So, I bought a barcode scanner:
https://smile.amazon.com/Automatic-Barcode-Scanning-Bar-code-Adjustable/dp/B00406YZGK/

I'm wanting to do something abnormal, of course, because using an electronic device as-is just isn't good enough. It works great when plugged directly into a computer via USB. When I try to solder leads directly onto the VCC, GND, D+, and D- pins and then connect those to a Raspberry Pi, it doesn't show up. I then tested continuity in the cable it comes with it - basically one of these:
https://sc01.alicdn.com/kf/HTB109vi...LS2208-Symbol-BarCode-Scanner.jpg_350x350.jpg

I get continuity between VCC and GND but not D+ and D-. Does anyone know what's going on, here?

 

Let me see if I follow you.

1. Connect device A with cable B and things work.

2. Connect device A with 1 to 1 wires to something else and things do not work.

3. Test cable B and 1 to 1 wiring is not observed.

Observation:

1. The assumption the cable is 1 to 1 is not supported by your measurements.

 

Almost correct - the "something else" is not necessarily true:

2. Connect device A with 1 to 1 wires to something else and things do not work.

Otherwise, yes, you described this correctly. If I connect the scanner to a computer with the provided wire (10c10p "RJ50" to USB), it works. If I use 1 to 1 wiring to connect it to the same or another computer, it does not. When I test the cable, the power wires are 1 to 1, but the D+ D- wires are not. There are only four wires in the provided cable, and looking closely at the RJ50 plug, they are standard red, black, green, and white for USB. The green and white do not show 1 to 1 continuity.

What's more, I'm soldering my 1 to 1 wires to pins on the scanner's PCB. They are labeled as VCC, GND, D+, D-. These pins correspond with the RJ50 wires in the plug. However, as described, if I plug in the cable and measure from PCB pins to the USB plug, I can test the power for continuity just fine, but the data wires don't show continuity. Weird...

 

Hmm... it appears that this is my exact scanner. I'm going to TRY using it in serial mode, but I've never really interfaced with RS232...
http://briankhuu.com/blog/self/barcode scanner/2014/12/02/upc-barcode-scanner.html