MacBook Pro m5 can’t recognize two external monitors with same EDID binary serial (only one works at a time)

One additional note — when I have both plugged in, and I unplug the one being recognized, it immediately recognizes the other one and applies whatever screen settings I had set on the now-unplugged monitor to the other one, suggesting it thinks it’s the same monitor plugged back in.

Have you filed a bug on this and, if so, what's the bug number?

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Kevin Elliott
DTS Engineer, CoreOS/Hardware

Yes: FB21777337

First off, jumping back to here:

One potential root cause might be that both monitors report identical binary EDID serial numbers (0x01010101), and the MacBook Pro M5 appears to use this value exclusively for display identity rather than combining it with other more detailed information (e.g., port, or alphanumeric serial number).

Have you contacted the display vendor about this? Setting all other issues aside, at a minimum the duplicate value is "weird" and possibly out of spec. The OS should handle this, but that doesn't mean the monitor should be doing this.

Yes: FB21777337

Looking at the bug, are you absolutely certain both monitors were plugged in when you captured the sysdiag?

My concern here is that, based on the registry state at the time of capture, I can't see ANY evidence of the second monitor at ALL. That's very strange, as the management of the bus itself is totally separate from the higher level display logic. It's possible that I'm overlooking something, but if both monitors were plugged in at the time you triggered the sysdiag then this would imply that something is going on at a much lower level.

If you can, what I think would be most helpful here is a sysdiag captured with both devices attached via USB AND with a USB device attached to both monitors. This helps validate the general bus configuration outside of the display system.

Has anyone run into this issue and found a successful workaround that is not one of the approaches I've described above?

I'm not sure of the exact hardware you need, but EDID emulators/adapter dongles are widely available and I suspect the "right" one would make this work fine. Basically, you need to force a configuration to the mac that's indisputably "different" (the right KVM switch might work as well).

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Kevin Elliott
DTS Engineer, CoreOS/Hardware

I have the same issue with my m5. Matching Asus monitors on a CalDigit ts4. Both using DP to USBC adapters from Cable Matters.

26.2. Works fine on my M1 Pro. Not on my M5. I contacted CalDigit and they're blaming macOS. very frustrating since my M5 is work issued and I can't use my second monitor.

I have the same issue with my M5. Matching Asus monitors on a CalDigit TS4. Both using DP to USB-C adapters from Cable Matters.

Have you filed a bug on this and, if so, what's the bug number?

Critically, I'm still looking for a sysdiagnose that's been collected with this configuration:

"If you can, what I think would be most helpful here is a sysdiag captured with both devices attached via USB AND with a USB device attached to both monitors. This helps validate the general bus configuration outside of the display system."

Finally, going back over all the data, there is a difference between the maximum display configuration of the Mac Mini M4 Pro (working):

Supports two external displays in any one of these configurations:

One display up to a native resolution of 8K (7680 x 4320) at 60Hz or 4K (3840 x 2160) at 240Hz over Thunderbolt or HDMI, and one display up to a native resolution of 5K (5120 x 2880) at 60Hz over Thunderbolt

One display up to a native resolution of 8K (7680 x 4320) at 60Hz (or 4K at 240Hz) over Thunderbolt, and one display up to a native resolution of 4K (3840 x 2160) at 60Hz over HDMI

Two displays up to a native resolution of 6K (6144 x 3456) at 60Hz or 4K (3840 x 2160) at 144Hz over Thunderbolt or HDMI

And the MacBook Pro M5 (failing):

Supports one display in the following configuration:

One display up to a native resolution of 8K (7680 x 4320) at 60Hz or 4K (3840 x 2160) at 240 Hz over Thunderbolt or HDMI.

Supports two displays in the following configuration:

Two displays up to a native resolution of 6K (6144 x 3456) at 60Hz or 4K (3840 x 2160) at 144Hz over Thunderbolt or HDMI.

Unfortunately, that difference might be enough to prevent the MacBook Pro from working.

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Kevin Elliott
DTS Engineer, CoreOS/Hardware

Unfortunately the duplicate binary EDID issue is a known, common practice among monitor manufacturers - they'll often use the same binary edid for a full batch of monitors to cut on complexity or cost. Asus does not offer a tool that can let me patch this on the monitor, and I couldn't find a DIY solution that lets me patch it myself.

I'm 100% certain both monitors were plugged in. I've replicated this again just now to get you a new sysdiag as well.

It's worth noting - I am dropping the monitor to "1920x1080 (low resolution) @ 30Hz" specifically to mitigate the risk that it's a bandwidth issue.

It's also worth repeating that when I disconnect one monitor the other one is immediately recognized and connected, and it adopts the profile of the first monitor (i.e., it adopts the 1920x1080 @ 30Hz setting, which is non-standard for this monitor), which makes me think the system believes they're the same monitor.

Also, worth repeating that if I plug in a Apple Studio Display ("5K Retina") and one of my monitors, it detects both of them successfully.

Anyhow, I've replied into the bug report via Feedback Assistant with a new sysdiag result attached.