Beolab 3500 Mk II
Tested with SW 3.33

When the Beolab 3500 Mk II is in PL mode, it behaves like a normal BL speaker (BL6000, BL8000): no IR reception, no SelectSource, and also no Vol+/Vol-. A speaker like that having its own display doesn't really make sense in the first place.

⚠️ Big warning: PL-level audio signals are much lower than a typical Line In / audio jack level. Since the Beolab 3500 Mk II's volume can't be controlled over PL (see above), feeding it a normal Line-In-level signal directly risks destroying it. Attenuate the input first — a voltage divider, or a level control on the source device — before connecting it.

The basic idea

Without display
Simple, two things needed — tie PL4 to +5V, and send 0011000111100111111100000000100 on PL6 / PL7.

With display
It gets more complicated — two additional requirements:
Mute BL Pin4 must only go HIGH after that init sequence (0011000111100111111100000000100) has finished sending — it has to stay LOW for the whole duration of the sequence.

⚠️Remark : Sending PL commands to BL3500 its different than usual PL commands:
After the frame's normal Stop (t4), one more t1 pulse (3.125ms) is required!
During dev  Beolab3500MKII_PL i recognize that the additional t1 in only needed for the init sequence. All other PL commands are as usual

Congrats!

@Klaus thanks and Made the first Beolab 3500 MK2 (PL) standalone

First of, very nice with these relatively simple solutions for making the BL3500 works as stand alone speaker :-) Might have to try that out when I have some spare time. A little comment to your earlier statement:

link

Just to clarify from my own and other peoples testing the BL3500 MK II  can only work as a normal Powerlink speaker when used with a Beolink Wireless 1 which send the correct signal to wake it up. I have not been successful at getting the BL3500 MK II going with other Powerlink sources.  

Yes, and the device (ESP32) does exactly the same as the BeoLink Wireless 1. See the post: https://forum.beoworld.org/forums/topic/bl3500-and-the-mcl-pl-connector/page/8/#post-125117. What I wanted to point out with that post was that with the MK1 you have volume control / source selection, but with the MK2 you don’t — just like with the BeoLab 6000. BR

link

What I wanted to point out with that post was that with the MK1 you have volume control / source selection, but with the MK2 you don’t — just like with the BeoLab 6000.

That makes sense. One can only wonder why B&O choose to make the complicated" activation of the MK II via the Powerlink connecter instead of just using the 5V power on signal like other Beolabs. My best guess is the B&O only needed to redesign the Plug and Crossfield PCB and could leave the rest of the internals of the BL3500 like the MK I.

0011000111100111111100000000100

This is a curious command. If I'm decoding it correctly, this reads as follows:

00:1100:01111:0011:1111:1000:0000:0100, or "Send to 'All' an 'AND' command with N-Count of 4" + data. 'N-Count of 4' is a reserved command space listed as dedicated IR test data in the original specification for Datalink.  Perhaps it has some other benefits in this scenario.

Madskp wrote: Just to clarify from my own and other peoples testing the BL3500 MK II  can only work as a normal Powerlink speaker when used with a Beolink Wireless 1 which send the correct signal to wake it up. I have not been successful at getting the BL3500 MK II going with other Powerlink sources.

That brings me to one more use case. If I connect the MK2 version of this project between a Powerlink source and the BL3500, and PIN 4 (5 V+) is enough to power the ESP32, I can transform a BL3500 into a normal Powerlink speaker. just send the init sequence to BL3500 when ESP32 boot. (and it boots every time Pin4 gets +5v :) will test it soon