(googledocs)[INTERNAL] How is Modbus data captured?
A bit of techie detail about how Modbus data is captured, and how this is different to Long Energy. Also, an edge case where collection may fail, and how to identify it.
When our devices collect data for reporting as Long Energy, we aggregate lots of individual 5 second samples into a 5 minute INTERVAL entry. What this means is that each entry of Long Energy only captures the changes between the last 5 minute interval, and presents these as consumption or average values depending on the measurement quantity.
Modbus data is collected every 5 minutes, but represents a snapshot of the instantaneous values at the time the snapshot is taken.
For the 6M+One, where we pair the 6M+MB device with a CET pattern-approved meter (the PMC220, PMC340A or PMC340B devices), this means the cumulative values at that point in time. It might help to think of this as similar to an odometer on a car. It keeps recording the total mileage, however, it doesn't necessarily store information such as how long you travel on the last three days or the past month.
This is outlined in more detail in the Modbus data section of our API documentation.
For other Modbus applications, such as the data we collect for Fusion HVAC from their commercial-scale air-conditioning systems, this data is a snapshot of the current settings and diagnostic values of the device at each 5 minute interval.
But doesn't the PMC store a log of interval values
PMC devices do contain configurable logs to record snapshots of registers at a time interval (eg: every 1 day or 30 days). However, these are not accessed by the Wattwatchers device for this purpose.
So what happens if the 6M+MB loses communications with the PMC meter/Modbus device?
If there is some kind of error or communication issue—for example the cables are unplugged, or there is some other anomaly that causes the Auditor to lose connection—then the Auditor will not store information from the Modbus device.
Even if the Auditor regains connection later, it will only pick up the next instantaneous reading from the Modbus device—the next cumulative value in the case of the PMC meters.
This is sometimes characterised as the Auditor "losing data," which is technically incorrect, but from our customer's perspective that is not a very helpful answer.
What about that edge case?
Early versions of the 6M+One used an option board (an extension within the Auditor) that was implemented in a non-isolated way. Once in the field, it was identified that this version of the Modbus board had some hardware issues that required rectification. When those devices exhibited these faults, they have been replaced under warranty with an updated version of the Modbus board that corrected these faults. These boards are known as the 'non-isolated Modbus boards'.
These can be identified by reviewing the _option value in the DMS for a device. When a non-isolated Modbus board is present, this value will start with MB+2SW before the forward slash (/).
The isolated Modbus option board that replaced it can also be identified by the _option value in the DMS, as these devices have an _option value starting with the value starting with IMB.
What can we do about it?
Since identifying this fault, we have implemented further checks and balances in the Mercury system to alert the engineering team when data goes missing.
However, if a customer reports missing data from a Modbus device, we encourage you to check the DMS to see if it is a non-isolated device (the MB+2SW option). If this is the case, you are authorised to replace the unit under warranty, provided the warranty is still in place.
If this occurs for a customer and they express concern about the period of missing data, we recommend the response:
Unfortunately the data for this period was not captured and we are unable to retrieve the historical data to backfill for this period.
Our suggestion is that you use available data (i.e. prior or after the missing data) to do a manual, like-day substitution process to estimate the data for the missing days.
In addition, we recommend installation of a CT(s) connected to the Wattwatchers device to monitor the same circuit(s) as the PMC meter, so that you have an additional data source to inform your estimates.