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I do not believet that there is any way to directly get a Unitronics PLC to talk BACnet. Just as a quick education for those who may not have ever heard of BACnet. BACnet is a building communication protocol that was developed as a "desired" standard for the United States market by a group comittee process from ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). The standard has not been wholey adopted and has been in fairly heated competition with the other communication protcols (LON works, Johnston N2-Metasys, proprietary Siemens protocols, and modbus). Whether or not BACnet will ever achive the desired conformity is unknown but unlikely. LON has been much more successful to date at getting their standard adopted with a much wider variety of gateway products available from multiple manufacturers.

At this time the easiest solution is to buy a gateway product such as a Fieldserver product such as the "Protocessor" and either go through the painful process of mapping everything yourself, or paying Fieldserver to do the maping for you. In general if you have Fieldserver do the work for you it will run in the $5000 range for a single project for the hardware and sofware. There are other solutions available but it is not an easy slam dunk solution that I would be able to say definately do it this way or that way.



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  • MVP 2014

Hi Nacho,

I have been requested for BACNet comms a couple of times also. My initial impression was that the physical layer of the communications would be the main problem. However if BACNet can operate on RS485 then that obstacle is removed. The next question would be to make sure the target devices are using RS485 compared to the other types of physical layer available. Once the question of Physical layer is resolved, the next issue is implemention of the protocol.

As per Keith's post above, I don't see an easy off-the-shelf way of doing this directly out of Unitronics. If you can obtain a BACNet slave to test with, maybe you can get a limited number of messages working.

Unitronics protocol is generally quite flexible. A couple of areas have caused me concern:

* use of the NULL (0x00) character as a string terminator. If you are trying to implement a binary ASCII protocol that can contain NULL characters inside the string, the Unitroncis PLC will stop transmitting (or receiveing) once it sees the NULL. This can result in only part of the string being transmitted or received.

* DLE-doubling and other escape sequences. The 3964-R serial protocol uses the character 0x10 as an escape character. If it appears in the middle of a string it must be repeated to ensure it is not interpereted as an escape sequence. There is no built-in way that I can see to implement this type of feature in the Unitronics Protocol blocks. I think it could be done in ladder, but would not be pretty.

I haven't gone over the BAC protocol to see if either of the above issues arise, but that is a summary of my experience so far.

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