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More details about the analog input signal of ACE PLCs



We move a logic output every x ms connected to an analog input. The input signal is square. The climb has a slight rounding at the end of 50 micro sec. There are no visible imperfections in the descent.
When the output goes to zero, at each lap of the cycle, we record the analog value in an array of 100.
We also calculate the average cycle time over 10 seconds.

The results are in the following table:

It takes about 20 ms for the read value to pass from max to min (15 ms to pass from low signal: 0.7 volt to high signal: 3.3 volts). And vice versa.
If we make a 10 volt slot, we’ll probably get close to 50 ms.
Sampling of an analog input with this test program: 342 μs (Cycle time)

How do the analogue inputs work internally?

The analog input speed measurements are going to vary somewhat, depending on how many analog inputs the PLC has. The CPU has two A/D converters that are operating at 500,000 samples per second. The different analog inputs are shared by the two A/D converters. In other words, for the Ace 222, each A/D converter does conversions on 6 input channels. For an Ace 3090, on A/D converts two input channels and the other one converts one.

The A/D converter automatically averages the last 64 readings, per channel. Then the PLC firmware does further averaging. In addition to this, there is a low pass filter on the input hardware.

I don’t know which PLC you did your testing with (BTW, its a very good test & good information). I would expect the Ace 3090 to be a little more responsive than the Ace 222. I’ve never actually performed the test that you have.