×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

RS-485 vs Fieldbuses
2

RS-485 vs Fieldbuses

RS-485 vs Fieldbuses

(OP)
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

RE: RS-485 vs Fieldbuses

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.

RE: RS-485 vs Fieldbuses

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources