I work in a water plant that is looking to upgrade from a Allen Bradley PLC-5 with analog signals to Control Logix with profibuss communication. I've read many discussions about the benifits of profibuss, but to me in an existing installation your not saving money on cable and really all the functionallity (for example a valve high torque to close message) will never get used by operations. So the question is when does a profibuss installation become an advantage. Is it IO count, cable distance? Currently we have 400 DI's, 250 DO's, 115 AI's and 50 AO's. Our longest cable is about 500 ft. So its not a big operation, and I'd like some advice from someone who is not trying to sell me something. Thanks.
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Analog vs Profibuss
- Thread starter jpduffy
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Skogsgurra
Electrical
What is the reason for the upgrade?
1. Is existing equipment getting old and unreliable?
2. Does maintenance personnel think that diagnosis will be simpler and faster?
3. Are you running out of spare parts for the existing system?
4. Difficult to find people that can handle your existing system?
5. Need to expand I/O or functionality beyond what existing system can handle?
If none of the above is valid, I wouldn't upgrade.
Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
1. Is existing equipment getting old and unreliable?
2. Does maintenance personnel think that diagnosis will be simpler and faster?
3. Are you running out of spare parts for the existing system?
4. Difficult to find people that can handle your existing system?
5. Need to expand I/O or functionality beyond what existing system can handle?
If none of the above is valid, I wouldn't upgrade.
Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
controlsdude
Electrical
i think your confusing analog in reference to profibus. Its an apples to orange comparison.
Analog is a universal signal that is used to transmit slow changing signals by voltage or current.
Profibus is a communications signal that is used for distributed systems where a common bus archetecture is used to gather/control all different types of signals to a common PLC. The signal can be analog, digtial, or some specialty module like an encoder signal.
Analog is a universal signal that is used to transmit slow changing signals by voltage or current.
Profibus is a communications signal that is used for distributed systems where a common bus archetecture is used to gather/control all different types of signals to a common PLC. The signal can be analog, digtial, or some specialty module like an encoder signal.
- Thread starter
- #4
Yea it'll be digital communication if we go to profibuss. So get out your laptop instead of a simple multimeter to troubleshoot. Maybe I'm just old school but perfer a multimeter. I'm more intrested in knowing if there is an IO count where digital becomes an advantage. Thanks.
Skogsgurra
Electrical
No. There is no such limit. Other things decide what technology to use. I mentioned some of them in previous post.
I very seldom use analogue technology nowadays. Except for front ends in measurement devices. Digital is so low cost and so flexible and reliable that you cannot avoid it. It has been so for decades. Adapt - or retire!
Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
I very seldom use analogue technology nowadays. Except for front ends in measurement devices. Digital is so low cost and so flexible and reliable that you cannot avoid it. It has been so for decades. Adapt - or retire!
Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
There are advantages with using fiedlbus instruments. Many such instruemnts report back to the PLC health and status signals with can be used to alert operators to issues with the instrument that may not be discovered with just 4-20 ma analog signals.
Just in the middle of starting up a waste water treatment plant that is all fieldbus. In the entire plant there is only six analog signals and less than 100 discrete inputs. Everything including motor starters and VFDs are on the 'net.
Another advange to fieldbus instruments is the ability to go "online" with them using some type of 'asset management' software. Not all instrument have this capability.
Just in the middle of starting up a waste water treatment plant that is all fieldbus. In the entire plant there is only six analog signals and less than 100 discrete inputs. Everything including motor starters and VFDs are on the 'net.
Another advange to fieldbus instruments is the ability to go "online" with them using some type of 'asset management' software. Not all instrument have this capability.
The advantages to digital comms are fairly well documented, my company even has an ROI calculator for my workgroup (MCCs) that shows it compared to discrete control. I've done a few already, so far the ROI is always favorable.
But that would be for NEW installations. If you are retrofitting, that's another story. Replacing every analog control component in the field with an equivalent fieldbus version would be very very expensive. I don't see how it would be advantageous except, as Gunnar said, there was some other compelling event. If all you are wanting is to upgrade from the PLC5 before it goes obsolete to the new Logix platform, I would not add to your headaches by simultaneously changing every analog device to a digital one. You will fall into a trap of not knowing which system was not working right.
But... adding a fieldbus comm point more local to the analog devices may have some advantages, not only in troubleshooting but in reliability as well. In other words leave your existing analog devices in the field, have them tie into local I/O modules, then use a comm bus to talk back to the PLC. That's done all the time. If you like troubleshooting with a multimeter, you still do that at the local level. But from that point on the comm link either works or it doesn't and if you set it up correctly, it tells you WHY it isn't working when it doesn't.
But have you considered Ethernet as opposed to Profibus? I would now. Maybe not 10 years ago, but a lot has changed.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
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But that would be for NEW installations. If you are retrofitting, that's another story. Replacing every analog control component in the field with an equivalent fieldbus version would be very very expensive. I don't see how it would be advantageous except, as Gunnar said, there was some other compelling event. If all you are wanting is to upgrade from the PLC5 before it goes obsolete to the new Logix platform, I would not add to your headaches by simultaneously changing every analog device to a digital one. You will fall into a trap of not knowing which system was not working right.
But... adding a fieldbus comm point more local to the analog devices may have some advantages, not only in troubleshooting but in reliability as well. In other words leave your existing analog devices in the field, have them tie into local I/O modules, then use a comm bus to talk back to the PLC. That's done all the time. If you like troubleshooting with a multimeter, you still do that at the local level. But from that point on the comm link either works or it doesn't and if you set it up correctly, it tells you WHY it isn't working when it doesn't.
But have you considered Ethernet as opposed to Profibus? I would now. Maybe not 10 years ago, but a lot has changed.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376
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