Urgent Problem: Analog input PLC Siemens S7 300 goes to 0mA blinking


Thread Starter


I actually have a PLC VIPA 200V which is exactly the same as a S7-300 CPU3152DP of Siemens (I even use Step7 to programm it). This question more than a Siemens question is a general question about analog input problem.

I have an universal analog input card (mV, RTD, TC, 4-20mA, 0-20 mA). All the inputs are passive inputs. I have connect a 4 wire brix transmitter (which is power supplied with 220 VAC) with an active analog output 4-20 mA to the channel 0. But I get the right value of the analog input for a few seconds (let's say I get a PEW280=4092 decimal)and then the reading goes to a 0 mA value (PEW280=-4096. I think this value is the one that shows). The reading is OK but the value is not stable. it's like the loop current wire get's a 0 mA signal.

I've checked the wiring and it's ok. there is no loose wire. To make sure, the transmitter is OK, I emulate a 4-20 mA signal loop with a Fluke 4-20mA emulator (I use a 24 VDC power with a Fluke 4-20 mA emulator and a external multimeter to measure the current in the loop I'm emulating) and the same not stable problem happened. When I did this emulation, I connect the wires directly to teh PLC to make sure there is no problems in the wiring. Then, In order to make sure the input card of the PLC is OK I disconnect the Brix transmitter and connect a 2 wire pH transmitter in the same analog input channel and the reading was ok and stable (there is no problem).

I really don't know what else to try. I've make all the possibles actions.

Please in case any of you had a similar problem before please let me know or at least give me your ideas or comments what else to try.

This is a new project and this brix reading value is very important for the process and I'm stuck on this problem.

Thanks in advanced for your help.
Your statement, "reading is OK but the value is not stable" does not make sense.

The reading is either OK or
- off by some percentage or
- noisy or
- inverted or
- some other problem,
but 'OK' and 'not stable' do not go together.

1) Loop powered pH transmitter to AI works OK.

2) Active output 4-20mA from 4 wire brix transmitter momentarily reads OK momentarily, but then saturates at some extreme value (negative 4096)?

3) Fluke emulator with 24Vdc power supply mimics the 4 wire problem. It isn't clear how this is wired with the Fluke.

My suspicion is that the common mode voltage between the 4 wire circuit and the PLC's AIs amplifier builds up over that 3 second time interval until the common mode tolerance is exceeded at which point the PLC's input circuit saturates, going full scale negative.

The pH works because its power supply either floats or is common to the AI input ground.

My approach would be to install a 4-20mA loop isolator, which will isolate the 4 wire brix 4-20mA output signal from the PLC AI. Loop isolators are available anywhere in the world.



Hello David,

First of all thanks for your answer. When i say the measuring is OK but not stable I mean: the process value is 16.0% brix, the brix transmitter is showing a 16.0% in the LCD, the analog input is showing 16.0% for like 2 seconds and then the analog input value goes to 0 mA. Then the signal comes back again to the 16.0% and 2 4 seconds later the signal again goes to 0 mA (showing in the PLC analog input word a -12287 decimal value).

So far I have added in the PLC program a restriction so when the value is negative (when the signal goes to 0mA) the PLC will not use that value. So in this way the signal is still blinking but I get the right value (since it always comes back to the right process value).

A signal isolator is a nice solution. I will try to buy an isolator. Well, so far at least I have solved this problem with this PLC additional program.

Thank you very much for your help.

Best regards
Hello Frank19842003,

I have this same problem, signal goes down to 0 every 4 min for 30s. All the wiring is OK. Have you found any solution?


we found the problem. One signal connected to the IO module was disturbing the whole card.


> I have this same problem, signal goes down to 0 every 4 min for 30s. All the
> wiring is OK. Have you found any solution?



Make sure you're not overloading the loop power supply.
I've seen that cause an intermittent problem.

Robert D. Wagner, P.E.