Nut upsdev - Mar 2014 - Developing the UPS side of the UPS-NUT equation (via usbhid)

Things are going along well, but there is one remaining area of
confusion...."units".  I understand exponents now...it's basically
how you pass decimal values.  That's easy enough.

But "units"....in other words, I have a voltage I want to put in a
report.  If I include in the report descriptor the fact that it is actually a
voltage, with units of "volts", then I'm suddenly also inheriting
an exponent of 7?  So to represent 120 volts, I'd actually have to send the
value 1200000000, so that on the NUT side it would apply a -7 exponent to get it
to come out as 120?

I know I could add a -7 exponent to "counteract" the units exponent,
but I'm trying to understand WHY it's like this, based on the table in
3.2.3 of the PDC HID doc.  Why did they think that volts needs an exponent of 7
in them?

I also noticed that current (amps) is listed as centiAmp, with a -2 unit
exponent in the HID doc....however, the table in libhid.c which contains
HIDUnits does not include anything for amps, just voltage and power.

I'm wondering if there is even any real value in specifying units, because
it seems like more of an obstacle to easy value understanding than anything
else.  Any why does the NUT library use the 7 exponent for voltages, but not the
-2 for amps?

Thanks as always.

Rob
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On Mar 18, 2014, at 10:37 AM, Rob Groner wrote:
> Things are going along well, but there is one remaining area of
confusion?.?units?.  I understand exponents now?it?s basically how you pass
decimal values.  That?s easy enough.
>  
> But ?units??.in other words, I have a voltage I want to put in a report. 
If I include in the report descriptor the fact that it is actually a voltage,
with units of ?volts?, then I?m suddenly also inheriting an exponent of 7?  So
to represent 120 volts, I?d actually have to send the value 1200000000, so that
on the NUT side it would apply a -7 exponent to get it to come out as 120?
>  
> I know I could add a -7 exponent to ?counteract? the units exponent, but
I?m trying to understand WHY it?s like this, based on the table in 3.2.3 of the
PDC HID doc.  Why did they think that volts needs an exponent of 7 in them?
Sorry for the delay in getting back to you.

I think most UPSes add that -7 exponent in the descriptor - I'd have to
check. I think it's one of those weird SI base units things, where some of
the units will be absurdly large unless they have an exponent.

Also, which PDC HID document are you looking at? I see a "Table 1" in
pdcv10.pdf.

The first column is the unit that they expect to be measured, with the second
column being the "standard" unit. The exponent column assumes you are
using the units from the first column.
 
> I also noticed that current (amps) is listed as centiAmp, with a -2 unit
exponent in the HID doc?.however, the table in libhid.c which contains HIDUnits
does not include anything for amps, just voltage and power.
 
I think that means that if you are measuring the UPS current in centiAmps, you
would apply an exponent of -2 to scale to amps. It's a question of how many
digits you want to the right of the decimal point.

To that end, we don't do a good job of indicating how many significant
figures are in a measurement. (On the other hand, Arjen, one of the usbhid-ups
developers, always used to say "an UPS is not a measurement device",
so...)
> I?m wondering if there is even any real value in specifying units, because
it seems like more of an obstacle to easy value understanding than anything
else.  Any why does the NUT library use the 7 exponent for voltages, but not the
-2 for amps?
 
I think the main motivation is to eliminate extra math operations from the UPS
side. If you are computing values in a microcontroller anyway, I agree, it just
adds confusion.

-- 
Charles Lepple
clepple at gmail