When and Why Do We Need an End of Line Termination?

By Matt

End of Line Termination Concept Most of the buses we use are specified with end of line termination in mind. In the field these terminations are not always present, especially in short runs. Do we need them? Here’s an analogy I like that illustrates why we need end of line terminations: a bus behaves like a rope held between two…

What is a BACnet Instance Number?

By Matt

A BACnet Instance Number Is a Globally-unique Address An often misunderstood and neglected topic is the BACnet instance number of a BACnet device. Neglecting to think about this number at installation might not bite you today, but it remains as technical debt that complicates future work. It’s a number that identifies a device uniquely on the entire interconnected BACnet network.…

RS-422 – The Other Bus-meat

By Matt

I encountered an RS-422 bus for the first time recently and it threw me through a bit of a loop! The device was a Modbus/RTU slave device and so I anticipated a RS-485 connection as per usual, but that was not the case. It’s the same cable, and the same essential signalling standard, but if you try to interface with…

RS-485 Bus Details

By Matt

Low-voltage communication buses are critical to networked automation systems and when they fail they are a pain to troubleshoot. We use them because the cables are cheap and you don’t need expensive switches, but what you save in parts you may pay in labor later. Ethernet/IP based communication is on the rise, but is still expensive as an up-front cost.…

IP-based Protocols and Building Automation Security

By Matt

When installers build automation systems getting it working is their main focus. They see building automation security as an obstacle to overcome rather than the goal. If authentication is getting in the way, they disable it. When firewalls are blocking communication, they disable that too. Even more problematic is the fact that several popular automation protocols do not even support…

Visual Look at RS-485 Without Termination

By Matt

RS-485 bus grounding and termination seems like a magical ritual. Some people get lazy with the details of termination because buses often work even when termination is done incorrectly. I had some untwisted, shielded AWG22 lying around, so I used it to show a quick comparison between bus wiring in a controlled environment. I used a single MS/TP BACnet controller…

What is a BBMD, and Why Do We Need It?

By Matt

BACnet networks are getting big. Infrastructure is getting complicated. BACnet is used to connect campuses, not just buildings. When you connect several networks together, BACnet/IP Broadcast Management Devices (BBMD) are necessary to bridge them. They’re important, but often misunderstood. Use them wrong, and you’ll get a broadcast storm. What is a BBMD? Broadcasts are efficient. Broadcasts are public. They are…

Modbus Is Hard, Then Tedious

By Matt

Modbus is one of the simpliest protocols you’ll find in the field. It’s the closest protocol “to the metal” of the chip on a controller since it often just directly exposes the controller’s memory. There’s no point discovery here; there are no point names or descriptions to help you integrate. All you have are datasheets and fat-finger data entry. There…

A Case Study on the Importance of RS-485 Grounding

By Matt

RS-485 grounding is frequently missing from commercial automation installations. It’s especially egregious when you are using your shield as your signal reference, because that reference is now noisy as heck. The shield’s job to collect interference. It’s up to you to make sure that interference goes somewhere. The Case Study I was looking at the RS-485 (MS/TP BACnet) buses for…

The Bus Problem Superweapon: The Oscilliscope

By Matt

I had the opportunity to troubleshoot an MS/TP BACnet bus using my Rigol DS1052E oscilloscope. Oscilloscopes are much more useful for bus diagnostics than a multimeter. Although they don’t tell you where the problem physically is, they can immediately confirm that you are in fact looking at a bus fault instead of a software problem and also identify when it…