RS-485 vs Fieldbuses 2

[ELEphilippe]

Can PLC communicate only with RS-485? What i mean is does PLC's only communicate by fieldbuses, which are higher level protocols than RS-485, or if they can communicate with only RS-485? If both are possible what are the differences between the two? You can probably send less complicated data with RS-485 than with fieldbuses... And if a Siemens and an OMRON PLC communicate with RS-485 can they understand each other? Also, how is the data acquisition done by a software if it receives an RS-485 communication.
Thanx
Philippe St-Pierre

 

[ICman]

I'm no expert, but I believe RS-485 is a serial hardware/communication standard. "Fieldbuses" usually refer to software protocols that define the signals.

PLCs can communicate via RS-232, RS-485, and other communication links, using Ethernet, Controlnet, Devicenet, Foundation Fieldbus, Profibus, Modbus, etc., etc. Just about whatever you want.

The basic communication is ASCII over RS-232. It works, but is slow and very limited. A lot of vendors are headed toward Ethernet, running at 100MB/sec, built in HTML, and easy to interface.

 

[jsalls]

It really depends on the PLC that you are working with. RS-485 is an electrical standard that allowed for greater distances between devices and also allows for multi-drop configurations. This is really a legacy solution from the time before Ethernet or the Fieldbus's became common. As far as the actual communications on the RS-485, it seems like everybody has developed their own standards. They all have their own start and stop characters, different message formats, etc. Usually if they both talk RS-232 or RS-485 there is a way to make the devices talk, but it really depends on the devices and it is almost never easy. Some PLC's don't have the ability to strip out the start and stop characters or to take the relevant data out of a serial message.

Fieldbus's the advantage is that the governing associations have imposed some discipline as far as how the devices communicate and how date is sent. Usually there is a specification as far as the physical medium and there is a specification on the software. When you get to this level, if both devices claim they can use a particular fieldbus, chances are very good you'll be able to get the devices to communicate.

Good Luck