Communication preference

[Lunatech]

Hello All,

The new forum rules have me a little lost, so hopefully this is the right spot to post. A few months ago I got some excellent help related to this question, and I'm back for more.

This is about integrating devices with building monitoring systems (such as BACnet).

We're an OEM of duct-mounted air treatment devices. We've been getting requests from a few customers to add functional status feedback so our unit can be integrated to building monitoring systems. This is still new territory for us, so we need some hand-holding. Bear in mind our units will only communicate two states -- they're either functioning or they're not. Also our units are "dumb" -- they'll just have a couple of terminals with a rudimentary output to which installers can connect their preferred communication hardware.

So we have a choice of either a DC voltage output or relay contacts. For voltage, the output would be near-0V for non-functional, and whatever upper voltage for functional. That upper voltage could most easily be 3V, 5V, or possibly 9V. I'm not sure how well that jibes with standard 0V -- 10V communication protocol...? The relay contact output seems to me to be more "universal" in how it could be utilized, but that's just my assumption. Contacts closed for functional, open for non-functional.

Any advice on which is the best way to go? In theory would could offer both, but we'd probably like to pick one or the other.

[Cagey57]

Lunatech,
your choice of terminology is a bit misleading.
My suggestion would be to chose the "relay contact output" aka. Dry Contact. From a Controls perspective a Form-C contact would be optimal.

The only issue I see is that you will have to define/account for both a "Failure" state and a Loss of power condition.

In my mind what you should provide is Contacts Open = Device is "Off", Contacts closed = Device is "On", how you handle the internal change from "Off" to "on" is another matter.

[Lunatech]

Hi Cagey57,

Thanks very much for the reply. Sorry if my terminology is a bit off. But what you describe seems to be what I have in mind. Contacts closed = Device is "On" / Open contacts = Device is "Off".

The way the unit's electronics work, the contacts will only open when there has been a specific component failure OR as you point out if there is a loss of power condition. And by loss of power I mean a catastrophic electrical failure inside the unit or, say, a rat chewing through the external mains power wires. Either way, the relay coil will lose power and the contacts will open. No matter if the failure is internal to the unit or due to external power interruption, the building's maintenance crew would be alerted that they need to go check the unit to see what's going on.

I'm not as comfortable with 0V - xV plan, since I don't know much about the 0V-10V communication / control scheme as it's used in HVAC applications. That is, I don't know how difficult it is for the building maintenance crew to set up their BACnet (or whatever) hardware so that -- for example -- 3V = "On" and 0V = "Off"