In order to connect two small electrical circuits, I needed 17 pins. Because of its availability and compact form, I chose the HDMI connector.

The two circuits are connected using an HDMI cable that was provided with a computer screen and everything worked as expected.

I tried with another cheap HDMI cable but it doesn't work and I see some missing connections. However, this cable works for its intended purpose: connecting a screen to a computer.

If I take a multimeter and test the ends of each cable, the results vary.
For the "good" cable, I get the "beep" for all 19 wires whereas, for the "bad" one, more than a few show no connection.

From this SE link, I understood that there are no unused pins in the HDMI standard. Is there any kind of redundancy function or something that could explain why a functional cable seems to have pin conductors between some pins?

Connections are as follow (bold is connected):
19 17 15 13 11 9 7 5 3 1
18 16 14 12 10 8 6 4 2

  • \$\begingroup\$ What is the type of HDMI connector? \$\endgroup\$ – User323693 Aug 16 '19 at 16:43
  • \$\begingroup\$ @Umar: HDMI type A, the "bad" cable is V1.4, while the "good" simply says high speed hdmi cable with ethernet. \$\endgroup\$ – Haboryme Aug 16 '19 at 18:09

Which pins are not connected? All pins should be connected. But as the TMDS clock and data pins are differential pairs, they run as twisted pair where the shield is connected to ground. It might be that the diff' pair shield is not actually connected to the respective ground pins but to the connector outer shell. I would not count on having 19 separate connections between connectors. Basically, as some pins are differential pairs, some are shields for differential pairs and some are just standard wires, it is a bit tricky to determine which pin should carry which signal.

But just to avoid confusion, I do not recommend using connectors of standard interfaces for any custom purposes like this. What if someone plugs a computer or monitor to your device and damages something.

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    \$\begingroup\$ Upvoted because you discourage using commonplace cables for custom purposes if that could lead to equipment damage. Good advice. \$\endgroup\$ – TonyM Aug 16 '19 at 22:33
  • \$\begingroup\$ @Justme: thanks for your input, I edited to add which pins work. The results surprised me as I expected the non-functional pins to be assigned to the shields. I agree that using a standard interface for a custom purpose might lead to unintended use but here availability was the decisive factor. \$\endgroup\$ – Haboryme Aug 17 '19 at 12:29
  • \$\begingroup\$ @Haboryme I think you have the pin numbers the wrong way around (it does not work if those pins are non-connected). If the pin numbers are interpreted correctly, then indeed, it is all the ground pins that are missing. \$\endgroup\$ – Justme Aug 17 '19 at 19:48
  • \$\begingroup\$ @Justme I followed the numbering from the wikipedia page. First line is the upper part, while the second is the bottom one. Do you have another source? \$\endgroup\$ – Haboryme Aug 17 '19 at 19:57
  • \$\begingroup\$ @Haboryme I have no reason to doubt Wikipedia here. If you look at TV connector, pin 1 is on right. If you disconnnect a cable and turn it towards you, pin 1 is on the left. The 5V pin is recessed on the cable so you know which pin is 18. \$\endgroup\$ – Justme Aug 17 '19 at 22:58

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