We're working on a grass root project and are looking a cheap way of bringing groups of temperature signals that are in close proximity into a DCS. Individual wiring is not an option so we need to Hart multiplex or use remote I/Os.

I've been looking into an economic way of getting the digital PV into the DCS of a bunch of non critical temperature signals by the use of Hart multiplexors. The info would be available to the DCS via a Modbus RTU protocol (providing the mux has a port for that) and the mux outputs would be jumpered on the mux output connector side(not hardwired to I/O cards).

We think using Fieldbus only for this would be over-killing. Would this be economically feasible or there is a more simple and cheapest way? Any suggestion? Thanks a lot.

 

There are less expensive temperature scanners than HART multiplexers. Thermocouple scanners are available up to about 64 points and often use Modbus interfaces. If you use HART, you will need to invest in HART Temperature transmitters for each point. Very expensive, unless you already have the HART transmitters. You can also use several of the Remote I/O devices intended for PLCs, that have thermocouple or RTD inputs and some kind of Ethernet connection to the DCS.

Dick Caro
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> groups of temperature signals <

I would think that you mean thermocouple direct because if you'd already paid for a transmitter, why wouldn't it be wired into your DCS?

Are you talking about T/C signals or transmitter signals?

> signals by the use of Hart multiplexors. via a Modbus RTU protocol <

Why use HART and Modbus? Why not Modbus alone?

> the mux outputs would be jumpered on the mux output connector side(not hardwired to I/O cards). <

I'm really lost now. Mux output? You already have a HART mux?

> We think using Fieldbus only <

Where did Fieldbus come from? Do you mean Foundation Fieldbus?

What exactly are you working with?

David

 

FOUNDATION fieldbus is not overkill. On the contrary, there is a killer application for FOUNDATION fieldbus for what you want to do:

Rosemount 848T high-density (8 channel) temperature transmitter using FOUNDATION fieldbus:
http://www2.emersonprocess.com/en-U...eldbus/documents/productoverview_848thdm.html

Rosemount 3420 FOUNDATION fieldbus gateway:

http://www2.emersonprocess.com/en-US/brands/rosemount/Accessories/3420/Pages/index.aspx

Use any number of 848T you need to cover all your points. If your DCS does not support FOUNDATION fieldbus, use one or more 3420 to convert the values to Modbus/RTU, Modbus/TCP, or OPC as accepted by your system. Connect AMS Device Manager to keep an eye on sensor/transmitter diagnostics for you.

Cheers,
Jonas

 

I tend to agree with David, I suspect that you are dealing only with temperature elements, please confirm this, if this is the scenario then a multiplexer with modbus output can be used especially if these are non-critical indication loops only.

Remote I/O would be another feasible option which will eliminate any integration and communication issues although modbus capabilities is standard with most automation systems nowadays.

 

The Emerson FFB 848T is a possible solution for this type of application and a good one. I use them on Ammonia plants. However dependent on what stage you are at on the project regarding detailed engineering (for systems I/O layout, cable schedule etc), the quantity of temperature points being considered, the FieldBus 848Ts may not be feasible because of cost considering installation works etc.

 

>> groups of temperature signals <<

> I would think that you mean thermocouple direct because if you'd already paid for a transmitter, why wouldn't it be wired into your DCS? <

Yes, direct T/C is the cheaper option to go with...No wired because as I said Client doesn't want it and is not the cheaper option right?

> Are you talking about T/C signals or transmitter signals? <

T/C signals...transmitters would add cost....

>> signals by the use of Hart multiplexors. via a Modbus RTU protocol <<

> Why use HART and Modbus? Why not Modbus alone? <

Use of Hart Txs is the standard for the project but would consider a different option for this specific application.

>> the mux outputs would be jumpered on the mux output connector side(not hardwired to I/O cards). <<

> I'm really lost now. Mux output? You already have a HART mux? <

I do not have Mux, I AM just planning to have is...(grass root project)

>> We think using Fieldbus only <<

> Where did Fieldbus come from? Do you mean Foundation Fieldbus? <

Sorry, I meant ANY Fieldbus.

> What exactly are you working with? <

A grass root project.

Thanks,
David

 

HART is an FSK signal superimposed onto a 4-20mA signal from a field transmitter. The HART signal is generated by a HART compatible field transmitter. Without a field temperature transmitter you have neither 4-20mA nor a HART signal.

A HART Mux multiplexes HART signals. You have raw thermocouples. Attempting to use a HART mux when you don't have any HART signals to multiplex doesn't get you any further to a solution. You cannot connect raw thermocouples to the HART muxes that I'm aware of. They take only conventional HART signals. Furthermore, your DCS and your HART mux have to talk the same protocol to handle the data from the MUX.

A solution for T/C signal concentration depends on which 'bus' and the associated programming your DCS can most easily handle.

Someone in this thread suggested a Foundation Fieldbus mux, which would work if your DCS is
setup for FF.

Someone else suggested commercially available converters that take analog thermocouple signals and convert them to Modbus. I happen to have used an Acromag 965EN box that takes 6 T/Cs and puts them onto Ethernet as a Modbus TCP slave. I'm sure that there are others brands and models.

So what digital protocol can your DCS easily handle to talk to external data devices?

David

 

There are many kinds of multiplexers: there are HART multiplexers and
temperature sensor multiplexers. I think the person was referring to temperature sensor multiplexers. A temperature sensor multiplexers is pretty much a remote-I/O with temperature input cards.

A temperature sensor multiplexer can itself be low cost, but the total cost is high because it is mounted centrally and long thermocouple compensation wires (expensive to buy and lay) must run from sensor to the central location. Moreover, the temperature sensor multiplexer may not support redundancy so one fault knocks out the whole thing. Since the thermocouple signal is low level, noise can be picked up along the way.

A Foundation fieldbus solution actually does not require the DCS to support Foundation fieldbus because you can use the Rosemount 3420 fieldbus gateway that provides Modbus/RTU, Modbus/TCP, and OPC to the DCS. The 848T multi-channel temperature transmitter has the advantage it can be mounted close to the sensors. This reduces sensor wiring substantially resulting in overall lower cost. The other advantage is that by doing signal conditioning close to the sensor, you are less likely to pickup noise.

Cheers,
Jonas