View Full Version : PLC analog input output rate

March 9th, 2004, 03:59 AM
Hi All,
some PLC analog input modules allow the user to program the output rate from 10Hz to 50/60Hz to 1Khz. I understand why somebody might want to select 50/60Hz to reduce 50/60Hz noise. But why would anybody want to select the higher output rate of 1Khz in cases where the 3dB point of the module is specified as low as 17hz.

Put another way is there a reason why you would want information at a much higher rate than the bandwidth suggests is necessary? Is it something subtle to do with PID loops?


March 9th, 2004, 04:03 AM
It would help if you defined "some PLC analog input modules".

March 15th, 2004, 05:28 AM
An example is the Foxboro I/A series FBC01 module. Specs on internet by googling FBC01. It is a 32 channel non-isolated analog input 0-20mA input card.

f3dB = 5Hz
Settling time 0.55 seconds.
Conversion time is 6mS.

Why have a 6mS conversion time when settling time is 550mS. This seems to make the conversion time 100 times faster than is actually useful.

March 15th, 2004, 06:09 AM
If you put a sinus signal on a circuit that dampens the signal at its frequency, then you will still have an output from the circuit, only it will be dampened to a certain degree.
I guess that the 50 og 60 Hz filter is an additional narrow band filter or a lowpass filter that blocks noise from the electrical supply, whereas the 1kHz filter is a lowpass filter to block the large part of noise that has a high frequency content.