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Quick Look: APC Back-UPS and Back-UPS LS BP350U, BP500U and BP700UC Exploration and Design Curiosities
keeping with the recent spate of American Power Conversion product and
assorted UPS related articles I've posted, here's (at least) one more.
While performing a massive upgrade to a client's computing
infrastructure, I found a dead APC Back-UPS model BP500U dating from
about 1999 or so. When they are new, APC seems to have advertised these
units as being particularly targeted towards Windows 98 users with USB
equipped personal computers. Although it was conceived around 1996, USB
didn't really start to hit the market until a few years later. It seems
safe to say that Windows 98 was probably the first fully USB capable
operating system that most people used. (Windows 95 in its OEM Service
Release 2.5 form shipped with USB support tacked on. Few USB devices
ever worked with the Windows 95 USB stack, as Microsoft soon decided
that USB devices should comply with the then-new Windows Driver Model.
Windows 95 never got WDM support.)
Macintosh users (System 9.04 and
greater, although 8.6 also supported USB) were also welcomed into the
fray with a unique clear (well, translucent) cased variant of the
BP500U, known as the BP500CLR. (Pictures: 1, 2 and 3, originally from RadioReference forums.) This variant seems to have some with an iMac-like colored speaker grille for its front panel. APC totally
ought to make another clear, or at least translucent, UPS model at some
point! (Hey, anything could happen. After all, Crystal Pepsi is rumored
to be staging a comeback!)
APC's UPS products have generally
been separated into two major lines: the consumer oriented Back-UPS and
business/professional oriented Smart-UPS. Smart-UPS models typically
all have true sine wave output inverters capable of running just about
any electrical device that falls under their output limits when running
from battery. A few models blur the lines, such as the Smart-UPS 420
and 620 products. These communicate using the APC UPSlink smart serial
protocol, but do not support SNMP management devices or output a true
sine wave when running on battery.
The Back-UPS BP500U that I'd found
seemed to similarly blur the lines between product classes. Like a
Smart-UPS, it refuses to power up without a good battery in place.
(Mine came right back to life as soon as a serviceable battery was
installed.) Yet it does not have a true sine wave output. I had also
noticed that after being unplugged for a while, that the BP500U would
turn off power to its internal logic by throwing a relay. I wondered a
little about how deep the similarites ran and forgot about it
until I answered someone's questions about a BP700UC. BP700UC is the
biggest of three models, with the smallest being a BP350. After Windows
98's popularity began to fade in favor of Windows 2000 and XP, it seems
that APC rebranded these "Windows 98 centric" UPS products as "Back-UPS
LS" models and continued their production for a little longer.
Screwing around with a variable
autotransformer indicated that the BP500U was capable of boosting low
AC line voltage in the face of minor to moderate sags without going to
battery. It did not have the same capability for bucking high AC power
line voltage. More and more it seemed like the BP500U was some sort of
curious cross between the consumer oriented Back-UPS product line and a
second generation Smart-UPS product. It clearly had features that APC's
consumer grade products of the time did not have.
I readily admit that most people
would never care about this sort of thing. I'm not most people. And so
I decided to investigate my suspicions.
There is a lot of second generation
Smart-UPS in this design. It's likely that some of the same people that
designed the second-generation Smart-UPS product also designed this.
Several components here are seen in both places, particularly the
analog to digital converter at the upper right hand corner of the PCB
and the 87C51B microcontroller. It is this microcontroller that likely
makes all the major decisions and oversees the operation of this UPS.
The same part is seen in DIP style packaging within the
second-generation Smart-UPS and in PLCC format here as well as in some
third-generation Smart-UPS models. Likewise, the ADC is also seen in
the second-generation Smart-UPS product, where I believe it digitizes
internal temperature sensor data for consumption by the microcontroller.
What's really curious is the second
microcontroller that's also visible here. This is an STMicroelectronics
ST72T771N and appears to be a custom part. The closest relative I can
find data on is an ST72T774/54/34, which is a microcontroller designed
for use in monitor (display) applications where USB capability is also
desired. While I would be surprised if the ST72T771N is similarly
intended for use in monitor/display applications, it wouldn't be the
first time I'd seen an IC used in an application incongruent with its
"obvious" purpose. A nearby 24 MHz oscillator reinforces the suggestion
of some USB related purpose.
My best guess is that perhaps the
ST72T771N is being used as a protocol translator for the 87C51B
microcontroller, perhaps taking the RS-232 serial oriented UPSlink
"smart" protocol and translating it for relay over the USB bus. Note
the curious large black jumper sitting on the white header assembly
next to the ST72T771N. Its purpose is inscrutable. The actual pins are
likely some sort of factory test, programming or serial communication
A sticker formerly attached to the ST72T771N read "PD236-0002 REV 4 BP350U/500U" with a copyright of 1999.
Here's a look at the back of the printed circuit board, just for the sake of completness.
On this there are a few curiosities
as well. Note the presence of multiple relays, designed RLY1, 3 and 4.
Exactly where relay two ended up is unknown. Note also the outlines for
the old-style "U" shaped heatsinks from the early Back-UPS 200-500 VA
product. This inverter appears to work better, although it has a
stepped squarewave output just as the older 200-500VA Back-UPS models.
It has a much easier time running highly inductive loads such as motors.
The board in mine is marked "BP350/500U".
This photo also calls out why taking
the BP500U apart is probably not the best idea. It comes apart like a
sandwich, starting with the outlet panel and working your way down to
the large transformer that is responsible for isolating the inverter
output and handling the voltage boost functionality. The screws
actually fit into two metal "collars" with threads cut into them. One
is visible at the mid-right portion of the above picture. As these are
only press fitted into the plastic casing, they can fall out quite
easily. The one you see is actually incorrectly positioned, and should
have placed within the plastic cutout that would have held it in place.
Don't make the mistake I did and try to put the unit together like
this, it'll only cause you more grief when you realize what a horrible
mistake you've made and try to take the unit apart. Strong language may
be required and patience is a virtue.
Unlike a Smart-UPS and some other
Back-UPS models, the periodic beeping that occurs when running on
battery cannot be muted from the front panel controls. Whether it can
be muted in software is presently unknown.
And here this was supposed to be a
quick article. Oh well. As always, your comments are appreciated,
especially if you have more information I could add to this page. Contact me right now!
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