the timer card are available
for this purpose. For example, assume that clock inputs occur every 1
microsecond and an initial value of 1000 is loaded into the timer. Then,
‘external gate’ input going active causes the timer to start. When ‘external
gate’ goes inactive, the timer stops and an attention interrupt occurs. The
timer is read and it is found to contain a value of 500; therefore, the pulse
width of ‘external gate’ was 500 +1 microseconds.
Functional Description of the External Timer Signal Lines
There are four external signal lines per timer. These signal lines permit
control of the timer with a user-provided time base and gate. The
following table lists the external signal lines:
Active
Signal Direction level*
Timer X customer clock To timer Down
Timer X external gate To timer Down
Timer X run state From timer Down
Timer X external gate enable From timer Down
* All signals are down-level (minus) active.
3-6 GA34-0033