Joseph Taylor


So, I recently purchased a new UPS after my old one stopped working (not the battery’s fault).

I did a bit of shopping around and I settled on the APC BackUps RS 550 from Amazon Warehouse Deals, where I got it for £102.45. A risk you take with buying things from Amazon Warehouse Deals is that occasionally there are things missing from the box. In the case of this UPS, the power cable was missing; which wasn’t a problem as it’s your standard kettle lead, but the USB cable was also missing. A cable that I didn’t have lying around.

So I did my research and found that these cables aren’t exactly cheap (from Amazon at least). I initially thought I’d be able to make my own one as it appeared to use an RJ45 connector to connect to the UPS. I was wrong. It uses an RJ50 connector. The difference being RJ45 has 8 contacts, whereas RJ50 has 10 contacts. Okay, so how about checking to see if the two extra pins are used? Nope, I need both, as you’ll see in my diagram later on.

I ended up informing Amazon of the missing parts, resulting in a 20% refund, bringing the cost of the UPS down to £81.96. I’m happy with that.

I still wanted to have a play with the USB interface of the UPS, so I looked at making my own cable. A seller on eBay who sells RJ50 cables. I had an option of buying an RJ50 cable with the wires exposed on the other end, or I could buy an RJ50 to RS232 (female DE-9 connector) cable. I opted for the cable with the DE-9 connector on the end, thinking that I could buy a DE-9 male break-out connector to test with, then eventually make a proper cable.

The break-out connector is still on it’s way, but that hasn’t stopped me jamming wires in to the end of the DE-9 connector to test with. First I had to determine which pins on the RJ50 connector were connected to the DE-9 connector.

The diagram below shows my findings:

Notice that contact 10 of the RJ50 cable is missing from the DE-9 connector. The DE-9 connector only has 9 contacts. No biggie, hopefully I won’t need contact 10. Yes, yes I do need contact 10. As you can see from the table below, contact 10 is used for USB +5V.

UPS Contact USB Connection
5 Data- (white)
8 Ground (black)
9 Data+ (green)
10 +5V (red)

I cut a slit in the cable so that I could access the 10th wire. Not ideal, but not much I can do about it.

I cut the end off of an old USB cable and exposed the wires. I soldered the +5V wire to the 10th wire, and tinned the rest of the wires, poking them in the the DE-9 connector in the relevant holes. Here is the result:

It needs tidying up and making more robust, but it works. And it worked first time! Windows recognised the UPS without any problems, adding a battery icon to the taskbar, similar to on a laptop.

Here is the APC PowerChute Personal Edition software, that connects to the UPS:

I’ve not had a play with it in much depth, but at first glance it looks quite interesting. The next step is to have a look at what else I can do with this cable. It’d be interesting to see if I can interface with the UPS using an Arduino.

To be continued...

If you have any questions about this project, or any others on my site get in touch!