<img height="1" width="1" style="display:none" src="https://www.facebook.com/tr?id=2854636358152850&amp;ev=PageView&amp;noscript=1">
4 min read

What is a communications bus?

By Phil Zito on Jun 11, 2017 8:47:48 AM

Have you ever wondered what a communication bus is? When I was starting out, I didn't know what a communication bus was. You've probably heard the term though but maybe you heard it called something else... Comm bus, Comm Trunk, Trunk. MS/TP...

The thing about our industry is that we are never short for names and acronyms. The thing is, understanding the communication bus (I'm going to call it trunk from this point forward to save on the typing) is vitally important.

Want to integrate a BAS? You need to know what kind of trunk it uses...

Controllers dropping offline? It's most likely an issue with the trunk...

Creating a guide spec or design spec for your projects/buildings? You want to specify how the trunk is wired and what kind of wire is used.

I mean who knew that there could be so much to cover for just 3 to 4 little wires? But that's the thing, without a trunk your controllers can't talk and if they can't talk you can't know what is going on!


What is a trunk?

Quite simply a trunk is a series of wires connecting field controllers to one another and to the supervisory device. There are multiple kinds of trunks but the most common one we in the BAS world deal with is called the daisy-chained serial trunk.

Usually, this trunk utilizes the RS-485 standard. The RS-485 standard defines how the controllers and supervisory devices communicate across the wire. Now first things first, RS-485 is a layer 1 standard. It simply defines how the messages are electrically transmitted across the wire.

Now the trunk itself can support multiple different protocols.

At this point, if you are completely new to BAS you may be asking yourself what is a protocol? Simply is a set of rules for how messages are formed and transmitted across the wire.

You see all the signals going across a trunk, they are just ones and zeros... Protocols make sense of those 1's and 0's and turn them into messages.

How does a trunk work?

Ok, but how then does a trunk work.

In this article, we are going to focus on daisy-chained trunks. These are the main kind of trunks you will encounter in the field.

Essentially you have a one-way run from the supervisory device to the first field controller. then the trunk continues in a "daisy chained" fashion from one controller to the next.

Once you've reached the end of the "run", this is what a single trunk is called, you need to tell the electrical signals running through the wire that they have reached the end of the trunk. You do this through the use of an end-of-line (EoL) terminator. Usually, the EoL is a resistor that indicates to the trunk that the end of the trunk has been reached.

One of the most common mistakes new BAS professionals make is improperly terminating the EoL to early or altogether forgetting to install the EoL.

Why is a trunk used?

So why is a trunk used?

Why don't we just use wireless or ethernet?

These are valid questions especially since everything seems to be going IP these days...

Back when trunks started to become the norm in the world of BAS it wasn't economically feasible to use ethernet and wireless standards for communication. Heck wireless wasn't even commercially available when trunks first started being used.

We needed a way to reliably send data to BAS controllers across relatively long distances. We met this by using trunks. Trunks provided, and in most cases still provide, a low-cost, reliable method for communicating between controllers.

There is another, not so obvious reason why trunks are still so prevalent even though so many other industries have turned to IP and wireless systems...

What about Ethernet and wireless? Are those trunks?

So in the last section, I left you with a teaser...

And you probably figured out that the "other reason" has something to do with Ethernet and wireless networks.

You see, wired comm trunks have very little support or setup requirements. If I want to run a daisy-chained MS/TP bus then I just run my wires and make sure everything is addressed and terminated properly.


If I want to use Ethernet or wireless networks, then the whole world changes!

Now all of the sudden I need to work with IT to ensure that I have IP addresses, I'm on the right subnet, I'm using the right wireless channel... and that's just the design portion!

Add to that the actual configuration requirements, which often involves logging into a switch via the Command Line Environment (CLI) and I've got myself quite a pickle?

How many of you reading this could confidently log into a Cisco or Juniper switch and configure the CLI? See... That's why I know this IT knowledge gap is going to smack the BAS industry hard!

Ok... with that segue aside, let's get back to the actual topic of this section. Are Ethernet and wireless networks trunks?

In my opinion yes and no.

Let's deal with the yes first. Ethernet and wireless networks do transfer data from a host device (controller) to other devices.

Score one for networks!

In addition to this, Ethernet and wireless networks have the potential for passing data from multiple field devices using technologies like MRP rings and Mesh wireless networking. Those technologies, however, are the exception to the norm!

Now, onto the no.

Ethernet and wireless networks, for the most part, are "managed networks" you may have heard this term before.

A Managed network is a network that is managed via a programmed device. In most cases, these devices are high-end switches and wireless LAN controllers. With management comes complexity. As I said earlier you and I could easily read a control manufacturers wiring diagram and hook up an MS/TP trunk.

It's a little more difficult for someone who isn't trained on managed network systems to just "hook them up".

So for that reason, I consider Ethernet and Wireless systems to be these weird hybrid systems that aren't truly trunks but aren't truly something else.

And there is the challenge that the BAS industry will face in 2017 and beyond. How to adapt to a changing infrastructure that requires a blend of BAS and IT.


I'd love to interact with you on your thoughts. Comment below and let me know what your experience with comm trunks is and what you think the future of BAS will be!


Phil Zito

Written by Phil Zito

Want to be a guest on the Podcast?