SM 1231 High Limit Exceeded

As a test, reconfigure the hardware to 0-10 or +/- 10V rather than 4-20mA.

Now across the input terminals for that channel, add in a 500 Ohm resistor.

Now measure the channel signal in voltage right across those terminals.

If it reads properly and stays in range while you operate the field device, your field device is working properly.

If you find the voltage runs high and out of range, your field device may not be configured properly, or the wiring may have been done incorrectly.

If it works and you are happy with it, you may decide to simply add that resistor to your drawings and move on.  However, I would urge you to contact the manufacturer of the field device, send a copy of your wiring diagram, any configuration settings, and make 100% sure their device is working as you need it to and you have it wired and configured as you need it to be.

I cannot tell you how many times I find problems with the wiring, or the configuration settings still set to default even after running through the setup, or sometime during the commissioning, I reset the hardware to default or changed it by accident, and now my input channels are over-ranged.

This little cheat saves a lot of time.  Convert it to voltage, and if the voltage stays in range, you have eliminated a lot of potential problems as a contributing factor to the result  you see.

JH9

It is like in fig3.

I measured L+,M:

23,99Vdc

29mA

I read from datasheet:
Input impedance ≥ 9 MΩ (voltage) / 280 Ω (current)

Should I get 280Ω, between Ch 0+,0- or...? It gives me 0L on multimeter and when +24Vdc L+,M is off, I get >3,5MΩ (rising).

Change to 0-10V, add 500 ohm R and measure voltage.

If it works, you are wired properly.  I suspect a wiring problem.

For a quick check, measure the voltage of a battery with the analog input by temp wiring a couple of wires to a voltage input, or a current input with the 500R, and see what it reads. If it reads close to what it should be, then the wiring is correct on the plc.

JH9

Thanks, that 500R trick worked.

Next problem is getting Asco Joucomatic 833-150301, air pressure sensor (2-wire) +24Vdc, 0...20mA, 0-1Bar work via Ch 1+, 1-. It too gives high limit if using 0-20mA setting.

I think it is past time for a picture of the wiring of the I/O on that module.  It really sounds like it is miswired, ungrounded, or in some way not working right.

As a test, connect a standard Dc battery (1.5V or 9V) to your analog input while it is set up for 0-10VDC.  What readings do you get in the PLC?

Keep in mind the values are scaled 0-27648, not 0-32767 nor 0-65535.

You can insert a NORM/SCALE block and the proper voltage will appear if you scale the output of the SCALE_X block to 0-10.0.

JH9

I get high limit on Ch1, with 500R, pressure indicator and 0-10V setting. Multimeter reads 15,43Vdc on Ch1(+1,-1)?

I tested Ch1+500R by adjustable psu, not faulty.

I wonder why these indicators won't work with mA settings as they should be?

EDIT:

I tried 100R with that indicator. 4,45Vdc, no more high limit, but it did not change, when pressure changed?

EDIT2:

I found other pressure indicator, Trafag 8891. 77 which is +24Vdc, 0-10V measurement, 0-6bar, 2-wire. That should work with 0-10V setting, without any resistor? I get 0,94Vdc in Tia portal and with multimeter 0,443Vdc. Readings remain the same, though I put some pressure?

Active versus passive loop current devices... active ones like yours require external power and common, and then provide loop current. 

I really dislike 3 wire devices.  I really like 4 wire galvanically isolated mA devices because you provide the device power, and it provides mA (or differential voltage) output that does not affect the bonding of the various other analog signals.

I routinely run into problems collecting accurate analog data when I have to use 3-wire devices.  My current choice is to run that signal through a galvanic isolator (signal conditioner), which tends to protect my common signal in the field from being artificially changed as the current signal increases.  This problem is amplified when several mA signal all shift in the same direction at the same time, and they all affect each other.  What you end up with is a lower than expected value for the real-world pressures/flows/and so on.

This is detectable by performing a calibration on a single signal.  Then do the same thing with all the other signals at the same time while leaving the calibration source stable on that questionable signal.  If it begins to deviate suddenly with no change in the calibrated source, your common wire is causing voltage / current drop.  When that happens, increase the common wire size by at least 2 wire sizes. If it still causes a drop of significance but is less, make it one size bigger still.

You can end up with a #12 (2mm) wire on what was originally #18 (1mm).

JH9

I got it to work. It was 3-wire, not 2.

Yet I wonder why those mA sensors didn't work with mA settings?