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

Question

Created 5d

Replies 5

Boosts 0

Participants 3

My MacBook Pro M5 running MacOS Tahoe 26.3 beta fails to detect two identical ASUS ROG Swift OLED PG32UCDM monitors simultaneously. Only one display is recognized at a time.

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).

I've verified that the monitor EDID binary serial numbers are in fact identical -- however the alphanumerical serial numbers are not identical.

NOTE: This behavior is specific to the MacBook Pro M5 — when connecting both monitors via usb-c to a Mac Mini M4 Pro running the same MacOS Tahoe 26.3 beta, the monitors work fine. The OS detects both and assigns different names to them (PG32UCDM (1) and PG32UCDM (2)).

NOTE: I could be wrong about this root cause, I don't have a way to disprove it, though the fact the monitors work fine on a Mac Mini is suspicious.

What I have tried:

Connecting the two monitors using different monitor ports (one on DisplayPort, another on HDMI, etc.), and different MacBook ports (one on HDMI, another on USB-C, etc.)
Bumping down the resolution on the monitors to "1920x1080 (low resolution)" and 30Hz to rule out bandwidth issues.
Connecting one, or both, monitors to CalDigit TS5 Plus dock. Neither alternate configuration yields the device recognizing both screens.
Using BetterDisplay to import a manually-edited EDID for the screen, with a different binary EDID value, manufacturer name, etc.

I've also verified that if I plug in my Apple Studio Display as one of the monitors, then the MacBook recognizes both one of the PG32UCDM monitors and the Studio Display at the same time. The issue seems to occur only when both monitors plugged into it are the same PG32UCDM model.

When I have both monitors plugged into my MacBook, each time I disconnect the cable to whichever monitor is currently recognized, it immediately recognizes the other monitor. Plugging the cable for the disconnected monitor back in has no effect.

I'm at a loss.

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

Boost

Answer 1

Melog OP

5d

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.

0

Answer 2

DTS Engineer OP

Apple

5d

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

__
Kevin Elliott
DTS Engineer, CoreOS/Hardware

1

Answer 3

Melog OP

3d

Yes: FB21777337

0

Answer 4

DTS Engineer OP

Apple

1d

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).

__
Kevin Elliott
DTS Engineer, CoreOS/Hardware

0

Answer 5

Miloian82 OP

6h

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.

0