Splitting a 4-20mA signal

Jim G.

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Nov 2007
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Georgia
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I have a system which has 4 large storage tanks holding a liquid. The level of each tank is sensed by a Senix ultrasonic sensor with a 4-20mA signal going into the analog card of a SLC 500 plc. The sensor has 4 wires, with 2 being the power supply and 2 being the signal out. The common is shared. Everything works great. NOW... we are going to add another production line and use these same 4 storage tanks as a supply to the new line. Completely separate system but I need to share the 4-20mA level signal with this new system. I know this can be done with a signal splitter but I am not sure what exactly to buy. Two way or three way isolation. Any recommendations? Thanks.
 
You shouldn't have any trouble finding one you like.
Everybody sells them: Phoenix, AB, Action Instruments, etc.

I've used all of the above and they work fine.

Some are configurable where you can select 0-10V on Ch1 and 4-20mA on Ch2.
 
A 4-20mA signal can be looped through multiple devices. As long as the loop is complete each device (PLC) will read the mA signal.

You just have to cut one wire of the existing loop and loop it through the other PLC.
 
"Multiple" is limited by the power supply. Typical is 24DC and 250 Ohm inputs. That puts the maximum at four full scale. If the input module uses 500 Ohm resistors, the limit is only two devices.
 
A 4-20mA signal can be looped through multiple devices. As long as the loop is complete each device (PLC) will read the mA signal.

You just have to cut one wire of the existing loop and loop it through the other PLC.

Aabeck, I agree that what you stated is possible but it probably is not a good idea in this case.

If he has two different lines (??Production??) then I would probably want to make sure the sensor is powered from a source different from each production line. You want to make sure that line 2 will still get the signal even if line 1 is off.
 
A 4-20mA signal can be looped through multiple devices. As long as the loop is complete each device (PLC) will read the mA signal.

You just have to cut one wire of the existing loop and loop it through the other PLC.

While possible, I suppose, that's a terrible idea.
 
I would either add a communication bus between them. No good will come of splitting a analog sensor between two independent sensors.

Alternately, drop a small PLC in that does nothing but read the shared sensors and send that data to both lines. Then you have the data even if one line or PLC goes down.
 
There is absolutely no problem with "sharing" the same 4-20 signal between different devices. I'm not sure what all the worry is about here but that has been a valid 4-20 loop design since the days of single loop controllers. It will work fine for two PLCs. Keep in mind because it is current, you are wiring the loads in series. Keithkyll is correct in that you will need to take into account the load when connecting devices. Isolated inputs would also be recommended. You can add some type of splitter as recommended by some above, but your power source for the transmitter will still be as it is now and turning off that power will remove the signal to the splitter device also. Not sure I see the point in that. Communicating between processors is fine too, but you already said that is not going to happen.
 
A 4-20mA signal can be looped through multiple devices. As long as the loop is complete each device (PLC) will read the mA signal.

You just have to cut one wire of the existing loop and loop it through the other PLC.


By "loop" you are referring to wiring all devices in series?

I have been successful doing that in the past. But it's very confusing to troubleshoot
 
In my book, the slight nod goes to the splitter. If cost is a huge concern, then passing the analog through is understandable. Keep in mind that you can't always do that, such as with non-isolated input modules that have a shared common.
 
As AAbek says, you can run the signal through ~3 devices w/o concern for whether the devices are powered. Analog input channel impedance is typically 250 ohms. If the impedance becomes too high, the 4-20mA Xmitter can no longer make it to 20mA...no damage from that...just bad signal. I would be comfortable running 3 channels in series if and only if[this is important] the channels are not commoned (they often are in the Analog input card). If this is this the case, then your're SOL with 4-20mA w/o a splitter. I'd switch to voltage (0 to 10 VDC perhaps).
 
Another reason you don't want to route a 4 to 20mA signal through multiple devices would be if the devices are not isolated from ground. Been there and had to abandon it when upgrading a VFD with a pressure transmitter than went through it and an RTU circuit board. The new drive did not have isolated inputs and there was nothing we could do about it in a pinch except add another pressure sensor.

My vote is for a signal conditioner with isolation and two outputs.
 

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