Lenovo ThinkVision P40w-20 Issues

[Jip-Hop]

Lenovo ThinkVision P40w-20 Issues

I bought this 39,7" 5120x2160 ultrawide monitor and planned on using it in the following 3 modes (while being able to quickly toggle between modes):

1. 5120x2160 in 2560x1080 HiDPI as second monitor
2. 2x 2560x2160 in 1280x1080 HiDPI as second and third monitor
3. 1x 2560x2160 in 1280x1080 HiDPI as second monitor (for when 3 monitors or ultrawide is too much)

The goal is to do this while connecting just a single thunderbolt 4 cable between my MacBook Pro (M3 Pro) and the monitor.

Unfortunately the True Split feature of the screen, which should be able to support the above configuration, only works with Windows. But ‘thankfully’ this monitor supports daisy chaining another monitor via the downstream thunderbolt port. So I came up with a workaround to connect to two inputs of the same P40w-20 monitor with just one cable to the MacBook:

MacBook —> thunderbolt cable —> P40w-20 —> usb-c to DisplayPort adapter —> DisplayPort cable —> P40w-20

Now when I put the monitor in Picture by Picture (PbP) mode I can use both halves of the screen (mode 2) as individual monitors and thanks to BetterDisplay I can disable the right half to work in mode 3.

However I’m still running into some issues.

When I disable PbP, then the MacBook is still connected to the screen through 2 inputs and another 5120x2160 monitor shows up in display preferences. I don’t want to use this screen in mode 1. I can disable this one through BetterDisplay, but BetterDisplay can’t show me which one to disable. There’s no way for me to identify which screen is the one connected directly (thunderbolt) and which one is connected via DisplayPort. They’re called the same with a number suffix. However confusingly this number suffix doesn’t correspond with the number suffix in macOS Display settings.

In the macOS Display settings I can find which screen I’m looking at in the Arrange Displays window. By clicking the screen a red rectangle appears. I then set the thunderbolt input as primary screen and only then I can visually identify in BetterDisplay which screen I want to keep, and which one I want to disable. But this means I need to go through display settings each time I toggle between the PbP and normal setup.

Could BetterDisplay show a red rectangle when hovering the screens from the menubar? That would be a great help to visually identify identical displays.

Thanks for the useful tool and I’m looking forward to any suggestions!

[waydabber]

I am just adding this feature:

But will add a toggle that will enable identification when hovering over the display header.

[waydabber]

#3747

[Jip-Hop]

What I've tried to get Picture by Picture (PbP) working on my Lenovo ThinkVision P40w-20 with a single cable between monitor and Mac.

Connect a USB-C male to DisplayPort female adapter in the Thunderbolt downstream port of the P40w-20 and then connect with a DisplayPort cable to the DisplayPort input of the P40w-20. Now you have both sides of the PbP connected through a single cable.

However there are issues.

When not using PbP there's always a second display connected (the one via DisplayPort) which you can't look at, since the P40w-20 will be showing the Thunderbolt input signal (or vice versa). I tried to solve this by buying a USB-C Cable with hardware On/Off switch. While this cable worked surprisingly well when connecting it directly between the Mac and the P40w-20, it didn't work (no signal) when used between the Thunderbolt downstream port and DisplayPort input (I tried with a USB-C female-female adapter and the USB-C male to DisplayPort female adapter as well as with a dedicated USB-C female to DisplayPort male adapter). Fortunately with BetterDisplay I can turn off the 'hidden' display, but still it's annoying this 'hidden' display is present on any device I now connect to the P40w-20. Besides unplugging cables at the back of the monitor (or using two cables from monitor to Mac), I can think of one last option: a powered DisplayPort switch with one input and two outputs. But I didn't try this.

Using the P40w-20 in regular and PbP mode means the monitor will be connected to the Mac in different ways.

1. 5120x2160 in 2560x1080 HiDPI (full screen)
2. 2560x2160 in 1280x1080 HiDPI PbP left half
3. 2560x2160 in 1280x1080 HiDPI PbP right half

In all three cases the P40w-20 identifies itself with the same serial number, week of manufacture, vendor ID etc. This may cause macOS and/or BetterDisplay to confuse the displays and swap them.

To distinguish the regular full screen mode from the PbP mode, BetterDisplay can now use the hash of the EDID as a display identification option. This helps maintaining display layout preferences. But this still doesn't fully solve the swapping issue.

This could potentially be solved by writing a custom EDID to the screen. I didn't go that far, as I did't want to risk bricking the screen. But I did confirm I could disable the EDID write protection by going into the P40w-20 Factory OSD menu (disconnect power cable and press + hold MENU while inserting the power cable then choose the little f menu option).

I did buy the Lindy DisplayPort 1.4 EDID Emulator to give the DisplayPort a unique EDID. Without it, connecting both the thunderbolt and DisplayPort inputs of the same P40w-20 in PbP mode to the same Mac may result in displays being swapped. However when using the Lindy I can't communicate over DDC with the DisplayPort connected 'half' of the monitor. But more importantly, the Lindy doesn't seem to support writing a custom EDID. BetterDisplay reports writing the EDID is successful but Lindy continues to report it's own default EDID. It seems the Lindy can only clone, which doesn't help in my case: cloning from my P40w-20 would result in the same EDID and I need a distinct one. So I can use the Lindy with the default EDID and then use BetterDisplay to override the EDID from the Mac, with an EDID I prepared myself (new serial number, week of manufacture, vendor ID etc.). This then does work to uniquely identify the DisplayPort input of the P40w-20.