Discrete I/O and Fieldbus

Dear Industry Experts,

We are considering a field communication network of Foundation Fieldbus (Or Profibus) for our plant.

There are several discrete I/O's in the plant, and having them all connected to the FF seems to be very expensive.

I was just considering some alternatives instead:

1- If we go for Profibus, there we can have Profibus PA for the main analogue bus and Profibus DP for the digital I/O's which supposedly make a saving on the digital part.

2- Make use of another bus for digital network, say AS-i to have cheaper hardware.

3- Use conventional wiring along with the FF network for discrete I/O's such as limit switches.

An expert advice is highly appreciated.

Thanks
Hamid

Hamid,
I have just recently finished on a project where we used Foundation Fieldbus for all the "Analogue" and ASIbus for the descrete devices such as valves, switches, lamps etc.

FF devices we used the Pepperl & Fuchs(P&F) Segment protectors.

Asi is cheaper and we used P&F G5s to convert the descrete device to ASI or wired a simple device to spare teminals on an ASI on/off valve.

Regards,
Carmel

Carmel,
It is exactly what we are considering! Are you satisfied with the whole system? What was the main controllers? Did you have a DCS or SCADA or something else? What was the number of digial and analogue I/O?

It would be wonderful if you could provide me some more information about the details.

Regards
Hamid

using Foundation Fieldbus for discrete IO is not advisable. PROFIBUS DP is best suited for such applications.

regards
Ravi Mahajan

Dear Hamid, Keep in mind that on/off sensors and actuators do not connect direct to PROFIBUS-DP. You still need a traditional panel to house conventional I/O card to which you run point-to-point wires to each sensor/actuator. You could do remote-I/O to reduce the cable length, but you still have all the terminations etc. ASI or DeviceNet can run at the sensor/actuator level. DeviceNet even allows intelligence in the sensors and actuators.

In my personal opinion FF is a better choice than PA for process control loops involving dynamic elements such as PID controllers, filter, lead-lag, and integrator/totalizer etc. because it is precisely periodic "dt" and control is scheduled and synchronized with communication. Both provide digital closed loop control, but FF has separation between real-time and non-real-time channels. FF also has automatic addressing which makes it easier to commission new devices: plug-n-play. Another neat feature only FF has is online bumpless firmware download so you can upgrade devices. Both PROFIBUS and FF use Enhanced EDDL (www.eddl.org) which is very good. With FF you will in a good control system also get sensor-to-actuator status propagation, that is, sensor failure propagates through PID and brings valve to a predetermined state.

To learn more about the above aspects of fieldbus take a look at the yellow book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" buy online: http://www.isa.org/fieldbuses

Cheers,
Jonas

Dear Hamid,

Your best choice for a mixed system that you described is PROFIBUS.

PROFIBUS DP high speed backbone give you easy and cost effective digital I/O. Plus there are many effective interfaces to AS-i. AS-i and PROFIBUS complement one another.

PROFIBUS PA connects easily into PROFIBUS DP (they are the same protocol just different physical layers) and is very good for process control. For PID functions, PA works great and gives you the added feature that you can easily design the network so that your loop update times are well under 300ms (something that is very hard to do in FF).

Having worked with both FF and PROFIBUS, I find PROFIBUS considerable easier to use and far more cost effective.

Regards,

James

We intend to upgrade the control system of our plant which is "Pneumatic" & "Electronic". In order to find the best choice I need help in comparison of DCS, FieldBus, Profibus or other new systems. Could anyone help me?

I discuss many of these issues in my book, "The Consumer's Guide to Fieldbus Network Equipment for Process Control", that is referenced below. The investment in this book can save you many $$$ on your project.

I recommend using Foundation Fieldbus with any of the popular DCS. The manufacturers of these DCSs will configure the number of controllers that are
capable of handling ALL of your control loops, but since ALL single and two-level cascade control loops can better be located in Foundation Fieldbus
instruments and control valve positioners, you are free to reduce the number of DCS controllers by 60-80 percent. This is the savings to which I have referred in the last line of my signature block below. If you choose to do this, your DCS supplier will still support you, and you will not only save money, but obtain better control. Many users of Foundation Fieldbus have reported such performance, but only obtain the savings from the reduced number of controllers on their second or later systems. It really works and can reduce your capital costs by a very large number.

Dick Caro
===========================================
Richard H. Caro, Certified Automation Professional, CEO, CMC Associates,
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +.978.764.4728
Fax: +1.978.246.1270
E-mail: [email protected]
Web: http://www.CMC.us
Buy my books:
http://www.isa.org/books
Automation Network Selection
Wireless Networks for Industrial Automation
http://www.spitzerandboyes.com/Product/fbus.htm
The Consumer's Guide to Fieldbus Network Equipment for Process Control
Buy this book and save 50% or more on your next control system!!!
===========================================

Thank you for your concern.
I have heard that FieldBus can not support all the signals and becuase of this matter some recommneds to use a composition of DCS and Fieldbus.

Refer to previous discussed issues about this subject some said that using Fieldbus for all signals would bee expensive,some recommened to use Pro-Fi-Bus PA for analouge signals and Pro-Fi-Bus DP for digital signals.

Also another point which I think it should be considered is that the technology is in progress and id we use DCS, we might face some problems for maitaining the system in future,becuase every day a new type amd model is coming.
I would greatly appreciate if you can clear the above issues for me.

Regards,
Zare

The original post from Carmel was onto something.

Use Foundation fieldbus for "analog" regulatory control loops such as transmitters and valves used for PID control, etc.

Use ASI bus for discrete switches and on/off valves. Note the distance limitation of 100 m, though. DeviceNet can go further.

Use PROFIBUS-DP for variable speed drives, motor starters, and MCC, etc. You can lower the speed to meet your distance requirement.

Use (wired) HART to monitor the devices used by your safety system

Use WirelessHART ("WiHART") for measurements where it is too costly to run wires and to take the place of mechanical pressure and temperature gauges, level sight glasses, and variable area flow meters for points where clipboard rounds is too slow, hazardous, or inconvenient.

You may ask why not PROFIBUS-DP for the switches and on/off valves (digital signals) like you mentioned below. The reason is that switches and on/off valves do not use PROFIBUS-DP. To use PROFIBUS-DP for discrete devices you need to individually hardwire the digital signals to remote-I/O boxes (with DI and DO modules). PROFIBUS-DP then runs from there to control system. Switches and on/off valves are simple devices and they use ASI or DeviceNet. ASI and DeviceNet connect direct to these simple devices.

You may ask why not PROFIBUS-PA for the transmitters and control valves (analog signals) like you mentioned below. The reason is that PROFIBUS-PA updates/scan is not precisely periodic. Foundation fieldbus on the other hand is scheduled and therefore precisely periodic and this has an advantage for the control performance for time-based "dt" control functions like PID, lead-lag, and totalization (integration), etc. This is why FF-H1 is the preferred protocol for digital closed loop control.

PROFIBUS-DPv1 is clearly the protocol of choice for motor controls though.

DeltaV is a control system that can tie these buses together. AMS Device Manager uses EDDL (http://www.eddl.org) to integrate information found in HART/FF/PROFIBUS/WiHART devices into a single management tool.

Cheers,
Jonas

Thank you for your reply and high valued information.

Referring to explanation, could you clarify following points?

1 - Can we consider ASI as a bus based system? Does it have any distance limitation?
2 - Could we connect it to other bus based systems such as Fieldbus?
3 - Could you give me more details regarding DeviceNet? Which devices can be connected to this network? Which protocols are used in this system for transmission?
4 - Could you give me some examples that indicate the application of the DeviceNet and other bus based systems?

Once again I thank you for your concern.

Regards,
Vahid Zare

1. ASI is a bus protocol (not a control system) http://www.as-interface.com/ ASI connects to control systems through interface card, e.g.: http://www.easydeltav.com/pd/PDS_ASI_InterfaceCards.pdf It is limited to 100 m. It can go 300 m using repeaters.

2. ASI is a kind of "fieldbus", but there is no gateway between Foundation fieldbus ("The Fieldbus") and ASI. However, systems may support both ASI and Foundation fieldbus, e.g.: http://www.easydeltav.com/pd/PDS_Ff_IO.pdf

3. DeviceNet is a bus protocol (not a control system) http://www.odva.org/ DeviceNet is used for switches, solenoids, and even drives. Systems may support both DeviceNet, ASI, and Foundation fieldbus, e.g.: http://www.easydeltav.com/pd/PDS_DeviceNet.pdf

The application (and reasoning) of each bus protocol was explained in my message below. In summary:

Use Foundation fieldbus for "analog" regulatory control loops such as transmitters and valves used for PID control, etc.

Use ASI or DeviceNet bus for discrete switches and on/off valves

Use PROFIBUS-DP for variable speed drives, motor starters, and MCC, etc.

Use (wired) HART to monitor the devices used by your safety system

Use WirelessHART ("WiHART") for measurements where it is too costly to run wires and to take the place of mechanical pressure and temperature gauges, level sight glasses, and variable area flow meters for points where clipboard rounds is too slow, hazardous, or inconvenient.

There is no ideal discrete bus for hazardous areas. DeviceNet and ASI don't run long enough and are not intrinsically safe. FF-H1 is intrinsically safe and long enough, but not fast enough to have a large number of points to make it economical for simple discrete I/O.

EDDL is the technology used to integrate intelligence in HART, WiHART, FF, and PROFIBUS devices into the same single device management tool.

Cheers, Jonas

Dear Hamid,

We had implemented similar type of architecture in year 2003 with DELTA V, DCS system. This was
Hazardous area installation.

All analogue Inputs on FF BUS: 170 nodes
ALL DI/DO [MTL 8000/1 EX] PROFIBUS DP: 1700
Field installations [zone 1 area]
All Loadcells on Modbus RTU: 21 nOS

We had not opted for PA bus for discrete in 2003, due to PA/DP conversion was required & as a matter
of policy we avoid Gateway/protocol conversion for process control application. We are not really aware of today's scenario on PA --> card in
DCS system.

All control loops we had conventional 4-20 madc/Hart: 64 Nos ----> 250 ms scan

This was fairly successful commissioning, about
12 days, but you need to remember the following:

1] Initial cost of ownwership is higher than
conventional system

2] Initial Engineering cost is also higher than
conventional system.

Yes, this architecture does save the cost on manhours for installation/commissioning of plant.
about 80% in 2003.

We were handling very critical/toxic & Hazardous
area chemicals, where we had installed 2- 3 xtrs
for very few loop since shut-down was never permitted & diagnostics of xtrs were helping us to
isolate xtrs that were about to go in faulty mode.

We appreciate your brief conclusion after going through all experts opinion.

Regards,

Jari