Technology instructions
10.1 Counting (High-speed counters)
S7-1200 Programmable controller
582 System Manual, V4.2, 09/2016, A5E02486680-AK
When possible, avoid assigning multiple input functions of the same HSC to the same input
point. For example, assigning I0.3 to the Sync input and the Gate input of HSC 1 to
synchronize the count and disable counting at the same time is also a valid configuration.
You can make this configuration, but it could produce unintended results. Refer to the
Warning below for further information.
Risks with assigning multiple functions to a single digital input channel
Assigning multiple input functions of the same HSC to a common input point may produce
unpredictable results. When a trigger occurs on a point with multiple functions assigned to
that trigger, the order in which the functions are executed by the PLC cannot be known.
This is known as a race condition and is often an undesirable situation.
This race condition can result in unexpected machine or process operation, which can
cause death or serious injury to personnel, and/or damage to equipment.
To avoid a race condition, do not assign more than two input functions, of the same HSC,
to the same input pin. If an HSC has two input functions assigned to the same pin, set the
triggers such that they could never occur at the same time. Remember that a falling edge
occurs at the same instance that a low level begins and that a rising edge occurs at the
same instance that a high level begins.
Note
You assign the digita
l input and output points used by high-speed counter (HSC) devices
during CPU device configuration. When you assign input and output points to HSC devices,
you cannot modify the values of these points using the force function in a watch table. The
HSC has
full control of these input and output points.
Hardware output pin assignment
If you enable the Compare output, select an available output point. Once you configure an
output point for use by an HSC (or other technology objects such as a pulse generator), that
output point is owned exclusively by that object. No other component can use the output
point, and the output point cannot be forced to a value. If you configure a single output
channel for multiple HSCs or for use in an HSC and a pulse output, the program generates a
compile error.
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