Using single transmitter for both DCS & ESD


Thread Starter


I am assigned with my first project, where I got a P&ID which shows single pressure transmitter which is connected to ESD system for High-High shutdown and the same transmitter signal is fed to PIC of DCS for pressure control of the line.

I am not aware about connecting same transmitter to both DCS & ESD system (My school thought me to use separate transmitter for DCS and ESD :)).

1. Please let me know is it advisable to use single transmitter for both DCS & ESD.

2. How can I split/connect 2 wire signal from single transmitter to both DCS & ESD. The DCS is from Foxboro and ESD from Triconex.
Important thing is that client want to complete the project with minimum budget, so may be the reason for using only single transmitter.

Please do help me.

I have to assume that the power supply for the transmitter is separate from the two systems.

It would then be a matter of connecting in series the 2 systems with the transmitter connected between them. Positive of xmitter to one system and negative to the other with a wire between the two remaining terminals on both sides.

I have been forced to use xmitters like this before and it works well.
Hi Dasa,

Is there any link between your ESD and DCS system? (ususally there is modbus link between them). If yes, you can send the signal to DCS and then send the HH signal to ESD. (because the IO of DCS is usually cheaper than of ESD I propose that your signal first goes to DCS).

But if there isn't any connection between your systems (I haven't seen this type yet) your transmitter must be of some special types which have two outputs. In conventional transmitters you can't transmit one signal to more than one system.


Amitabha Thakur

First of all single transmitter connected to both DCS and ESD is not recommended for critical measurements. However if client wants to reduce cost (some times they do not understand the technicality), you can use single transmitter with sacrificing the reliability. Tell it to your client.

Now, best way to save cost is to connect the transmitter first to the ESD (provided your ESD accept analog signals) and fanout an analog output (AO) to DCS.

Another way is to use a signal multiplier / distributer (at the ESD panel) and use two multiplied signals, one to ESD and other to DCS.

A. Thakur
Thanks Thakur,

Can you tell me in what way it will affect criticality? I assume on safety point ESD system must have the signal availability all the time, so out of the two method you suggested 1st option of connecting to ESD and from ESD to DCS AO out will be fine. In this project we need to install new I/O cards, so to reduce the cost (putting new ESD AO card!!) it will be a viable option to
put a splitter as u suggest. If I use a splitter how much reliable will be the system ie splitter. Is I can live with the second option of putting a splitter??

Thanks djvol,

I couldn't understand that "power supply to the transmitter will not be from these two systems".

The xmitter is 4-20mA 2 wire loop powered. Do you advice that to provide a separate 24vdc supply which will be connected in series with ESD, DCS, Xmitter in a loop as described by you.
Hi Saeid,

Thanks for your reply. If it is in other way around, ie xmitter is connected to ESD (I will buy SIL2 transmitter)and I will out AO (Analog o/p) serially over the modbus link b/w ESD & DCS and this CAO(Serial Analog Output)from ESD will be used by DCS in its PIC controller to control my pressure control valve.

Is it will work and reliable? What will be the normal delay in a modbus communication? Will I can have a real time PIC control if above method is used?

I will be thankful for your reply.


One way you can do this is to wire the transmitter directly to the ESD system with a signal isolator re-transmitting the signal to the control system (DCS).

The signal isolator is load powered type, supplied from the load side (DCS) or a seperate power source. If the control system or signal isolator supply fails the transmitter still works with the ESD.

When you are trying to svae the cost , configuration or wiring should be made very simple.

Normally , I would have used signal splitter
from MTL, P & F , TURCK
You can procure with following specification

input: 4-20 MA DC , 2 WIRE
Output: 4-20ma dc [ 2 Nos ]

Mind you this ISOLATORS are SIL2 certified, so if your procure SIL2 certified Field transmitters, it could serve your purpose of cost. Please note that this is not treated as safe system the way it envisaged as per documentation of ESD. This would give advantage of loop response time which is required for ESD. Please note that this splitter also would give xtr power supply to field with isolation.

Other option is to procure Pressure switch which will be directly wired to ESD system. You also generate PSHH in DCS with PT & Hardwired to ESD take OR signal. you still achieve the functionality you require & Reliability.

Please avoid following

1] Serial communication between ESD & DCS for tripp commands.You are defeating all the purpose of guaranteed performance.

This is my school of thought in the event we have to compromise on cost.

iconcnl [at]

The answer to your question is simple, because you can use barrier with 2 analog outputs in your ESD and 1 output can be connected to the DCS.

We use this solution in our several applications and it works properly.

Important to note that we use this solution ONLY in applications where the ESD trip is generated by 2 independent sensors with 1oo2 voting. DANGEROUS can be the usage of single instrument for both control and trip! - in case of malfunction on the loop elements can cause the shutdown of the process -> loss of production, human safety, etc. - You should better to consult with your process department and check the HAZOP study.

The question is always the LOOP classification and it's SIL class and if there is HART signal for the selection of the barrier.

Good luck!


Doesn't your barrier with two outputs introduce another item that could fail?

We use a signal isolator to send the signal to the non critical DCS. The only portion of the isolator in the ESD wiring is a simple input resistor with its power coming from the DCS or a DC supply not part of the ESD system. The isolator can fail but still the ESD gets its signal.
Perhaps I misunderstood your post.


Mohamed Ragab


I am looking for a signal isolator similar to the one that you described here; one that does not introduce a common cause failure to both DCS and ESD systems if it fails. Can you please advise which make and model you are using and whether it is SIL rated?

My email is ragaboxy [at]


safety is priority if you have only one transmitter this one will have to be declare as safety and directly connected to safety system and all the application implemented in safety system for safety functions and actions.

First solution (preferred), a split barrier SIL certified will have to be insert in marshaling cabinets in helping to deliver a copy of the signal to the DCS system in helping process control to get information related to this sensor.

second solution, the safety system will communicate the value of the transmitter via the network to the DCS, be careful regarding the scan time limit for control function.
What solution did you choose? Can you send me some layout about the principals of wiring of the system, because i'am confronted to the same problem.

thank you
Eyal, eyal.szombat [at]
Hi guys,

im having the same problem. One single transmitter i need to wire to my ESD and to my DCS too...

I see, that priority is the ESD (in this case 4-20mA) but instead of putting a splitter, could I send this info through Profibus DP comm to my DCS. Profibus DP is the link between ESD and DCS. I think there should not be such a delay to cause control problems because the loops are not very fast. The thing is that I want to avoid 4-20mA loops to my DCS... I have spare for Fieldbus devices, but as this transmitter is for the ESD, it has 4-20mA loop.

Any known problems with this configuration?
It's akin to putting the fox in the henhouse to protect the chickens! I can produce a number of horror stories that are available upon request.

Regards, Phil Corso.
I want to know what are the esd signals?
and is it really a device or a plant called as emergency shutting device? in which areas it comes into the picture?

please answer my question. it is really confusing as. Designing of switchboards
Ok. firstly it is bad practice to use a single transmitter for control and protection (ESD). Install two transmitters. You should also SIL assess the protective function to determine the integrity level for the protective function and ensure you engineer to meet that integrity level. (See ISO 61508 / ISO 61511). For high SIL levels you might need more than 1 transmitter on the protective function !!

If you insist on using a single tx. then you have to ensure the shared signal does not cause ground loop problems so either :-

a) ensure that each system has isolated current inputs (i.e the sig com is not grounded). then you can series the two inputs with impunity. However you need to check the total loop resistance to ensure the tx can drive 20 mA around the total loop resistance(simple ohms law stuff).

b) use a signal isolator that will isolate and repeat the signal to the control system (make the ESD the 'master system' in the original loop, the DCS in the secondary loop)

c) use a common resistor (e.g. 250 ohms) to turm the 4-20 mA signal into a 1-5 volt signal, then use voltage inputs on both the ESD and DCS I/O, you can happily parallel them provided they share the same sig common (or are isolated),otherwise beware you will cause problems !!

d) wire the signal to the ESD and communicate it to the DCS across e.g. a modbus link. Note however this is likely to give control problems as its likely the control loop wont like the delays associated with the data transmission. it may not be reliable enough for the control loop. The loop may not perform well or be tuneable. OK if you just want a pressure indication, rather than closed loop control.

Also consider the maintenance tech who has to do the routine checks on the ESD loop. If he needs to verify or calibrate the tx, or to verify the input to the ESD system, he will also disable the control loop, this may mean you plant will be down during the checks. This may be unpopular with operations and costly!

All of which just shows that its easier to do it right first time (put two txs in)and avoid the potential pitfalls! Which is why its good practice.

I think in a few years ago this solution was impossible and not recommended, but not with the the new technology. We can use a SIL3 or SIL2 single splitter to transmit the value of the transmitter to the DCS & ESD. This solution is allowed if the master signal will transmit to ESD/SIS, and the and the duplicated signal to a control system (DCS, DCS), that allows the supervision of sensitive points of the installation.

good luck
It is acceptable to use a signal splitter to provide the same transmitter signal to ESD or BMS and DCS?

The ESD system must be the system that powers the signal splitter.

good luck