Rockwell Automation 1756-IF4FXOF2F ControlLogix High-speed Analog I/O Module - Manual (Page 45)

Rockwell Automation Publication 1756-UM005B-EN-P - January 2013 45
Module Features Chapter 3
Rate Alarm
The rate alarm triggers if the rate of change between input samples for each input
channel
exceeds the specified trigger point for that channel. Values are
configured in volts/second (V/s).
To see how to set the rate alarm, see page 77 .
Synchronize Module Inputs
With the Synchronize Module Inputs feature, you can synchronize the sampling
of inputs across multiple high-speed analog I/O modules in the same chassis,
allowing those inputs to sample simultaneously within 100
μ
S of each other. This
feature lets multiple modules synchronize the start of their RTS scans, enabling
their inputs to take a snapshot of an application at that user-defined interval.
For example, if you have 12 input devices connected to inputs on three
high-speed analog I/O modules in the same ControlLogix chassis, you may need
a snapshot of the input data available at each input terminal at a single moment in
time.
While setting the RTS to the same value on all 3 modules guarantees that each
module samples at the same rate, it does not guarantee that they will sample at the
same time. When enabled, the Synchronize Module Inputs feature provides each
module a synchronized starting point for its respective RTS scans. Because the
RTS values are the same, the inputs on the modules are sampled at the same rate
and
the same time.
To use this feature, the multiple high-speed analog I/O modules must have the
following:
• CST backplane master configured for the chassis, such as a controller or
1756-SYNCH module
• Same RTS rate
• Synchronize Module Inputs feature enabled (see page 77 )
EXAMPLE If you set the module to a rate alarm of 10.0V/s, the rate alarm will only trigger
if the difference between measured input samples changes at a rate greater
than 10.0V/s.
If the module’s RTS is 10 ms (sampling new input data every 10 ms) and at
time 0, the module measures 5.0V and at time 10 ms measures 5.08V, the rate
of change is (5.08V - 5.0V) / (10 ms) = 8.0V/s. The rate alarm would not set as
the change is less than the trigger point of 10.0V/s.
If the next sample taken is 4.9V, the rate of change is
(4.9V - 5.08V)/(10 ms) = -18.0V/s. The absolute value of this result is
> 10.0V/s, so the rate alarm will set. Absolute value is used because rate alarm
checks for the magnitude of the rate of change being beyond the trigger point,
whether positive or negative.