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29
Problem
Possible causes and solutions
Questionable alarm message
from digital input
- Go to the relevant screen in the I/O branch and check the status of the
input.
- Check for voltage on the input involved between terminal "C" and
corresponding terminal "Cn".
If 24V is measured, the input is closed; if 0V is measured, the input is
open (the controller normally outputs an alarm signal when the input is
open).
The controller repeatedly
continues to go into Watchdog
mode (it switches off and then on
again as though there were a
power cut for a few seconds or
randomly activates some digital or
analogue outputs).
- Check that power cables are not placed close to the controller.
- Check that the sizing of the power transformer is correct.
The user terminal is blocked (no
response on pressing the buttons)
- Check that the terminal is not disconnected. If it is, plug it back in
place and switch the controller off and on again with the terminal
connected.
4.3 Analogue input testing
Analogue inputs are used to read 0/1V or 0/10V voltage signals, current value 0/20 mA or 4/20 mA, or signals
varying their ohm value as the temperature changes.
The sections below provide instructions for checking, through signal reading, that the sensor is working
properly or the input is being read correctly.
4.3.1 Active voltage probes
To check correct operation of the active voltage probes, normally used for temperature or humidity
measurements, using a voltmeter take a measurement between the “GND” terminals and the “Bn” terminal for
the input connected to the probe to be checked; starting from this voltage value, it is possible to check its
correspondence with the quantity measured by means of the general formula:
(FS
max
– FS
min
) / S
max
x V
mis
+ FS
min
= T
mis
where:
FS
max
=
temperature value read by the probe at the admissible input signal peak;
FS
min
=
temperature value read by the probe at the min. admissible input signal value corresponding to 0V;
S
max
= Maximum
voltage value allowed by the input.
V
mis
= voltage value measured in Volt;
T
mis
=
temperature value read on the controller.
Example: Use a probe having an operating range between 0°C and 100°C, and a signal between 0V and 1V.
Take the measurement between terminal “GND” and the connected input 150 mVdc: the temperature read by
the controller is calculated as follows.
(100 – 0) / 1 x 0.15 + 0 = 15°C
4.3.2 Active current probes
Correct operation of the probes normally used for measuring pressures in live conditions is checked by placing
a voltmeter between the “GND” terminals and the “Bn” terminal of the relevant input. The value of current is
calculated indirectly based on the probe signal.
To make the calculation, use the formula I = V / R, where the input impedance “R” is 50 ohm.
The pressure value Ps that the probe is sending is calculated as instructed below.
(FS
max
– FS
min
) / 0.016 x (V
mis
/ 50 – 0.004) + FS
min
= Ps
where:
FS
max
=
pressure value read by the probe at the admissible input signal peak;
FS
min
=
pressure value read by the probe at the min. input signal admissible value;