APM Power Modules
I was having a problem with my 250 Mini Quad (need to write up the build log at some point) where I wasn't getting current measurements from the power module. Most of the parts for my 250 came from http://www.readytoflyquads.com/, including the mini APM and power module. I had checked and double-checked everything, but no luck. Some extensive digging on rcgroups turned up this post confirming that the ReadyToFlyQuads have the XT60 connectors on backwards, with the result that it can't measure current properly.
No problem, I thought - I'll just swap one in from one of my other drones. I grabbed the HobbyKing power module I use on my 450 and hooked it up. But when I plugged in the battery, the ESCs didn't give their final chime indicating a signal from the flight controller. After banging my head for a while, I noticed that the APM wasn't powering up. This was strange - the mini APM uses a different layout for the PM connector on the board itself, but I assumed the power module layout would be standard (because they're also used with full-size APMs). But no.
Standard power module layout (looking at the receptacle as you'd plug it in with the retaining tabs facing up), a standard power module puts out:
V V Is Vs G G
The ReadyToFlyQuads power module puts out:
G G Vs Is V V
[Vs and Is are measured Voltage and Current, respectively]
I re-configured the cable from the mini APM for a standard power module layout and everything now works like a charm. Hopefully google search will pick this up in case others run into the same problem I did.
Since I needed another power module (figured it was cheaper to buy another one than the hassle of re-connecting the XT60 connectors in order to get current sensor working), I tried going super-cheap and ordered a power module from eBay for $8 shipped. Standard layout, though the voltage out is slightly lower (5.37v vs 5.40v on the other two). Will see how it holds up. On the plus side, it comes with a 6-pin DF13->JST-SH cable for use w/ the mini APMs, including the correct wire order. I need to put a load on it in order to confirm the current sensor works.
I ordered another power module from ReadyToFlyQuads because, why not? This one came with the XT60 connectors on the right way but still the backwards layout for the power module connector.