USB - two black wires - why?


#1
  1. VCC (red)
  2. D+ (green)
  3. D- (white)
  4. GND (black)
    ??? (black)

For example, looking at this MadCatz 4716 PCB you can easily see that there are in fact 2 black USB wires. The 5th connection appears to made using a heavier gauge wire. However according to most any USB pinout diagram I have read, there is no 5th pin. So is this a shielding wire? A redundant ground? Why is a 5th wire being used on the PCB??


#2

Yes, it’s a shield ground.


#3

Yes it is a shield ground and is redundant. Why? I don’t know. Perhaps hooking up the shield ground prevents EMI (Electro-Magnetic Interference) noise from coming out the wire. But you shouldn’t worry about that unless you are playing Street Fighter on a plane.


#4

Not being able to play on a plane seems like a deal breaker to me


#5

thanks guys. i’m wiring up a dual pcb stick & on my 360 pcb (one referenced) i accidentally broke the solder on the 5th pin. so i was wondering if it was worth my while to re-connect it… and whether it was worth tying the two pcb’s “5 pins” together.


#6

Now thats an idea, some-one needs to make a street fighter plane tournament. Any millionaires in the forums?


#7

Your question is answered on Wikipedia:

The connector construction always ensures that the external sheath on the plug makes contact with its counterpart in the receptacle before any of the four connectors within make electrical contact. The external metallic sheath is typically connected to system ground, thus dissipating any potentially damaging static charges (rather than via delicate electronic components). This enclosure design also means that there is a (moderate) degree of protection from electromagnetic interference afforded to the USB signal while it travels through the mated connector pair (this is the only location when the otherwise twisted data pair must travel a distance in parallel). In addition, because of the required sizes of the power and common connections, they are made after the system ground but before the data connections. This type of staged make-break timing allows for electrically safe hot-swapping and has long been common practice in the design of connectors in the aerospace industry.