Multitronics 100/200 connected to S7 PLC, working only on 2-wire mode?

We have a Siemens Multiranger 100/200 device that has the 4-20mA output connected to the AI input of a Siemens S7 PLC. However, when the analog input is set up for 4 wire mode it does not register the signal, when the PLC gets connected the mA and voltage signal drop down. However the cards and backplane etc are not in fault because we have injected into the PLC and it registers the signal fine (when having the 4-20mA signal supplied with voltage from the injector). Then, when the input channel is set-up for 2-wire mode, the signal works perfectly fine and registers the correct signal.

The multitronics appears to have the voltage (24V) on the line with the 4-20mA at all times, looking through the Siemens S7 manual, it shows the only real difference between the 2-wire mode and 4-wire mode as being the resistors, but there is more resistance on the 2-wire.

How is the Multitronics working only on 2-wire mode, when I thought it's output was active and powered by the Miltronics itself, not loop powered from the PLC?

You understand the Multi/HydroRanger 4-20mA analog output correctly; it is an active, powered output.  It does not need or want an external DC power supply to operate.  Your use of the term 'active' output is conventional and correct.

I'm an instrument guy and haven't a clue where Siemens hides their PLC documents.  Do you have a link to the AI wiring that you're referencing?

danw

You understand the Multi/HydroRanger 4-20mA analog output correctly; it is an active, powered output.  It does not need or want an external DC power supply to operate.  Your use of the term 'active' output is conventional and correct.

I'm an instrument guy and haven't a clue where Siemens hides their PLC documents.  Do you have a link to the AI wiring that you're referencing?

 There have been a few different wiring diagrams seen for Siemens S7 cards, and I don't know why, the manuals I have been looking through are on the link to the PDF below; the first being the module data, the second being the installation and set-up, both for s7-300 version PLCs:

https://cache.industry.siemens.com/dl/files/629/8859629/att_55794/v1/s7300_module_data_manual_en-US_en-US.pdf

http://home.hit.no/~mortenp/A289/Manualer/Simatic/S73bhb_e.pdf

Still unsure as to why it works 2 wire mode only, and not in 4-wire mode like it should do.

> it shows the only real difference between the 2-wire mode and 4-wire mode as being the resistors, but there is more resistance on the 2-wire.

I'm not sure which wiring diagrams you're looking at, but the wiring for either a
- passive 2 wire loop powered input or
- active 4 wire input
have exactly the same resistance in the loop.

Here's the wiring for passive 2 wire loop powered:
https://s11.postimg.org/tyzkeww77/2_wire_loop_powered_wiring_to_S7_300_AI_card_edi.jpg

Here's the wiring for an active 4 wire input:
https://s8.postimg.org/qex9xn4h1/4_wire_active_4_20m_A_input_to_S7_300_AI_card_edi.jpg

(from page 325 pdf of the first manual listed above)

Each 250 ohm resistor (on the ones/tens numbered wiring terminals) is effectively directly across the input of the amplifier (the triangle) on the twenties/thirties number wiring terminals.  It is there to create an IR drop of 1-5Vdc that the amplifier can 'see'/'read'.

That 250 ohm resistor is there for any milliamp current input, whether passive loop powered or active input.  So the resistance for either is the same.

Referring to the 2 wire loop powered diagram, I don't see how you can wire the (-) terminal from your active AI output to an even numbered terminal and then wire the odd numbered terminal to M (ground) and get any current flowing through the resistor; there is no current loop out of the field device because there is no connection to the field device (+) terminal.


 

danw

I'm not sure which wiring diagrams you're looking at, but the wiring for either a
- passive 2 wire loop powered input or
- active 4 wire input
have exactly the same resistance in the loop.

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Referring to the 2 wire loop powered diagram, I don't see how you can wire the (-) terminal from your active AI output to an even numbered terminal and then wire the odd numbered terminal to M (ground) and get any current flowing through the resistor; there is no current loop out of the field device because there is no connection to the field device (+) terminal.


 

 Yeah I realised after I put about the resistance that there was still a 250 Ohm in the 4 wire as well, so that doesn't make any difference.

The wiring diagram I need is slightly different to the one you are giving from a manual (I have seen that drawing but not in the manuals I have used recently, not sure how), the wiring I need is on pg 372 of the first manual.

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The current flows in the loop because the transmitter isn't using the voltage and generating it's own signal, it only regulates the current flow from the voltage source, it is basically a varying impedance (mostly resistance). Then once the signal reaches the PLC, as can be seen from the diagrams you posted, it only passes through a resistor and then to ground, in one full circuit. Although it is hard to tell if it should be a common ground or if they should be separated. There is a great document on 2-wire transducer method, found here:

https://www.acromag.com/sites/default/files/Acromag_Intro_TwoWire_Transmitters_4_20mA_Current_Loop_904A.pdf

I will eventually figure out how it is working the way it is when I get back to it