In automation there are several communication protocols. Some are entirely proprietary and others are open. Despite the friendliness of the word, “open” isn’t always good news. Some of the older proprietary protocols are easy to integrate with off the shelf integration products. Two versions of the same protocol often look different. It’s a zoo out there, so here’s an overview of commercial automation protocols to help you sort it out.
Modbus is about as bare bones as a protocol can be. It’s simplicity is a strength and a weakness. Since it requires so little it’s easy for vendors to support Modbus, and as a result it’s quite popular. Modbus does little work for you, which makes it difficult to integrate. To integrate a Modbus device you’ll need good documentation and a good understanding of how computers store numbers. If your tools allow you to copy integrations you’ll only have to do it once, as the addresses don’t vary between identical devices. Modbus is more of a simple data transfer protocol without much in common with other automation protocols.
We wrote an article that goes into more depth on Modbus.
Like Modbus, OPC is common in industrial environments. It can be straightforward to integrate as it doesn’t require you to know low-level details. Discovery is possible, and advanced features such as alarms and historical data access are possible for devices that support them. The tricky part with OPC is connecting devices. The usual form uses a proprietary Microsoft technology that relies on DCOM. DCOM settings can be difficult enough that Matrikon developed an OPC Tunneler that allows you to skip all of that headache for any OPC client-server relationship.
Lonworks is a protocol developed by Echelon. The protocol understands an air handler and what kind of points they should have. Many vendors implement their Lonworks interfaces in a generic manner though and don’t take advantage of this standardization. Other times they will use the standardization, but some of the points will be duds or their implementation will be more confusing. A Lonworks installer needs to understand the protocol deeply since it’s an advanced protocol. If you work in a company with Lon products, you’ve probably had someone bring an entire building down because they synchronized the database the wrong way. It can be a very labour intensive protocol.
BACnet is a popular protocol created by ASHRAE that is “open” but not very standard in practice. It’s replacing other commercial automation protocols. There’s a grab-bag of object types and parameters in the protocol for fire monitoring, lighting, scheduling, and so on. BACnet only requires that vendors are implement a few object types. Some will implement the bare minimum in strange ways. When done right, BACnet is a pleasure to work with. Objects are discoverable and contain lots of data that help the integrator understand what they are looking at. Points can be overridden on different priority levels, allowing for schedules to command a fan directly and an operator to override that without any extra programming logic.