At present we have AB SLC's controlling and monitoring processes in a wastewater plant. New motorized valve actuators are to be installed replacing older equipment. Some of the connection wiring is faulty, as is the underground conduit. It will be much simpler and less expensive to replace all of the control wiring with a data cable, right? My question is which protocol will be best for this application.
Control cards are available for both protocols from the actuator manufacture. It seems to me that Allen Bradley is pushing the DeviceNet connection, while I have been told that the Profibus will work and be less costly. Between the two, does one take up anymore overhead than the other in the SLC processor? Any other pitfalls I need to know about?

Thanks

Terry Murphy

 

If you do use Devicenet points to consider:
-amount of nodes (64 limit scanner counts as one
-people working on the system (replacing equipment in the devicenet system can require someone to configure the device before it will work. Allen Bradley has Automatic Device Recognition which will do the configuring work for you making the system 'plug and play' but this feature is not supported on all devices for this network.
-there is length limitations to the network
-Devicenet system is nice if it is well thought out otherwise it can be a real headache.

 

Profibus DP is a very fast master slave network.
DeviceNet is more intelligent but slower.
An SST Profibus DP master card can update 6000 I/O points a millisecond. However a Profibus DP system usually has a PLC that scans the I/O and then must echo data to a HMI. This is wasteful and not very intelligent. DeviceNet is much slower since it is only transmitting at 500 KB/S but it is more efficient. Device can be setup so the I/O devices transmit only on a status change or at periodic intervals. When the I/O device transmits the data both the PLC and the HMI can receive the data. This is a producer/consumer network in stead of the Profibus DPs master/slave network. I would go with Profibus DP for systems that have a lot of I/O that change quickly. For sytems where the I/O changes infrequently DeviceNet may get better response as a device does not have to wait to be polled to transmit a status change.

 

What is the max length limit?

 

I have a customer (tobacco processing) that uses both technologies.

He always tells me that installing devicenet is much more easy than profibus.

And believe me, if you are planning to mix vendors (siemens and ab) may be a good idea if you have time and budget.

Good luck

Juan De los Santos

 

Another consideration in the choice for between DeviceNet and Profibus for this application might be maximum cable length. The maximum cable length
for both field buses varies with their transmission rates. However, at 500Kbaud I believe DeviceNet is limited to 100m where Profibus is 400m. This could be a major issue in a "wastewater plant."

Jeff Pinegar

 

Terry,

Profibus will likely be cheaper for your initial installation to valve positioning devices because:

1. The cable is cheaper, mostly because RS-485 wiring is more widely used than CAN.
2. SST's Profibus DP scanner module is probably comparable to A-B's 1747-SDN in cost.
3. SST's Profibus configuration software is probably comparable or cheaper than RSNetworx for DeviceNet.

The wiring topology will not be too different; Profibus is principally a daisy-chain, and DeviceNet is a trunk/drop but the drops are limited to about 20 feet. Profibus will give you a longer available network length if you run at low data rates, because you don't have to worry about voltage drop like you do with CAN.

The SST module and the 1747-SDN both take up about the same amount of I/O memory in your SLC processor; 32 words of discrete I/O, and then a larger I/O file (128 words for Profibus, 150 words for DNet) in the "M-file" portion of module memory. All the scanning and diagnostics are done onboard the scanner; the only thing that affects your SLC program scan is when you access M-files, so do it sparingly if possible.

You will have messaging capability from ladder logic and passthru capability for slave device monitoring (slow but useful) if you choose
DeviceNet; A-B necessarily has better connectivity to networks they work with closely. The SST module may provide some messaging capability, but I know it doesn't allow Passthrough.

If you have a strong A-B distributor locally who knows DeviceNet and can help you learn it's details, you're clearly money ahead on aspirin. It's hard to find expertise on open networks when each player is just getting a small bit of your business.

If you're also doing motor controls with these SLC's and will be getting DeviceNet products from A-B (or Cutler-Hammer, ABB, or Control Techniques), you'd do best to stick with one kind of device network for your whole facility.

Weigh my advice with a grain of salt, too; I'm a DeviceNet expert but don't use much Profibus.

Good luck,

Ken Roach
Rockwell Automation / Seattle
[email protected]

 

There are many differences between DeviceNet and Profibus. And you cannot get an unbiased comparison from the major control suppliers. Allen Bradley promoted DeviceNet. Siemens promotes Profibus.
InterlinkBT (www.interlinkbt.com) 888 546 5880 provides both Profibus and DeviceNet we have no preference and can suggest the best network for your application in an unbiased way. Give us a call and ask for myself or an application engineer.
Thanks
Bob

> Another consideration in the choice for between DeviceNet and Profibus for this application might be maximum cable length. The maximum cable length
for both field buses varies with their transmission rates. However, at 500Kbaud I believe DeviceNet is limited to 100m where Profibus is 400m. This could be a major issue in a "wastewater plant."

Jeff Pinegar