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Quick Look: APC Back-UPS and Back-UPS LS BP350U, BP500U and BP700UC Exploration and Design Curiosities

In 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.

APC BU350U / BU500U PCB Front Scaled (click to see full size)

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 arrangement.

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.

APC Back-UPS BP350 / BP500U printed circuit board back side (click to enlarge)

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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