LonWorks to BACnet - A Difficult Upgrade
Alerton Technologies, Inc.
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:
Then why is LonTalk in the BACnet specification?
A good question. There are several reasons. For one, LonWorks
devices may be developed which will communicate on LonTalk LANs
with BACnet messages - but to convert an existing LonWorks installation
to BACnet would require replacing every single unit in the system
with the new devices. This would be a rather expensive conversion.
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:
What about a LonWorks to BACnet gateway?
It is possible that a LonWorks to BACnet gateway (translator)
might be developed to make an entire LonWorks system look like
a BACnet system (i.e., talk "BACnet" to BACnet devices).
These types of gateways exist today, though as a rule they are
rather limited and setup and programming tends to be an expensive
So if I specify LonWorks now with an eye to adding BACnet
You will have two options
It would be far easier and cost less to just specify BACnet right
from the start.
- 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.