Where is the Unit value deriving from?

[wmmihaa]

We're trying to make sense of the Unit field of the data record, which seem to lack consistency. For instance:

• "Volume (m m^3)"
• "Volume (my m^3)"
• "Volume (1e-1 m^3)"
• "External temperature (1e-2 deg C)"
• "Energy (100 Wh)"
• "C"
• "Time Point (time & date)"

It seems to me that the Unit element is based on three parts:

• Type - E.g "Volume"
• Scale - E.g "1e-2" or "0.001" or "100"
• Unit - E.g "m^3" or "deg C"
  ... but sometimes the scale comes as scientific number, sometimes as a decimal value and sometimes not at all

I understand that reverse-engineering the field might never get fully accurate, but if feels like it's just a free text field. Having a look at the mbus-protocol-aux.c, it looks like pretty structured, but perhaps the Unit value is derived from somewhere else?

In an effort to understand if we would be able to parse the Unit element, I created a function that tries reverse-engineer the text, and then wrote another program that went through all test files in the libmbus repo and extracted every Unit element. Lastly I ran my function on every one of those Units to create the table at the bottom of this issue.

The report shows three columns:

Unit field - The Unit value as part of the XML telegram
Predicted Scale - The identified scale value in decimal
Predicted Unit - The identified unit value

The reverse-engineering function works as follows

1. Ignore Unit fields with no parentheses
2. If the Unit field has parentheses…
   a. …and the parentheses value can be split on double spaces, then assume the first value is the scale and the second is the unit
   b. …or the parentheses value cannot be split on double spaces, then assume the value is the unit

Unit field (unparsed)	Predicted Scale	Predicted Unit
Energy (kWh)	UNKNOWN	kWh
Flow temperature (deg C)	UNKNOWN	deg C
Return temperature (deg C)	UNKNOWN	deg C
Volume flow (m m^3/h)	UNKNOWN	m m^3/h
Power (W)	UNKNOWN	W
Time Point (time & date)	UNKNOWN	time & date
C	UNKNOWN	UNKNOWN
c	UNKNOWN	UNKNOWN
Fabrication number	UNKNOWN	UNKNOWN
Volume (m m^3)	UNKNOWN	m m^3
Time Point (date)	UNKNOWN	date
Operating time (days)	UNKNOWN	days
Firmware version	UNKNOWN	UNKNOWN
Software version	UNKNOWN	UNKNOWN
On time (days)	UNKNOWN	days
Error flags	UNKNOWN	UNKNOWN
Volume (my m^3)	UNKNOWN	my m^3
Volume (1e-1 m^3)	0.1	m^3
Temperature Difference (1e-2 deg C)	0.01	deg C
m^3	UNKNOWN	UNKNOWN
-	UNKNOWN	UNKNOWN
m^3/h	UNKNOWN	UNKNOWN
s	UNKNOWN	UNKNOWN
°C	UNKNOWN	UNKNOWN
W	UNKNOWN	UNKNOWN
Wh	UNKNOWN	UNKNOWN
K	UNKNOWN	UNKNOWN
Energy (Wh)	UNKNOWN	Wh
1e-1 V	UNKNOWN	UNKNOWN
V	UNKNOWN	UNKNOWN
Reset counter	UNKNOWN	UNKNOWN
Manufacturer specific	UNKNOWN	UNKNOWN
A	UNKNOWN	UNKNOWN
cust. ID	UNKNOWN	UNKNOWN
bat. time	UNKNOWN	UNKNOWN
Flow temperature (1e-1 deg C)	UNKNOWN	1e-1 deg C
Return temperature (1e-1 deg C)	UNKNOWN	1e-1 deg C
m A	UNKNOWN	UNKNOWN
Temperature Difference (m deg C)	UNKNOWN	m deg C
Customer location	UNKNOWN	UNKNOWN
Digital input (binary)	UNKNOWN	binary
1e-2 %RH	UNKNOWN	UNKNOWN
External temperature (1e-2 deg C)	0.01	deg C
Special supplier information	UNKNOWN	UNKNOWN
Averaging Duration (hours)	UNKNOWN	hours
Digital output (binary)	UNKNOWN	binary
V	UNKNOWN	UNKNOWN
Energy (10 Wh)	UNKNOWN	10 Wh
1e-1 A	UNKNOWN	UNKNOWN
Power (10 W)	UNKNOWN	10 W
Volume (1e-2 m^3)	0.01	m^3
Units for H.C.A.	UNKNOWN	UNKNOWN
Temperature Difference (1e-1 deg C)	0.1	deg C
Power (100 W)	UNKNOWN	100 W
On time (hours)	UNKNOWN	hours
Volume ( m^3)	UNKNOWN	m^3
Operating time (hours)	UNKNOWN	hours
Volume flow (1e-2 m^3/h)	0.01	m^3/h
Power (1e-1 W)	UNKNOWN	1e-1 W
Energy (10 kWh)	UNKNOWN	10 kWh
Volume flow ( m^3/h)	UNKNOWN	m^3/h
On time (seconds)	UNKNOWN	seconds
Operating time (seconds)	UNKNOWN	seconds
Power (kW)	UNKNOWN	kW
Temperature Difference ( deg C)	UNKNOWN	deg C
1e-2 V	UNKNOWN	UNKNOWN
dimensionless / no VIF	UNKNOWN	UNKNOWN
Energy (mWh)	UNKNOWN	mWh
l	UNKNOWN	UNKNOWN
reserved but historic	UNKNOWN	UNKNOWN
External temperature ( deg C)	UNKNOWN	deg C
Energy (100 Wh)	UNKNOWN	100 Wh
Volume (1e-4 m^3)	0.0001	m^3
Volume flow (1e-4 m^3/h)	0.0001	m^3/h
Flow temperature (1e-2 deg C)	UNKNOWN	1e-2 deg C
Return temperature (1e-2 deg C)	UNKNOWN	1e-2 deg C
Volume flow (1e-1 m^3/h)	0.1	m^3/h
Energy (0.1 MWh)	UNKNOWN	0.1 MWh
Model / Version	UNKNOWN	UNKNOWN
Parameter set identification	UNKNOWN	UNKNOWN
Reserved VIF extension	UNKNOWN	UNKNOWN
kWh	UNKNOWN	UNKNOWN
Unknown (VIF=0x7B)	UNKNOWN	VIF=0x7B
Energy (MJ)	UNKNOWN	MJ
Unknown (VIF=0x79)	UNKNOWN	VIF=0x79
Medium (as in fixed header)	UNKNOWN	as in fixed header
Actuality Duration (seconds)	UNKNOWN	seconds
Averaging Duration (seconds)	UNKNOWN	seconds
Averaging Duration (minutes)	UNKNOWN	minutes
Reserved	UNKNOWN	UNKNOWN
J	UNKNOWN	UNKNOWN

PS. http://www.rscada.se/ is not accessable....(internal server error)

[wmmihaa]

After further investigation it seems I'm using the VIF output from mbus-protocol.c, rather than mbus-protocol-aux.c. Is there a setting or compile flag I can use?

[lategoodbye]

In order to use mbus-protocol-aux decoding, you can use test/mbus_parse_hex which support the parameter -n. The default output of mbus-protocol is more intended for human. AFAIK the M-Bus protocol wasn't designed for machines.

[wmmihaa]

We're just using the mbus_is_secondary_address method. Is there a compile flag or some other global options that I can set to force using mbus_vif_unit_normalize?

PS(2). http://www.rscada.se/ is not accessable....(internal server error)

[ptorrent]

any news ?