Safely applying power to unlabeled, unknown USB PC board connector - Page 1
Forum section: electronics / projects

author: PP3331
author_rank: Contributor
timestamp: February 10, 2020, 04:34:26 am
content: Hi all. I have a device here that exposes its power and data interface through a single PC board 4 pin connector. I know that it is a USB interface. I cannot easily (that is, non-destructively) examine the board to trace the pins (e.g., to establish which one connects to earth/ground) nor is there a metal case, etc. to use as a GND test point. While it is logical to assume that Vcc and GND are on the outer pins and the data lines on the inner ones, I don't know that for a fact, nor do I know the polarity even if Vcc/GND are on the outer pins. Can anyone suggest a safe procedure for establishing the correct way to hook this up to a USB port, without releasing the magic smoke, using only a common multimeter and/or a cheap scope? Thanks!

author: PP3331
author_rank: Contributor
timestamp: March 04, 2020, 06:01:07 am
content: Hi. The actual readings (which I just took) don't match up exactly with the above. Hopefully they're close enough to prove that this IS a USB port and not something else (like a serial port). And of course I want to get the polarity correct. Meter specs diode mode as:Open Circuit Voltage: < 2.8VdcTest Current: 1 mA TypicalOn the port in diode mode:---- red meter lead (gnd?)---- 1.760v (data?)---- 1.760v (data?)---- 1.494v (+5?)Any thoughts? Thanks!

author: RiffRaff
author_rank: Newbie
timestamp: March 07, 2020, 09:45:18 pm
content: You cannot determine the pins in this way.If you are looking into a "black box" that you can't open up and look at the PCB, then you could try the following:First find the GND pin.Then, with GND grounded, apply a 1k resistor to pull a pin up to 5V with respect to GND, while measuring the voltage on the other two pins. The ESD protection likely includes diodes which will steer to the 5V pin. If they do, then you will find that pulling 2 of the 3 pins up to 5V will pull up the third pin. Pulling one data pin up shouldn't pull the other data pin up, unless this device has some dedicated charging mode handshaking circuitry in it, but you seem to indicate that the port is for communication, so I am guessing this is not the case and if anything it has what is called CDP which will turn off when you apply the 1k pull up.If this is successful, then you know which is your 5V wire and which are your data pins, and you can find your data pin polarity by either a process of elimination, or by applying GND/5V and looking at the data pin voltage. Assuming this is a high speed capable device, then you will likely see a pull-up to 3.3V appear on the D- pin right after you apply GND and 5V.

author: PP3331
author_rank: Contributor
timestamp: March 07, 2020, 10:08:32 pm
content: Thanks for this. I can't even access the ground plane in this (microprocessor) device, so I have to determine ground only by reference to the other pins. My biggest concerns are potentially blowing this out if it's actually a 3.3v device (not 5), or somehow still getting the polarity incorrect. If this turns out to be a serial port and not a USB port, the entire circuit being powered off 3.3 is not impossible of course.

author: Gyro
author_rank: Super Contributor
timestamp: March 07, 2020, 10:13:52 pm
content: It wouldreallyhelp if we knew what the device is, what micro it is using, picture of the PCB etc. It's normally fairly easy to trace the power and data lines if you know what to look for.

author: ogden
author_rank: Super Contributor
timestamp: March 07, 2020, 10:16:32 pm
content: Quote from: PP3331 on March 07, 2020, 10:08:32 pmYou said you have photos. Just show what you have, including picture of whole device. Now it is just guesswork because your measurements do not tell ANYTHING.

author: PP3331
author_rank: Contributor
timestamp: March 07, 2020, 10:49:19 pm
content: Ah, fed up fighting this. It's going to get opened up. Thanks all!

author: ogden
author_rank: Super Contributor
timestamp: March 07, 2020, 10:53:00 pm
content: Quote from: PP3331 on March 07, 2020, 10:49:19 pmFed up fighting advice of more experienced than you, people?

author: Simon
author_rank: Unknown
timestamp: March 08, 2020, 10:36:53 am
content: https://www.eevblog.com/forum/projects/is-this-a-usb-port-(based-on-measurements)/new/#new