Can I connect one 4-20mA sensor to 2 PLCs?

[Serhioromano]

For example I have pressure sensor 0-6Bar (4-20mA). I have PLC that have to relay pump based on pressure and I also have digital indicator that display pressure.

Do I need sort of signal splitter to connect the same sensor to PLC and Indicator? What would be wire scheme?

 

[MrAnonEMoose]

They can be connected in series provided the total loop impedance is less than

(Supply Voltage - Min. sensor operating voltage - V_wires) / 20 mA

 

[BryanG]

Sometimes you can connect them in series, sometimes you can't, it depends on how the Pressure Sensor, PLC and Digital Indicator are designed. I would expect that the pressure sensor will be loop powered, meaning it has no other supply than the 4-20mA signal, is so then that shouldn't cause any issues.

Somewhere in the loop there has to be a supply voltage, if both the PLC and Digital Indicator are trying to supply the loop at the same time there is likely to be an issue. If the power supply of both the PLC and Digital Indicator are grounded, that will cause issues.

So unless you are very confident in using 4-20mA and know the design of the parts, you would probably be better to use a signal splitter.

 

[Serhioromano]

Thank you very much. Probably I'll go with splitter. I am not confident that I understand what I am doing in terms of 4-20 loop and other things like voltage.

 

[RonJohn]

Do your PLCs already communicate with one another (Ethernet, ProfiNet, etc.)? If so, you could run the 4-20mA pressure signal to the pump control PLC and then send the pressure data via message to the display PLC.

If you're not sure how to send this data, please respond with info on PLC make/model numbers and communication format. It's likely that someone will be able to help.

 

[osmanmom]

i see only 1 plc,

if another plc have free analog output then use it to another plc.

 

[Saffa]

Simple is best. What you need is called a loop isolator. Your local electrical wholesaler should be able to supply one.

They can still be a little tricky to wire up but if there are no good drawings in the product manual then come back here with make and model of the sensor, the isolator your PLC.

The other good reason for using a loop isolator is that if your display and your PLC both have their negative analog input legs tied to DC common then you can't make this work without one. However most displays are either loop powered or have a floating input

 

[Serhioromano]

Do your PLCs already communicate with one another (Ethernet, ProfiNet, etc.)? If so, you could run the 4-20mA pressure signal to the pump control PLC and then send the pressure data via message to the display PLC.

If you're not sure how to send this data, please respond with info on PLC make/model numbers and communication format. It's likely that someone will be able to help.

I do not have 2 PLCs. I have digital indicator and PLC. They do not communicate.

Simple is best. What you need is called a loop isolator. Your local electrical wholesaler should be able to supply one.

They can still be a little tricky to wire up but if there are no good drawings in the product manual then come back here with make and model of the sensor, the isolator your PLC.

The other good reason for using a loop isolator is that if your display and your PLC both have their negative analog input legs tied to DC common then you can't make this work without one. However most displays are either loop powered or have a floating input

Thank you for that. I'll study this one. Sounds right what I was thinking I am going to need.

 

[shooter]

the indicator has is isolated so you can use the display in series no problem.