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LonWorks to BACnet - A Difficult Upgrade

What is the problem?
Specifying engineers have been specifying LonWorks in the belief that they will be able to expand their system later by simply adding on BACnet devices. They will find, too late, that it is not that easy.

Why do they think LonWorks and BACnet are compatible?
ASHRAE's BACnet standard 135-1995 specifies the LonTalk LAN as an available transport medium for BACnet messages. At first glance this implies that LonWorks and BACnet are compatible.

However, the BACnet standard specifies LonTalk only as a transport medium for BACnet messages - it does not describe BACnet communications with LonTalk devices. This situation occurs with Ethernet also - though the BACnet standard specifies Ethernet as a transport medium for BACnet messages, BACnet messages are not understood by, say, Novell LAN devices. The standard simply defines how BACnet messages can be carried on the LAN without interfering with other kinds of messages on the LAN.

Why aren't LonWorks and BACnet completely compatible?
The key word here is "interoperability", which the ASHRAE standard defines as "the ability to integrate equipment from different vendors into a coherent automation and control system." BACnet and LonWorks devices are not interoperable and do not form a coherent system.

For example, LonTalk devices communicate through a mechanism called Standard Network Variable Types (SNVT), which are device variables and parameters readable and writable by other devices that speak LonTalk. The structure of the variables are quite unlike BACnet Objects and the means and messages by which they are read and written are quite unlike BACnet Services.

The telling point is that BACnet messages are conveyed across a LonTalk LAN in what LonWorks calls "Foreign Frames," which are "typically used by application gateways to other networks." "Foreign Frames" means LonWorks devices don't look at the messages themselves, they just pass them on.

What all this means is that the BACnet standard specifies a way for BACnet messages to be conveyed across a LonTalk LAN, but if a LonTalk device saw a BACnet message it will not be able to interpret the "foreign" message. In effect it is like placing an international telephone call: even though you speak only English, your call can pass through the German telephone network; if you actually reached a German-speaking person by mistake, the two of you would not understand each other. In the same way, the language of LonWorks devices and BACnet devices are mutually foreign; the inclusion of LonTalk in the BACnet standard simply allows BACnet messages to be conveyed between BACnet-savvy devices across a LonTalk LAN.

This is illustrated in the following diagram:

In some cases an existing LonTalk (or Ethernet or ARCNET) LAN may serve to bridge new BACnet LANs that are physically far apart, as in the following diagram:

So if I specify LonWorks now with an eye to adding BACnet later...?
You will have two options

• Add BACnet devices talking on the same LANs with separate BACnet and LonWorks controllers, or,
• Replace every LonWorks device with a BACnet device.

LonWorks and LonTalk are registered trademarks of Echelon Corporation.
ARCNET is a registered trademark of Datapoint Corporation.