Second monitor on Studio through KVM is always black

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or other issues.

Linux and Windows use the "if you think it's good enough, then it IS" standard. Apple makes not apologies for its high standards and does not garbage-match other vendors.

The only way to support TWO displays (of modest resolution) on one cable out of a Mac is for the port, the cable, and the first device (Dock or display) to be a genuine ThunderBolt-3 or higher device. USB and HDMI do not have enough bandwidth (in Apple's Opinion) to support multiple displays, so the Mac refuses to use them.

Obi-Wan-YJ wrote:

I'm not sure exactly what you mean by that last paragraph, @den.thed. Each of my monitors has only a single HDMI input, and no Thunderbolt ports.

Most displays have at least two inputs, HDMI and DisplayPort or HDMI and DVI if they are older.

The Connect a display to Mac Studio - Apple Support article, spells out the different type of cables the you need.

I've tried hooking the Dell directly to the Mac, and it worked fine then. When I re-inserted the KVM in between the two, it stopped working again.

Not sure what model the Dell is..? Newer ones should have both HDMI and DisplayPort inputs.

To use the DisplayPort input on any display with a newer Mac, you need a USB-C to DisplayPort cable.

you wrote:

<< The Studio feeds its HDMI port into the BenQ monitor and uses a USB-C-to-HDMI cable to feed the Dell monitor (both passing through the KVM). >>

I read that as your connecting the Mac to the BenQ, then connecting the BenQ to the Dell.

If that is NOT what you intended, you need to clarify that language a bit more.

If we mis-communicated, that is a problem -- not necessarily My Problem or Your problem.

Sorry but that's not the way I read it and you're not the first to complain about this.

The switch is the problem. While the Mac can see the monitor, it can not confirm and send video out to that monitor thru the switch.

The work-around is to connect monitor directly to the Mac using one of the monitors inputs and connect the switch from the other system(s) to the monitors other input. Then switch the monitors input when you want to use the Mac or other system(s).

Obi-Wan-YJ wrote:

I'm not sure exactly what you mean by that last paragraph, @den.thed. Each of my monitors has only a single HDMI input, and no Thunderbolt ports.

Most third-party monitors – other than ultra-high-resolution ones that require Thunderbolt input – have multiple inputs. Since you did not identify your monitors in much more detail than to tell us that one was a BenQ monitor and one was a Dell, den.thed probably assumed that they had multiple video inputs.

That was a reasonable assumption, even if, unfortunately, it turned out to be an incorrect one.

there is another factor that may figure into this:

The Mac does not rely on windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.

To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.

the Mac must see the EDID answer-back THROUGH the KVM to know that a display is present. The video connection alone is not enough.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)

The display-name comes in with the rest of the EDID information, so the EDID seems to be working.

Modern MacOS is getting more demanding about error-free operation, and may blank the display if the link incurs errors.

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

(especially for 4K over 60 Hz) “ULTRA High Speed HDMI cable" or that + "48G"

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

To expand on the connections that I mention earlier.

It looks like the BenQ GW2765 has HDMI, DisplayPort and DVI-D.

Use either an HDMI to HDMI cable or a USB-C to DisplayPort cable between the Mac Studio and BenQ.

It looks like the older Dell U2412M has DisplayPort, DVI-D and VGA.

Use either a USB-C to DisplayPort cable or a USB-C to DVI-D cable between the Mac Studio and Dell.

Obi-Wan-YJ wrote:

I didn't realize that you could feed USB-C into DisplayPort. I suppose I could have gone that route initially. By now, I already own the all-HDMI KVM and a slough of high-end HDMI cables, so I'd prefer to stick with HDMI at this point. Replacing all of that would cost almost as much as buying a new monitor.

More importantly, since we don't know why the current cable path is failing, we have no reason to trust that the DP option would work any better than the current HDMI path.

You would have the same problem using a DisplayPort switch.

What I would do, is connect one display thru the switch and the second display directly to the Mac Studio.