Our technical support in the field – Part 2

In the first service job, I installed the MUC500 and connected 400 meters. When comparing the data with previously maintained lists, I have noticed that, firstly, meters were still missing and, of the 400 connected meters, 150 could not be read out.

This led to my second service job, in which I was instructed to find the error for the non-readability and, at best, to connect more meters.

Service job 2:

A broken wiring is one of the most frequent reasons for errors regarding the automated read-out via M-Bus networks. Debugging the wiring is then needed. A good starting point for analyzing the network are wiring diagrams with details on cable lengths and position of junction boxes. In general, it is recommended to use a cable of type J-Y-S-T-Y 2 x 2 x 0.8 mm.

Already on the way I thought to check and run the repeaters first, because it is very untypical that all of a sudden 150 meters are not readable anymore. As assembly points where parts of the network converge, you can analyse both sides very quickly at these. Voltage and current flow must be detectable both to the meters and to the bus master.

Elementary faults can easily be detected and located by using a multimeter.

So I took a multimeter and checked the input and output voltage of the repeaters.

Here is a small sketch of the voltage measurement: 

voltage measurement M-Bus

The input voltage should be around 40 VDC and the output voltage should be 36 VDC according to the data sheet of the repeaters used.

Following measurements are showing the normal case: 

Normal measurements

I found what I was looking for. An old repeater from another manufacturer that had been installed years ago only had an output voltage of 6 volts. Even when disconnecting the bus load, the voltage remained at 6 volts.

So it was defective. I installed one of our repeaters; installed the power supply, looked for a 230 VAC output and wired everything.

Then I read out the entire bus from the central master. And lo and behold, the meters were readable again.

That is a wrap.

Troubleshooting in large networks can take time. After all, everything has to be run down and the accessibility of the installation is not always given, since you first need the key from the building services department.
Also, the readout in large networks takes quite a long time and you need a lot of time to find out after the repair if the error is fixed now.

And so, there was a third service job to integrate the remaining 400 meters into the system.


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