Internet PLC Forum

I ran into the same issue protecting ADC inputs on soil sensors and solved it with a 10k resistor, 3.6V Zener, and small cap — worked well in practice.
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Sorry the TRi PLC currently do not send PING command to another device.

However, if the device has an open TCP port you can try to make a connection to the TCP port and check if connection is successful by examining the STATUS(3).  You can then disconnect from the device after making an initial connection.

I am very certain how I destroyed the 1st PLC.  I am using inexpensive 4..20 mA signal generators that can, also, output 0.20 VDC. I screwed up and sent 20 VDC into both ADC inputs.  Not too bright on my part.

I tried to be much more careful with the 2nd NANO-10.  Both ADCs worked with 4-20 mA signals. I then worked on refining my digital signal processing software using ADC[1].  I then added in ADC[2] to my test code snd found that ADC[2] was not working.  I don’t know how I damaged the 2nd PLC, but I did.

I have found a 3rd NANO-10 in my collection of PLC stuff. I will see if it works and add the input protection circuitry.  If I can’t get it working I will order another NANO-10.

Regards,

Gary Dickinson

I've run into a bit of a problem with the NANO-10 Analog Inputs.  I have damaged the inputs on 2 of my NANO-10 boards.

I have attatched a schematic of a possible input protection circuit.  I am looking for feed back.

The sensors that I am working with output a 4 to 20 mA current source.  The 250 ohm resistor converts this to voltage between 0 and a bit less than 5 VDC.  I am aware of the 2K input impedance of the analog inputs.

The 5.1V Zener diode is an attempt to limit the voltage that the analog inputs "see". The Zener is supposed to clamp the voltage to about 5.1 VDC max and if the input voltage goes below ground the Zener will behave like a normal diode and clamp the low voltage to about -0.7 VDC.

The 120 Ohm resistor just limits the current that flows in the circuit and does not affect the accuracy of the measurement (the sensor outputs current and not voltage). The 0.01 uF capacitor is an attempt to bypass transient events to ground.

Gary Diuckinson

I am working with a Nano-10 to work with 4-20 mA sensors.

The Nano-10 does not appear to have an analog ground connection.  The best that I can do is connect the free end of the current sense resistor to my common DC bus.

I am concerned about minimizing noise pickup by the ADC circuitry.

Is there a spot on the Nano-10 that I could solder onto as an analog ground?

Gary Dickinson