lördag 26 april 2008

How does it work?

Consider a simple I/O device with some input and output signals.
This simple I/O device probably has two static process images; the input image and the output image.

Let's say we want a PLC to connect to that device. In that case we need to have mirror images in the PLC that represent the images of the device. These images needs to be defined and setup with some kind of tool developed by the PLC vendor. The protocol used to communicate between that tool and the PLC is not withing the scope of PieP.
This configuration could be done using a Web Server on the PLC.

The PieP protocol is used for transfering the process images between the I/O Device and the PLC; so that:

  • the mirror of the input image in the PLC is updated from the generated input image from I/O Device

  • and so that the output image content in the I/O device is the same as the mirror image in the PLC.

So, to do that we just open up a telnet client and connect to the PieP port of the I/O device.

On the I/O device we use PieP commands to:

  1. tell the device to send its input signals to the IP address of the PLC,

  2. and we tell it to get ready to receive an image from the PLC IP address and to put that image onto it's output signals.

Then we connect our telnet client to the PLC and use the same PieP commands to:

  1. tell the PLC to send it's output mirror to the IP address of the I/O device.

  2. inform the PLC that it shall get ready to receive an image from the I/O device IP address and to put that image in the input mirror.

  3. ask the PLC to start distributing it's images as configured.

Then connect the telnet client back on to the I/O device and:

  1. ask the I/O device to start distributing it's images as configured.

Now we are up and running!!

Can it be more simple?

(An Internet Draft of the protocol can be found on the PieP Project Site @ Source Forge)

Why Yet Another Fieldbus?

Thats a good question.
Another question is, why do all existsing Filedbus implementation turn to Ethernet?
Could it be because the populatiry of the internet protocols?
Could it be because it seems to be so easy to get devices to cooperate in an office enviroment? Just hook up your printer o scanner o camera to the network and it pops up as an icon in your PC.

Then here's another question: Why didn't the implementors of the existing fieldbus protocols try to extend the existing internet protocols, and reuse what already exists?

That's what PieP do. Piep uses TCP/IP to create a fieldbus protocol in the most simple way, just like all other internet protocols like FTP, SMTP,...