I'm building a product that will use HS 2.0 on USB C for data transfer and charging. But I want to make a dumb cable that just has 3 wires connected to Ecg leads to plug into the same USB C port. Can I keep those 3 pins on the USB socket always connected to my A2D, and simply ignore the data coming in on them if it happens to have a real USB cable connected (assumedly by looking for incoming power)?
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3\$\begingroup\$ Thunderbolt, DisplayPort, power… I don’t see why we can’t add ecg, just make sure the naming when you standardize it is confusing. (This is usb humor) \$\endgroup\$– BryanCommented Oct 4, 2022 at 2:26
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2 Answers
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No.
Please don't use the USB connector for ECG leads, because that's unsafe.
Let's do a spot of risk analysis. Imagine the ECG electrodes connected to a patient on one end, and to a USB plug on the other end. Imagine all the places where somebody could accidentally stick the USB plug into: laptops, smartphones, counterfeit Chinese wall chargers.
What kind of benefit can outweigh such risk?
More generally. Exposing common connectors (HDMI, USB, RJ-45, etc) for other purposes is usually a wrong idea. But [the opposite] using an uncommon or proprietary connector for common communication interfaces is fine.
answered Oct 3, 2022 at 20:56
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7\$\begingroup\$ If you have a USB plug, someone will find a way to plug it into a USB socket. Furthermore, if you have the budget to do custom injection molding, you have the budget to use a proper connector. You can use the same single-socket scheme to prevent the power cable and ECG leads from being connected simultaneously but I would check to see if that is a permissible level of protection. \$\endgroup\$– virCommented Oct 3, 2022 at 21:59
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2\$\begingroup\$ On a side note, RJ-45 is not even a connector. It's an unofficial name for a wiring standard which uses a 8P8C modular jack as the connector. \$\endgroup\$– JustmeCommented Oct 4, 2022 at 5:56
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2\$\begingroup\$ and (@Justme) that connector has been used for many things over the years - I've certainly seen RS232 using it, on devices that may even predate 10base-T. (and indeed that was standardised) \$\endgroup\$– Chris HCommented Oct 4, 2022 at 7:59
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3\$\begingroup\$ @David "Sound safe?" - more importantly, are you prepared to justify your choice at an inquest? fda.gov/medical-devices/general-hospital-devices-and-supplies/… \$\endgroup\$ Commented Oct 4, 2022 at 14:11
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2\$\begingroup\$ @OskarSkog I understand, but no counterfeit charger would ever connect to those pins. either the pins wont exist on the connector or they would be unconnected. However, the reason you give is a good reason why connecting the leads to ground wouldnt be ok. The circuit's ground may be at 230 V \$\endgroup\$ Commented Oct 5, 2022 at 5:26
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You will need to use the CC lines to negotiate entry into a newly-defined alternate mode, in a manner compliant with the specification documents. Your cable will have to complete this handshake according to the specification, so that it does not interfere with devices which do not implement your alt-mode.
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2\$\begingroup\$ There is the Audio Adapter Accessory Mode which requires no negotiation and is entirely passive. However, I believe that is the only fully passive "alternate mode" (note the small letters) and it is not a proper Alternate Mode. Indeed, AFAIK, all Alternate Modes require active negotiation. Audio Adapter Accessory Mode is its own special thing and cannot support active negotiation, since that would defeat its point. \$\endgroup\$ Commented Oct 4, 2022 at 9:40