To use the new 27-inch iMac as an external monitor requires a very specific cable to work: a Mini DisplayPort to Mini DisplayPort cable, to be exact. It
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Apple Mac Video Cable 590-0621-A RARE for Monochrome Portrait
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🌈The S500 triple portable monitor is equipped with two 14.1-inch 1920 * 1200P FHD displays, 100% SRGB color gamut, 16.7 million display colors, 60HZ
LIMINK 14 Triple Portable Monitor with 1 Cable Connect, S500 Laptop Screen Monitor Extender, 1200P USB AC Frameless Tri-Screen Compatible for
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Directly Connects Your Computer's DisplayPort Output to a 4K HDMI Monitor or TVThis active adapter cable lets you use a device with a DisplayPort connector with an HDMI monitor, television or projector. Because the P582-006-HD-V4A is an active adapter, it converts single-mode and dual-mode (DP++) DisplayPort output and can be used with graphics cards that don't transmit DP++ signals.
Tripp Lite P582-006-HD-V4A DisplayPort 1.4 to HDMI Active Adapter Cable 6 ft. (1.8 M)
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USB C Multiport Adapter Dual 4K HDMI, PD - USB-C Multiport
Tripp Lite DisplayPort 1.4 to HDMI Active Adapter Cable (M/M), 4K