Simple Question: Two PLCs to series connect to two 4-20mA

Guys,

No matter how much tutorial i read, I'm still a bit uncertain about earth loops. So need your help.

I want to connect two PLCs in series to two 4-20mA signals. The PLCs share the two 4-20mA.

1)Connect like this right?
http://www.geocities.com/bambinorama/2datalogger.JPG

2)To check whether the two dataloggers can be in series, at least one data logger must have floating point right? Then use a multimeter on ohms or continuity to see if either input connection is connected to the negative power supply connection like this right?

http://www.geocities.com/bambinorama/loop2.JPG

As long as one datalogger doesnt have its input connected to negative power supply, I can safely series the dataloggers?

Advance thanks guys.

signal duplicator

Use the MINI MCR-SL-UI-2I-NC (order no. 2864176) from Phoenix Contact.

It is a signal duplicator for electrical isolation and doubling of analog signals.
I have used it many times for splitting up analog signals for several PLC systems. Simpel to use and you don’t have to worry about earth loops.

Thanks for the reply. I did use that Phonenix contact on my remote weather station, but it is using too much solar power.

So I want to cut away the multiplier.

More then likely both PLC have grounded negative. In this case a signal will be grounded at first PLC and never reach a second PLC. To avoid this problem you have to use at least one signal isolator. The isolator must be installed in this case at first PLC, not in weather station.
If your PLC analog inputs are not grounded, then everything is OK.

Thanks guys.

Can you look at my diagrams to see whether my wiring is correct, and my method to check if the PLC analog input is grounded?

Your diagrams may be OK. Another way to test if PLC input is grounded or not, you may use mA meter in place of second PLC and see if its measure anything. It shoud detect 4-20mA if PLC input not grounded.

Check for common mode voltage (while Datalogger/PLCs are powered) between signal returns (grounds)


Check both AC and DC

Please advise one more configuration

Huge huge thanks.

Hey guys I got another wind sensor that gets its positive power from it 4-20mA positive terminal, not from a separte power input. Please have a look.

1) Is my wiring ok? This is the original wiring for one datalogger.


This the wiring for two datalogger in series


2)I am a bit worried about the second datalogger 3.5V AA battery's ground being connected to the 12V 75AH battery ground. But...they both are zeros...should be ok right?