WAC510, identifying serial pins with a multimeter

Hardware Questions and Recommendations
1

I am trying to flash a Netgear WAC510 via the TFTP method (because I can't get access via the stock web interface).

I have opened up the WAC510 and found the four serial pins. These are not labelled other than "J2" below them, with a white dot next to the first one next to the J2 label.

According to the WAC510 wiki, the order of the pins should be:

Serial: Header J2
PIN 	Description
1 	    3.3 Volt (Do NOT connect!)
2 	    TX
3 	    RX
4 	    Ground

I have a basic multimeter and I attempting to confirm the above order using the method in the wiki here.

However, when I take my readings I get:

pin 1 to pin2: 0
1-3: 3.3V
1-4: 3.3V
2-3: 3.3V
2-4: 3.3V
3-4: 0

This does not accord with the wiki. According to the wiki, I should get three 3.3V readings and three 0 readings. I have four 3.3V readings and two 0 readings.

I don't want to damage my WAC510 or USB TTL device, so if anyone knows how I can confirm the order of the pins here I would be very grateful!

Thank you.

2

pin 1 is probably marked in some way, square pad, or a small arrow printed on the PCB, post a photo.

photos at https://fccid.io/PY316200355 aren't clear enough.

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3

Just a thought.
You know both outer pins (1 and 4) carry 3.3 V. The multimeter should indicate the polarity of the voltage it measures.

Measuring a positive voltage of 3.3 V on the outer pins identifies pin 1 (and 4).

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4

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The pins have the label "J2" below them. The first (or last, depending on which way you count them) pin has a white dot next to it. There are no other labels.

5

This is helpful but I had a few queries before relying on this method alone:

  1. Are serial pins always in the same order? Could it go GND, 3.3V, TX, RX, for instance?
  2. Why have I got four 3.3V readings and two 0 readings when I should have had three of each? What does it mean?
6
  1. I don't know but just for the pinout I would follow the wiki.
  2. Did you click on the port_serial link in the wiki (https://openwrt.org/docs/techref/hardware/port.serial). Does it match up?

Screenshot 2025-02-05 182119
Screenshot 2025-02-05 182119680×220 6.52 KB

7

The dot next to J2 would mark 3.3v.

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8
  1. No, it doesn't match up at all unfortunately. See first post - four of the pin pairs read 3.3V and two of the pairs read 0. According to the wiki (the image in your post) it should be three of each.
9

thnx for the photo, I added it to the wiki page.

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10

I'm not entirely sure the truth table in the wiki holds up under all circumstances and for all TTL/UART chips. Sure, UART protocol suggests that RX and TX are nominally supposed to be at 3.3V at idle, but those are not power lines but signal lines with miniscule current capabilities and internally connected to active and passive elements. IMHO, RX and TX are unreliable to be measured/identified with a simple multimeter.

Likewise one cannot find VCC by measuring random pins that carry voltage because one doesn't know why they carry voltage. It is reasonable to assume which pin is which on a connector, but one can not tell a signal line at 3.3V from a VCC line with a multimeter alone. That requires a signal probe or an oscilloscope (or the courage to simply try connecting an adapter), and others have already done that for you.

A dot on the silkscreen will always mark pin 1. No manufacturer will maliciously mark a different pin than pin 1.

If you need to confirm the orientation of the connector, the most reliable way is to confirm the location of the ground pin. With the device turned off and your multimeter in resistance/beep mode, put one probe to a known ground (the shielding of a USB or RJ45 port is a popular choice) and see/hear which pin on the connector gives you virtually-zero resistance and beeps when you touch it with the other probe. That tells you the ground pin, in your case pin 4.

You measured none of the connector pins higher than 3.3V, and you know which one is ground. As long as you connect this ground pin to your TTL adapter's ground and leave your TTL adapter's VCC unconnected, which you should anyway, there's nothing you can damage. Worst case is that it will not work.

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11

Thanks everyone for all the help and explanations. To report back - I think the order set out in the wiki is correct. I got a successful connection by connecting pin 2 (TX in the wiki) to RX on the USB-TTL adapter, pin 3 (RX in the wiki) to TX on the USB, and pin 4 (ground in the wiki) to GND on the USB.

12

That's exactly how it's supposed to work. "RX" stands for "receive", "TX" stands for "transmit". The transmit pin of the device is connected to the receive pin of your TTL adapter, and vice versa.

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