Profibus

I have a weird relationship with Profibus. I like a bunch of things about it but I’m not fond of other things.

Profibus is more and more important. It used to be that Profibus was only found in Europe where Siemens was dominant. DeviceNet was in the US, Profibus in Europe.

Well, that’s over now. Siemens has made huge strides in the US. So much so that it’s not hard to find manufacturing sites in the US that have standardized on Profibus IO.  Rockwell even had a module created so that their ControlLogix PLC platform could use Profibus (and Profinet IO) IO though its kind of a “hands off” module as they directed one of their close partners to make it and sell it.

Profibus had two huge advantages over DeviceNet. One is speed and the second is data size.

The speed is what I really like about Profibus. 12Meg. That’s really fast. Faster than most people need but generally automation engineers believe that when possible, it’s always best to go faster. I remember that a guy who had four injection molding machines that dropped a part every 27 seconds told me he was getting 100Gig Ethernet for communications with those machines. With a part dropping every 27 seconds I was thinking that 300 baud serial would do just fine.

Speed has its downside though and that’s cost. It costs more to go that fast. A special ASIC with the communications infrastructure on it (MAC – Media Access) and special transceivers, cabling and connectors. All that makes Profibus devices significantly costlier than DeviceNet devices. DeviceNet can actual be really done on the cheap. The controller chips are pretty cheap and you can use real inexpensive connectors if you are doing a device that operates off of CAN power.

That’s about the only time that DeviceNet has an advantage. Network powered devices that move a few bits from a device to the controller. Simple devices like proximity sensors, photoeyes and such will always be more cost effective on DeviceNet than Profibus. And usually with mechanical devices, the speed Profibus advantage isn’t that important since the mechanicals don’t move all that fast.

But when you get to more complex devices like drives, scales and the like, Profibus is clearly a better solution. These devices are more costly to begin with, have more data to transfer and sometimes can use the higher speed.

The data size advantage over DeviceNet is significant. Profibus has a frame size of 244 bytes. That’s monstrous compared to 8 bytes frames in CAN. Yes – DeviceNet has fragmentation but that eats up so much bandwidth it is hardly useable.

One of the things I dislike about Profibus is the data representation. All Profibus devices look like a rack of I/O to the controller. That means that a device is a series of slots, each slot with a module in it and all sorts of different size modules.

That’s easy for simple devices like I/O devices. You can simply decide that the first slot has a module that is a 16 Discrete Input device. The second is 8 bit of Discrete Output and so on. If you have Analog in/outs too, you could setup the device using four slots and four modules.

But how do you do a Scale? What is the structure for that? What is the structure for a barcode reader. This data representation doesn’t easily fit those devices. It gets kind of awkward.

We have a lot of DeviceNet gateways in our gateway product list. This month we will be adding a bunch of Profibus gateways; Profibus ASCII, Profibus to Modbus TCP, Profibus to Modbus RTU and more. I hope you will take a look at them.