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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?

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

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  • \$\begingroup\$ What is the type of HDMI connector? \$\endgroup\$
    – User323693
    Commented Aug 16, 2019 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
    Commented Aug 16, 2019 at 18:09

1 Answer 1

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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
    Commented Aug 16, 2019 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
    Commented Aug 17, 2019 at 12:29
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    \$\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
    Commented Aug 17, 2019 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
    Commented Aug 17, 2019 at 19:57
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    \$\begingroup\$ @Justme: I got it wrong indeed, 18 is right most. Thank you! \$\endgroup\$
    – Haboryme
    Commented Aug 18, 2019 at 12:14

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