Each BMS includes metering devices that collect information about energy, water, and gas usage. Those recordings are needed for more than just an invoice. As Peter Drucker said, “If you don’t measure it, you can’t manage it.” For metering, we should add one more statement before that sentence: “If you can’t collect it, you can’t measure it.”
We need all of the information from the building to measure the usage and then think about optimizations.
You can choose a meter with different communication protocols. Usually, it is M-Bus or Modbus. Of course, this decision depends on the market. But which one should you choose when there are two options available?
What are the differences?
Susceptibility to interference — M-Bus has a lower susceptibility to interference because the transmission is carried out by a current signal, not a voltage differential (as in RS485).
Number of connected devices — Both protocols allow users to connect many devices. In Modbus, it is usually up to 128 slaves (when one device equals ¼ unit load). In M-Bus, it can be even up to 250, but the engineer has to remember about the M-Bus Master power supply.
Length of the bus — M-Bus, depending on the speed, allows reaching the bus with a length of up to 10km (at baud rate 300), while in Modbus, it is up to 1.2km (at baud rate 9600 - support for lower speeds in Modbus is hardly seen anymore). Of course, one thing needs to be remembered, in both cases, is a longer bus may affect the number of devices (instead of 250 in M-Bus you can handle e.g. 220 devices on a long bus and in Modbus instead of 32 devices, e.g. 25 devices on the longest bus).
Power supply — Some manufacturers allow the meter to be powered from the M-Bus bus, which saves on the power cable. The second option is to buy M-bus meter with the battery, which allows it to work between two and five years, depending on the frequency of transmission. In Modbus, there are no possibilities like these.
Predefined facets — M-Bus has predefined types of devices (water meter, etc.) which allows communication drivers to automatically determine the measurement units without the need to set facets manually. In Modbus, an engineer manually sets up facets.
This is a short list with only a couple of examples of the differences between M-Bus and Modbus when it comes to metering. Instead of thinking M-Bus or Modbus meters, the best option is to find a device that allows you to use both of them. A popular option is a meter gateway, a device that can work with both protocols and send gathered information from meters to a controller.
One of those solutions is iSMA-B-MG-IP, a meter gateway that can cooperate with M-Bus and Modbus meters. The M-Bus interface allows to communicate and supply power to 60 M-Bus meters (up to 130 mA). The RS485 interface allows to connect up to 128 Modbus devices. Which makes it a perfect fit in many projects.
Editor’s Note: This article was written in cooperation with the GC5 Support Team and first appeared in the August issue of www.automatedbuildings.com. See the article in its entirety at http://www.automatedbuildings.com/news/aug20/articles/gc5/200723125404gc5.html.
