two PLC connect to single 4-20mA source

jnherm · Oct 15, 2006

hello everybody, is it possible to connect 2 PLCs analog(4-20mA) inputs from a single 4-20mA source? How? Is there available amplifier for the 4-20mA source?

Mickey · Oct 15, 2006

Yes it is possible. A typical PLC input resistance for current is 250ohms.(times 2= 500 ohms)
Most transmitters will have specs. that look like this

So if you have a 24VDC power supply (typical) you can have a total load of about 750 ohms.
I wouldn't push it, 500 should be ok.

Check the Specs. for your equipment.

Or use something like this
http://www.miinet.com/products/sg_signal.shtml (my preferred method)

jnherm · Oct 15, 2006

Hello Michey!
Have you experience it already that you connected 2 plc input from a single 4-20mA, because if we directly connect the two plcs the current would be divided into 2, so for instance the transmitter will output 10mA the 2 PLCs will receive 5mA each. for transmitter with 0-10V for example will be ok since both will receive the same voltage levels, but here we are dealing with current.

EKBist · Oct 15, 2006

Resistance Current --> Input U

50 Ohm +/-20 mA --> +/-1 V

250 Ohm +/-20 mA --> +/-5 V

250 Ohm 4...20 mA --> 1...5V

see PDF Automation System S7-400H Fault-tolerant Systems A5E00267695-02

Mickey · Oct 15, 2006

You connect them in series.( current is the same thru-out a series circuit) Again my prefered method would be to use some kind of isolator.

rPraveenkum · Oct 16, 2006

Dont connect 4- 20 ma in series to two analog inpus use signal multiplier avaliable with MTL make & phenoix contact many other

Gerry · Oct 16, 2006

If the PLC inputs are differential, it may work - depends on the electrical isolation between the two. If the inputs are single-ended, then you can't do it.

And yes, current inputs would be wired in series, voltage inputs in parallel.

jnherm · Oct 16, 2006

tnx!

To Everybody who contributes, thank you very much!
To EKbist, Can you explain to me in plain english what you contributed? I don't understand Greek. Thanks

jnherm · Oct 16, 2006

Hi Mickey! are you familiar with Siemens and GE PLCs? that's the two plcs im mentioning above. And the 4-20mA Transmitter im working with is a single-ended one. do i have any option not to use isolated system?

Calistodwt · Oct 16, 2006

You could connect both inputs to the PLC's in series from a single current source and as previously stated above provided that the current source can handle the loading but only if the inputs are floating (not earthed or connected to a common zero potential) otherwise it will not work. The safest is probably to use a current unit which accepts one 4-20 mA input and provides two 4-20 mA outputs.

Mickey · Oct 16, 2006

jnherm said:
Hi Mickey! are you familiar with Siemens and GE PLCs? that's the two plcs im mentioning above. And the 4-20mA Transmitter im working with is a single-ended one. do i have any option not to use isolated system?

Sorry, no I am not. I'm an AB and Controlmicrosystems SCADAPack
guy. I think single-ended is going to be a problem for you without
some kind of isolation.

danw · Oct 16, 2006

without isolation, you will likely have problems.

Here's model number and link to Phoenix Contact's 4-20mA splitter with 4 way isolation
MCR-FL-C-UI-2UI-DCI-NC 2814867

One 4-20mA input: two isolated 4-20mA outputs

Find link to data sheet on this page.
http://www.phoenixcon.com/products/interface/db/level1_2.asp?l0=MCR&l1=Isolators%2FConverters

jnherm · Oct 16, 2006

Thanks Danw!

two PLC connect to single 4-20mA source

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