CR4® - The Engineer's Place for News and Discussion®


Previous in Forum: Endress Hause FMD71   Next in Forum: Infrared Sensor and Heavy Amounts of G Shock
Close
Close
Close
14 comments
Participant

Join Date: Nov 2017
Posts: 4

Analog Signal Wiring to PLC

11/15/2017 1:15 PM

Hi,

I have a question regarding wiring analog signal to PLC . In my case Analog signals are connected to PLC via surge protector and HART multiplexer. My cable is shielded till Multiplexer. Is that okay to use just unshielded signal wire from multiplexer to PLC IO. There is a power supply module inside the panel which is 120VAc. But we are using seperate wire duct for Signal cables. Please advise

Register to Reply
Pathfinder Tags: Analog PLc
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting ["brexit" - ugh] as Little England and Wales (not too sure about Wales bit, either). Kettle's on.
Posts: 26779
Good Answers: 703
#1

Re: Analog signal wiring to PLC

11/15/2017 1:33 PM

It depends upon the applicable wiring standard for the installation, which cannot be seen from here. At CFAT [Client's Factory Acceptance Testing], the client's Acceptance Engineer will use that standard to assess the installation, and it it doesn't comply at that juncture, then it would be an incorrect situation that requires remedial work to achieve compliance.

However, assuming there is no applicable standard in force one needs to consider the style of the signals (which has been withheld from the forum):

  • If the analog signals were current-based (and a typical example is 4-20mA representing 0-100% of scale) into low-impedance inputs then in general the signals are very resilient to the effects of induction noise on the cables and the risk of using un-shielded cable is relatively low.
  • However, if they are low-level voltage signals into high impedance inputs a different state of affairs might exist, the behaviour of which is unpredictable from here.

Neither of the above bullet-points comes with making the risk of rework zero, which is why reputable organisations use standards so that things go right first time. Therefore in the absence of an applicable wiring standard, it can only be stated that shielded cable is preferable from a risk reduction aspect. As to the level of risk, well, that can only be determined after the installation has been put into service, and an applicable standard based upon experience can be written after any re-work has been completed for others to use going forward.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13777
Good Answers: 155
#2

Re: Analog signal wiring to PLC

11/15/2017 2:37 PM

This sounds a little bit off.

Usually the analog signal, either a voltage, or a 4-20 mA current loop (output from an analog device), is read in the local PLC panel through an analog module (converts signal to digital for transmission on the HART bus, essentially an ethernet bus, strictly for the PLC's LAN. The digital signal is not shielded as this is considered a high level logic signal, not low-level analog subject to electrical interference.

That there is separated wire duct for signal cabling is no surprise, although not entirely necessary in nearly every application.

Please do elaborate on this surge protector, as this is a bit different configuration?

__________________
If it ain't broke, don't fix it. Just build a better one.
Register to Reply
Guru
United Kingdom - Member - Not a New Member Hobbies - Musician - New Member Hobbies - Fishing - New Member

Join Date: May 2006
Location: Reading, Berkshire, UK. Going under cover.
Posts: 9575
Good Answers: 457
#3

Re: Analog signal wiring to PLC

11/15/2017 5:34 PM

Is the multiplexer to PLC link voltage or current loop? If voltage, it should definitely be screened twisted pair.

__________________
"Love justice, you who rule the world" - Dante Alighieri
Register to Reply
Participant

Join Date: Nov 2017
Posts: 4
#7
In reply to #3

Re: Analog signal wiring to PLC

11/16/2017 9:14 AM

Multiplexer to PLC is a current loop 4-20mA

Register to Reply
2
Guru
United Kingdom - Member - Not a New Member Hobbies - Musician - New Member Hobbies - Fishing - New Member

Join Date: May 2006
Location: Reading, Berkshire, UK. Going under cover.
Posts: 9575
Good Answers: 457
#11
In reply to #7

Re: Analog signal wiring to PLC

11/16/2017 10:23 AM

Given 4-20mA and (as you've posted elsewhere) the mux and the PLC are in the same enclosure, with about a 1m cable run, you'll be OK with twisted pair. But keep the wiring away from mains/motor drives/motor wiring etc.

__________________
"Love justice, you who rule the world" - Dante Alighieri
Register to Reply Good Answer (Score 2)
Guru

Join Date: Dec 2010
Posts: 1231
Good Answers: 91
#4

Re: Analog signal wiring to PLC

11/15/2017 6:13 PM
  1. What does the PLC manual say about cable shielding & separation?
  2. How long is cable run from multiplexer to PLC?
  3. What kind & amplitude of signal is used from PLC to multiplexer?
  4. What is the physical separation distance between "multiplexer to PLC" duct & ducts for 120VAC & any "non-signal" cables?
  5. Do you mean there is a surge protector on each analog signal to the HART multiplexer or on the Mux to PLC wires?
  6. How critical are your analog signals to your purpose? An occasional jump of an operator indication is less critical than a high/ low signal alarm, which is less critical than an automatic shutdown programmed from an analog signal.

Any unwanted noise between wires increases proportional to length of parallel run and reduces with increase of spacing between the wires (often on" inverse square" law).

It also increases with the area between wires of the "noise" circuit and the area between the wires of the "victim" circuit - it is always good practice to run live/neutral of power circuits or go-return of victim circuits in twin cables. Allowing any heavy current to flow in a large area loop is a way to maximise its magnetic interference potential!

Generally, there is no substitute for a separation distance, shielding cannot compensate close proximity, especially with magnetic fields. Separation is usually cheap - before the plant is built!: shielded cable costs significantly more.

Register to Reply
Participant

Join Date: Nov 2017
Posts: 4
#10
In reply to #4

Re: Analog signal wiring to PLC

11/16/2017 9:46 AM
  1. What does the PLC manual say about cable shielding & separation? - I can't find anything specific regarding shielding
  2. How long is cable run from multiplexer to PLC? - Both Multiplexer and PLC inside th e panel . Around 1 meter
  3. What kind & amplitude of signal is used from PLC to multiplexer? -4-20mADC
  4. What is the physical separation distance between "multiplexer to PLC" duct & ducts for 120VAC & any "non-signal" cables? - both ducts are running parellel
  5. Do you mean there is a surge protector on each analog signal to the HART multiplexer or on the Mux to PLC wires? - Yes there is a surge protector on each analog signal
  6. How critical are your analog signals to your purpose? An occasional jump of an operator indication is less critical than a high/ low signal alarm, which is less critical than an automatic shutdown programmed from an analog signal. - This is used for PCS
Register to Reply
Guru

Join Date: Dec 2010
Posts: 1231
Good Answers: 91
#13
In reply to #10

Re: Analog signal wiring to PLC

11/16/2017 12:08 PM

Thanks for reply. With 1 metre run, I agree with John DG post #11 - twisted pair but keep the separation.

I like to see 100mm separation. If your ducts are parallel & actually touching, there could be only 2.5 mm separation. Use cable ties to fix power cables to side of duct remote from signal duct & to fix signal cables to side of duct remote from power duct, so you can see the space between signal cables & anything else.

If wires are together in a duct, or worse, laced together, the noise one wire has, they all have. My "rough estimate" is that wires in the same PVC insulated cable have 0.04 microfarad capacitance between them over 100m run. Even at 50 Hz, it is possible that a 6VA 240V contactor coil series resonates with cable capacity, so opening a motor emergency stop contact does not stop the motor.

Since you have a multiplexer, I suppose a lot of analog values depend on that 1 metre run, so it is best to take precautions.

Along with "Iris" post, I wonder if Mux to PLC is serial digital. Since you have not given the Pepperl & Fuchs part code CR4 cannot tell.

Reference 6. - "PCS" is vague - do you mean Programmable Control System? If it controls something a corrupted analog signal might produce scrap or mix something that goes "bang" not "bubble".

Register to Reply Score 1 for Good Answer
Guru

Join Date: Jul 2010
Posts: 651
Good Answers: 172
#5

Re: Analog signal wiring to PLC

11/15/2017 8:55 PM

I'm confused. The input of a HART multiplexor is the HART data from multiple field devices/instruments which are connected to analog inputs where their 4-20mA signals are used in a conventional sense.

The output of a HART multiplexor is the HART data in the form of serial data, or maybe, nowadays, an Ethernet output, as shown in the graphic below:

So how is your Mux output "going to PLC I/O"? The field instruments connect to the PLC I/O.

Serial data deserves the same noise protection (ie shielding) as analog data, because if digital serial data is affected by noise, you don't get "data + noise", you get a no data and a 'comm error'. Comm protocols always throw away corrupted data and issue a comm error.

Register to Reply
Participant

Join Date: Nov 2017
Posts: 4
#9
In reply to #5

Re: Analog signal wiring to PLC

11/16/2017 9:41 AM

https://www.pepperl-fuchs.com/global/en/classid_2256.htm

We use pepperl fuchs multiplexer .So Field instruments are connected to Mux. Then from Mux its connected to the PLC IO.

Register to Reply
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13777
Good Answers: 155
#12
In reply to #9

Re: Analog signal wiring to PLC

11/16/2017 11:52 AM

Approximately how many analog devices (each has its own 4-20 mA signal output) and how many HART MUX are present in your system that brings data from local panels/boxes to which brand of PLC? What HMI are you using, and what brand of software is involved?

Who is responsible for programming/interfacing your system to the PCS?

__________________
If it ain't broke, don't fix it. Just build a better one.
Register to Reply
Participant

Join Date: Nov 2017
Posts: 1
#6

Re: Analog signal wiring to PLC

11/15/2017 11:32 PM

as my opinion, the signal cable is different,diffent PLC brand even different model has different cable.

Register to Reply
Guru

Join Date: Sep 2016
Posts: 763
Good Answers: 8
#8

Re: Analog Signal Wiring to PLC

11/16/2017 9:32 AM

Signal wiring should be shielded especially for lengthy ones. The EMF is there you can not discredit that.

Register to Reply Score 1 for Off Topic
Power-User

Join Date: Nov 2009
Posts: 289
Good Answers: 55
#14

Re: Analog Signal Wiring to PLC

11/16/2017 8:15 PM

P&F have an H System Mux and a K System Mux. Different flavors of the same thing.

The H System Mux 2700 is explicit about its connection to the Asset Management System, the intelligence that exploits the HART data, not the 4-20mA data:

The connection on the output side of the MUX is RS-485, a serial communication bus.

The diagram of the system topology confirms that the MUX output is serial, not analog 4-20mA:

The concept is the that control system uses the 4-20mA on its analog inputs, but for HART variables or diagnostics, the high level system interrogates one HART device at a time, starting with a 485 serial connection to the MUX, and then through the Mux to the device of interest on the far side of the MUX.

A HART mux is like a sniffer, letting the 4-20mA do its thing for the control system, and using HART on an as-needed basis, one device at a time.

You may continue to believe that you are getting an analog signal as the output of the MUX to your PLC, but it will be generally square wave shaped signal at some kHz frequency, AKA RS-485.

I concur with Iris's statement that the serial connection needs as much noise protection as the analog signals

Register to Reply
Register to Reply 14 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

67model (2); Carl_E (1); gutmonarch (1); Iris (1); James Stewart (2); JohnDG (2); PWSlack (1); qiwating (1); sherin21 (3)

Previous in Forum: Endress Hause FMD71   Next in Forum: Infrared Sensor and Heavy Amounts of G Shock

Advertisement