View Full Version : Fieldbus, Modbus, Profibus.....
May 12th, 2003, 05:49 PM
Why can't we just have one protocol??? Who owns rights to these protocols???
May 12th, 2003, 06:21 PM
This market is no different than any other, many design things and others apply them to their devices. Its called free enterprise.
If you goto the grocery store do you find one brand of milk? bread? soup?
Modbus was created by Modicon and I am not sure about ownership but it has evolved into a form of open architecture.
Profibus was a joint venture by many manufacturers (many european) and research facilities to develop a common communication protocol.
Fieldbus can get a little confusing because there are 2 of them. One of them is http://www.fieldbus.org/
May 13th, 2003, 08:37 AM
Modbus RTU is public domain. Modbus Plus is owned by Schneider/Modicon and is still proprietary.
Profibus is not owned by any one manufacturer, although Siemens was instrumental in its formation. There is a Profibus foundation that controls licensing and rights.
Foundation Fieldbus was put out by the Instrument Society of America. The standard formation process got so politicized that it ended up with several protocols, and is not being as widely adapted as a "universal" protocol as was originally intended.
There are lots of other protocols, most of which are proprietary to each PLC manufacturer. We may never see a true universal open protocol because each manufacturer tries to protect their installed base and market share with the communications link. The closest is Modbus RTU, which almost any brand can use in one way or another.
May 13th, 2003, 05:06 PM
As far as I know and I am sure I will be corrected if wrong, but modbus plus is the modbus protocol encased in an ethernet packet. Regards Alan
May 13th, 2003, 05:27 PM
Modbus RTU and ASCII are open but are very slow, although pretty reliable.
Profibus is open but seems to change regularly.
ASI is open but very limited.
Device Net is open and supported very well by manufacturers. The variety of devices available is amazing. Try www.odva.org.
I guess the next step will be Ethernet IP. Hope it works better than Ethernet.
Many manufacturers either have developed or are developing a communications layer to run on TCP/IP below Ethernet. Hopefully this will be more reliable but the implementation will be different with each manufacturer and will no doubt not be compatible with any other brand.
Many believed that Ethernet would be the common answer but this has not happened and it is almost impossible to set up communications between different brands of PLC this way, although it has been done.
May 14th, 2003, 08:55 AM
Modbus is an open protocol that determines how a device knows it's address, recognizes a message addressed to it, determines the kind of action to be taken, and the extraction of any data or other information contained in the message.
Modbus messages may be transported over many different networks and mediums.
Modbus serial networks may be RTU mode (8-bit bytes that contain two 4-bit hexadecimal characters, CRC error checking) or ASCII mode (8-bit byte of two ASCII characters, LRC error checking). Up to 255 devices may be addressed.
Modbus Plus is a proprietal high speed local area network. A standard Modbus message packet is contained within the Modbus Plus message, this allows the easy application of bridge devices to go between Modbus and Modbus Plus networks. Up to 64 devices may be addressed.
Modbus TCP is an open implementation of Modbus messages on standard TCP/IP networks. Again, bridge devices may be utilized to provide standard Modbus devices with Ethernet connectivity.