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Make sure that your iMac is turned on, and the other Mac is logged in to a macOS user account.<br />
Connect the Mini DisplayPort or Thunderbolt cable between the two computers.<br />
Press Command-F2 on the keyboard of the iMac that you want to use as a display.<br />
<br />
Or look at this youtube link<br />
<br />
<a href="https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=16&cad=rja&uact=8&ved=0ahUKEwihiPK00KDZAhXILMAKHb2dAPwQtwIIeDAP&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DGhRai_StDHU&usg=AOvVaw2h290-cYSsYDtP9L-m6ooV" rel="ugc">https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=16&cad=rja&uact=8&ved=0ahUKEwihiPK00KDZAhXILMAKHb2dAPwQtwIIeDAP&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DGhRai_StDHU&usg=AOvVaw2h290-cYSsYDtP9L-m6ooV</a><br />
<br />
hope this helps<br />
<br />
Justin
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so what is the exact cable I need to do this?
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The newer iMacs can use a display port cable, but, based on the 1st image there is no display port on this iMac. &nbsp;Your 2 images seem to be in conflict. &nbsp;The 1st image shows firewire 800 and firewire 400 ports that are available on a much older model of Mac. &nbsp;I also don't see the standard Mini Display Port connector, just the old mini-DVI video port. &nbsp;Your 2nd image shows a 2015 iMac which would no longer have those obsolete firewire ports or the older display connector. &nbsp;Are the images from 2 different Macs? &nbsp;The DVI port won't work for using your iMac as a monitor.<br />
<br />
<a href="https://www.cnet.com/how-to/how-to-use-your-imac-as-a-monitor/" rel="ugc">https://www.cnet.com/how-to/how-to-use-your-imac-as-a-monitor/</a><br />
<a href="https://support.apple.com/en-us/HT204592" rel="ugc">https://support.apple.com/en-us/HT204592</a>
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I want to hook up an old iMAC as a secondary monitor. &nbsp;Is this possible? &nbsp;If so, how do you do it. &nbsp; &nbsp;See the screenshot attachment for the ports on the old iMAC and a screenshot of the specs of my new iMAC.<br />
<a href="https://filedb.experts-exchange.com/incoming/2018/02_w07/1272810/IMG_0083.jpg" target="_blank" class="file-inline" title="old imac ports (488 KB)"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M67.508 468.467c-58.005-58.013-58.016-151.92 0-209.943l225.011-225.04c44.643-44.645 117.279-44.645 161.92 0 44.743 44.749 44.753 117.186 0 161.944l-189.465 189.49c-31.41 31.413-82.518 31.412-113.926.001-31.479-31.482-31.49-82.453 0-113.944L311.51 110.491c4.687-4.687 12.286-4.687 16.972 0l16.967 16.971c4.685 4.686 4.685 12.283 0 16.969L184.983 304.917c-12.724 12.724-12.73 33.328 0 46.058 12.696 12.697 33.356 12.699 46.054-.001l189.465-189.489c25.987-25.989 25.994-68.06.001-94.056-25.931-25.934-68.119-25.932-94.049 0l-225.01 225.039c-39.249 39.252-39.258 102.795-.001 142.057 39.285 39.29 102.885 39.287 142.162-.028A739446.174 739446.174 0 0 1 439.497 238.49c4.686-4.687 12.282-4.684 16.969.004l16.967 16.971c4.685 4.686 4.689 12.279.004 16.965a755654.128 755654.128 0 0 0-195.881 195.996c-58.034 58.092-152.004 58.093-210.048.041z"/></svg>IMG_0083.jpg</a><br />
<a href="https://filedb.experts-exchange.com/incoming/2018/02_w07/1272811/Screen-Shot-2018-02-12-at-8.39.21-AM.png" target="_blank" class="file-inline" title="new imac spec (32 KB)"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M67.508 468.467c-58.005-58.013-58.016-151.92 0-209.943l225.011-225.04c44.643-44.645 117.279-44.645 161.92 0 44.743 44.749 44.753 117.186 0 161.944l-189.465 189.49c-31.41 31.413-82.518 31.412-113.926.001-31.479-31.482-31.49-82.453 0-113.944L311.51 110.491c4.687-4.687 12.286-4.687 16.972 0l16.967 16.971c4.685 4.686 4.685 12.283 0 16.969L184.983 304.917c-12.724 12.724-12.73 33.328 0 46.058 12.696 12.697 33.356 12.699 46.054-.001l189.465-189.489c25.987-25.989 25.994-68.06.001-94.056-25.931-25.934-68.119-25.932-94.049 0l-225.01 225.039c-39.249 39.252-39.258 102.795-.001 142.057 39.285 39.29 102.885 39.287 142.162-.028A739446.174 739446.174 0 0 1 439.497 238.49c4.686-4.687 12.282-4.684 16.969.004l16.967 16.971c4.685 4.686 4.689 12.279.004 16.965a755654.128 755654.128 0 0 0-195.881 195.996c-58.034 58.092-152.004 58.093-210.048.041z"/></svg>Screen-Shot-2018-02-12-at-8.39.21-AM.png</a>
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Screen-Shot-2018-02-12-at-8.39.21-AM.png

IMG_0083.jpg

I want to hook up an old iMAC as a secondary monitor.  Is this possible?  If so, how do you do it.    See the screenshot attachment for the ports on the old iMAC and a screenshot of the specs of my new iMAC.

use imac as secondary monitor

A monitor or a display is an electronic visual display for computers, usually comprised of the display device, circuitry, casing, and power supply. The display device in modern monitors is typically a thin film transistor liquid crystal display (TFT-LCD) thin panel, while older monitors used a cathode ray tube (CRT) about as deep as the screen size. They are connected to the computer via VGA, DVI, DisplayPort, Thunderbolt, LVDS or other proprietary connectors and signals.

Displays / Monitors

Mac OS X is the desktop operating system from Apple Inc., found on Macintosh computers. OS X consists of a Mach/BSD-based kernel, operating system interfaces primarily based on FreeBSD and additional frameworks (written in C, C++ and Objective-C providing user interface and application-level services.