PLCs receiving 4 - 20 mA inputs

  • Thread starter Salai Kuberan E S, Manager/C&I
  • Start date

Thread Starter

Salai Kuberan E S, Manager/C&I

Comments are welcome on the Pros & cons of PLCs receiving 4-20 mA input. Also kindly enlighten on the successful players in the market.

Regards & TIA

Pro- Can use longer wire runs without signal strength loss. Lots of transducer outputs are 4-20mA. Noise immunity is also pretty good.

Con- Must be sure that you have compatible impedence so the source is not overloaded.

Salai Kuberan E S, Manager/C&I

Thanks for Comments on the Pros & cons of PLCs receiving 4-20 mA. My intention is to know the implications of using transmitter signals in PLC based Safety BMS etc.,



Rajesh Mehta

Hi Salai!

Now I understand what you want. Well, there is nothing wrong as such with using 4-20 mA (I would like to call analog signals) in a PLC even if that is used for protection systems like BMS or Turbine Protection.

The earlier designs had separate Data Acquisition Systems and Protection systems. The sensors themselves were separated for the two systems. This was definitely an overkill. As the technology improved, we moved on to combining the two in same system/ PLC. But the sensors for the two still remained separate. The information was extracted from analog signal whereas the protection was incorporated using field switches.

The trend nowadays is to have a common sensor for both protection as well as information. Depending on criticality of the protection system you provide 2 or even 3 signals and do a majority voting in the system. It is even common to have one of the elements to be analog whereas the other to be a binary (like pressure/ level switch) in the field.

The advantages of using analog signal is that it is constantly varying and gives you a fair indication as well as a trend before actually reaching alarm situation. Whereas a binary is either 0 or 1 so you never know if the switch has some problem till it reaches alarm stage. The other advantage is you can adjust the alarm limit using software rather than physically calibrating/ adjusting the setpoint/ differential. The differential setting may not be available in most switches but will most certainly be achieveable through software for setpoints created through analog signals.

The disadvantage is that you have another piece of electronics in the path and there can be delays due to dampening. I would avoid using an analog signal in Furnace Draft and still rely on Pressure Switches for fast action.

I hope this is of some help to you.


One of the distinct advantages of using a 4-20MA signal is the diagnostics you can do with it in a PLC. As mentioned in a prior reply, trending an analog signal(if using an HMI package) is a great way to see what is occurring in your process. But what I do is check for a broken wire, bad transmitter, or a span problem before I scale the signal to engineering units by utilizing clamp blocks or comparator blocks in the PLC. If using, say a GE Fanuc PLC, the signal will come in at 4000 to 20000 counts. If the signal is less than 10 counts, you could have either a broken wire or a bad transmitter. This is by far the best advantage of using a milliamp signal over a voltage signal. If you were using a 0-10v signal you would not have this capability since 0 volts equates to 0 in your process. If your counts are around 3000 to 3500, the zero on the transmitter could be off and likewise, if greater than 20000(ie, 21000 to 22000) would prove that the span of the instrument is off. Most of the clamp blocks have an "outside of limits" digital bit associated with it which you can then use an an alarm bit. Coding these alarms correctly will cut down the maintenance time needed to correct any issues.

Powerplus Engineers

We have used 4-20 ma signal inputs to PLCs. We have not faced any technical problem in using 4-20ma analog signals.However the designer has to take care of signal failure,wire breakage,electrical noise problem (software smoothening of analog signal can be considered)while designing the programme. The selection of right PLC is very important. The PLC has to be chosen based on what type of manipulation you want to do with the input signal viz 4-20 ma. The manipulation may vary from simple limit value monitoring to complicated analysis of process.
You have mentioned about BMS and BMS normally handles digital I/O s. However we have done PLC based BMS jobs where we have successfully interfaced analog signals 4-20 ma directly to BMS. For eg. the air flow. Hope the information is useful.
G.J/Powerplus Engineers


Johan Bengtsson

You can smooth the signal with analog components before it
reaches the PLC, this would normally give you better (or at
least equal) smoothing as if it is done in the program.

/Johan Bengtsson

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