Using Apple iMac mid-2011 (has Thunderbolt 2) as an external display for my Windows 10 Primary laptop

1. The Thunderbolt port on the iMac can output Mini DisplayPort, which is used by your HDMI adapter, but it can't input Mini DisplayPort, only Thunderbolt.
2. If your Windows PC/Laptop does not have Thunderbolt, there are no adapters, cables, or converters that will make it work with a Thunderbolt device. Some Dell laptops do have Thunderbolt. e.g.

https://www.dell.com/al/p/inspiron-15-7590-laptop/pd

has a Thunderbolt 3/USB-C port that can connect to older Thunderbolt via an adapter:

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

That doesn't guarantee it can use your iMac as a display, but it could at least connect to it.

I have been persistently trying to connect my Apple iMac mid-2011 (has Thunderbolt 2 for Video-in) to work as external display for my Windows 10 laptop. (has USB-C and HDMI) ...

Hi,

Firstly, your 2011 iMac doesn't have Thunderbolt 2, it has two Thunderbolt 1 ports. Not sure if that's what you meant.

And I assume your windows laptop doesn't have a Thunderbolt port? If the port is USB-C, and not Thunderbolt 3, (check for thunderbolt symbol near the port) then this doesn't work.

From the start, the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter has a very misleading name, it does not convert USB-C signals to Thunderbolt 2 (nor does anything else, it's just not possible). Nothing will work if you use that adapter on your USB-C only laptop with no Thunderbolt 3.

Also, since your iMac has a Thunderbolt port, it will only work in TDM with Thunderbolt cables connecting to Thunderbolt ports. So even if you got a USB-C to Mini DisplayPort cable, it would not work with your iMac as a display.

And as has been pointed out, Apple doesn't support Target Display Mode with non-Mac primary computers, but in my experience it works about as well as Apple's 27 inch LED display and Thunderbolt Display did with PC's, so ymmv, if you get a PC with Thunderbolt, that is.

hds6825 wrote:

I think Apple has programmed the Mac OSX to not allow Windows PCs to work with Target PC mode.

It's not that it's programmed not to work. It was never supported in the first place.

See: Use your iMac as a display - Apple Support

No mention of using a Mac as a display for a Windows PC.

hds6825 wrote:

The only challenge is solving either of the two -

Finding a mini-DisplayPort to Thunderbolt 1 converter/cable so it can work with this cable1. .

Cable for converting Display Port to Thunderbolt 1 so it can work with this cable1. .

There are no such converters or cables, but you can plug a Thunderbolt adapter

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

into the Thunderbolt 3 port on the Windows laptop and connect it to the iMac with a Thunderbolt cable.

https://www.apple.com/shop/product/MD861LL/A/apple-thunderbolt-cable-20-m

(Thunderbolt 1 and Thunderbolt 2 use the same adapters and cables.)

Can't promise that it will work, but the connections will be correct. You would have to look in the Windows setting to see if it shows the extra display when you enter Command-F2 on the iMac

Use your iMac as a display with target display mode - Apple Support

1. Yes. Malcolm's explanation is also correct, I don't know if it was intentional, but Apple using the same mechanical interface for mDP and TB1/2, and then asserting that the ports were compatible with mDP displays and claiming that TB signals included the DisplayPort signals to achieve this causes a lot of confusion. (I feel the same thing happened with USB-C and TB3...) When you plug a TV into the iMac, the iMac is a Thunderbolt "host" device. When a cable is inserted, the TB controller actively determines if it needs to output a DP signal, or if the connection is a Thunderbolt Bus with additional host/peripheral devices on it, and decide how to route each multiplexed signal to its destination. This is something only a device acting as a host device can do. In Target Display Mode, the iMac is basically a Thunderbolt Display. It is the same problem as many people had, including myself, when I attempted to hook up decommissioned Thunderbolt Displays to mDP sources. It is extremely inefficient and impractical to make it work. (Building a "dummy multiplexer" interleaving DP into a silent, but powered, PCIe bus cost ~$200, excluding chassis and power supply)
2. Unfortunately, no. You could build one yourself, but at the price and with no support or guarantee of it working, I really can't recommend this to you. The setup closest to yours that I've had success with is a Mac with DisplayPort output in Boot Camp, plugging it into my home-built Frankenstein DP+PCIe 3.0 x0 -> TB2 multiplexer into a TB Display, and it was in no way reliable. I'm not sure because of the model year, if you mean there are no machines you can request that has any Thunderbolt ports, or if you mean you can req models with TB2 or TB3 ports... If it's the latter, then if you can get IT to let you use one, you should be good to go with the adapter and cable you have. If you mean no machines with TB ports at all at your company, I would say sell the iMac to get a display, or if you still use it, get a cheap display panel.

If you're feeling up for a headache and think you might like tinkering with this stuff, you can check out some Thunderbolt Add-In Cards (AIC's) for PC motherboards, they are a source of a TB host device, a key component of multiplexing DP and (not)PCIe into thunderbolt. This will cost more than what a similar display to your iMac's will cost though, so keep that in mind.

Wait a second, so your laptop's port is actually a TB3 port? Confirm this in Windows, either Thunderbolt should be at least mentioned in device manager (possibly with a bunch of PCI bridges), or OEM Thunderbolt device manager/firmware updater software should be present. USB-PD also uses a thunderbolt-like logo (they claim it's lightning, what exactly is a thunderbolt anyways? Always thought it was lightning bolt or thunderstorm) so it might just mean the port is for charging.

Yeah... It must be the USB-PD logo. I couldn't tell you how to tell them apart, especially some vendors mark their Power Delivery ports way to similar to the TB logo (cyan-blue, circle, etc) to believe it is not intentional :(

But TB3 only reaches like, 1m? 3m if you have an active cable? (I don't know if anyone's tried the engineering QSFP samples for TB3 over SR fiber from a not-to-be-named company in that specific area of the industry, but interoperability is a joke don't hold your breath) And TB2 doesn't have spectacular cable lengths either, unless you buy optics and fiber, which are available but don't have the best history when it comes to compatibility even for TB1/2.

So I'm thinking, if you just plug in a regular old Cat 5e between the two machines, you could get a decent screen share task going, or if you really hate the picture quality, for a windows device, set up directshow and ffmpeg to stream the desktop directly to a udp multicast address on the link with only the iMac on it, and play that stream full-screen on the iMac?

I might be completely missing the point if closed-clamshell mode was your goal, but it's still a possibility. Anyway, give ethernet a try is what I'm trying to get at :) Chances are the laptop can encode h264 or hevc on hardware, and you don't need the massive bandwidth.

Great info Ted, thanks. :)