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I want to preface this with the fact that I don't know much on the hardware end of things.

We have a device at work that requires a serial port connection to communicate with the software that controls it. My boss has a stash of these little microcomputers that he found cheap, but they don't have serial ports (9-pin... RS232 most likely...)

He DOES have some VGA to HDMI adapters, so he went and hooked it up through that. He asked me to check it out (I'm the resident “fix my computer problem” guy) because they couldn't get the software to control some parts of the device. When I saw the little computer with a wire going from the device to the PC, I was somewhat annoyed that he had tried to connect it with an HDMI adapter and told him that was the issue. He then pulled up the software and showed me that it was recognizing the device and could get it to start and stop.

When connected, the software actually recognized the device, but it can't read a ton of the data (there are “No Response” type errors all over the place).

I am frankly flabbergasted that it even recognized that it was connected. I have to now get the thing to work somehow, and I can't get readings from half of the device. I have little to no knowledge of either of these connections, but I got a theory on my commute home: Obviously HDMI has to have some level of IO going on, and there is no driver for the device installed. The software is likely just scanning every available IO looking for some response, and whatever signal it's pinging gets a response from the HDMI, which is why the software is actually able to see the device. Unfortunately, it's then trying to send/receive on the other pins/lines that should be associated with it and can't get proper communication established beyond the one IO, so it just throws N/R errors all over the place...

Does this sound like it makes sense at all?

Should I just attempt a USB adapter or tell them to just order me a damned computer with a proper serial port / get a PCI serial card or something? Since the thing can be 'seen' by the software, it's gonna take some serious convincing to get him to actually shell out money for a computer that has the proper connections. I can attempt a USB adapter on my own as I can get one of those for a couple bucks, but if that doesn't work I'll have to have a sensible reason to get them to spend for what I need, and I figured this group will have the most reasonable reasons for me to cite...

But also, I'm honestly just curious to know what the hell is up with this actually kinda partially working. I would have never tried connecting through an HDMI port in the first place, and I'm just so utterly confused how it could even partially work...

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    \$\begingroup\$ What are "these little microcomputers"? They may have serial ports, but with "TTL" levels rather than RS-232, and with pin headers rather than DE-9 connectors. If they have USB ports, the easy solution is a USB<->RS232 adaptor. \$\endgroup\$
    – Peter Bennett
    Commented Jun 19 at 23:59
  • \$\begingroup\$ I believe they're Dell Optiplex Micros. He's somehow got his hands on a half dozen of them for about $150 or so each. Refurbs or something, but he pulls one out any time someone's like "I need a computer" and they're terrible. I should have clarified. They're x86 and they've got win10 installed. \$\endgroup\$
    – Ben Leatherman
    Commented Jun 20 at 1:18
  • \$\begingroup\$ Yeah, I think that's going to be my first attempt. Honestly, I just was so perplexed about it picking up the device connected through an HDMI port, I just had to ask people smarter than me about this, hehe. \$\endgroup\$
    – Ben Leatherman
    Commented Jun 20 at 1:55
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    \$\begingroup\$ It is physically impossible for any of that to work. Unless the adapters are passive and only change the connector, not the data protocol. Simply use a USB serial adapter, but remember that in a production environment it may not be a reliable solution. \$\endgroup\$
    – Justme
    Commented Jun 20 at 3:48
  • \$\begingroup\$ Note that the VGA and HDMI pinouts do have an embedded I2C link, called DDC \$\endgroup\$
    – Ben Voigt
    Commented Jun 20 at 16:15

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When you said "these little microcomputers" I assumed that you had Raspberry Pi or similar single board computers.

However, since you have Windows 10 PCs, the simple and obvious solution is to use an USB<->RS232 interface - these are readily available from Amazon, if not at your local computer shop.

You are not likely to find a modern computer with a real DE-9 serial port.

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  • \$\begingroup\$ Yeah, I think this is pretty much the only possible answer, heh. I just got my interest piqued about the whole "it kinda worked through the HDMI port" weirdness and I had to ask people smarter than me how it was even possible. It kinda blew my mind a little bit. \$\endgroup\$
    – Ben Leatherman
    Commented Jun 20 at 2:12

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