link

That could be usefull for some pepople. There have from time to time been questions on the forum on how to use the BL3500 as a center speaker for a TV where that solution could be used

  Unfortunately the PL power lines (Pin 1 and Pin 4) don’t provide enough current. However, with an external 5 V supply it becomes feasible. Of course the code must be adapted — but I streamlined it so it’s now under 100 lines.

/*

  BeoLab3500_MKII-Powerlink

  Minimal trigger board for a Beolab 3500 MKII on the B&O PL bus:

  hold GPIO5 HIGH and it sends the captured MKII power-on/init frame

  once on GPIO1 - nothing else. No RX, no MK1 support, no other

  commands. Split out of Beolab3500-Standalone's MclBusWriter (see

  link ) - only sendInit() is

  needed here, so it's inlined directly instead of pulling in the

  whole class/enum/MclData hierarchy.

  Per B&O MCL-2 Service Manual (Datalink '86"): the transmitting unit

  pulls the bus LOW for a fixed strobe width then releases it; the

  timing symbol (t1..t5) is the FULL period (LOW+HIGH), not the LOW

  width alone. t1=3.125ms t2=6.250ms t3=9.375ms (data), t4=12.500ms

  (Stop), t5=15.625ms (Start). MKII traffic has one extra trailing t1

  pulse after Stop.

*/

#include <Arduino.h>

constexpr gpio_num_t TX_PIN      = GPIO_NUM_1;

constexpr gpio_num_t TRIGGER_PIN = GPIO_NUM_5;

constexpr uint32_t T1_US = 3125;

constexpr uint32_t T2_US = 6250;

constexpr uint32_t T3_US = 9375;

constexpr uint32_t T4_US = 12500; // Stop

constexpr uint32_t T5_US = 15625; // Start

constexpr uint32_t STROBE_LOW_US = 1562;

// Captured MKII power-on frame (Command=49, unrecognized/literal -

// see Beolab3500-Standalone's main.cpp file header for context).

constexpr const char *INIT_BITS = "0011000111100111111100000000100";

void pulse(uint8_t tcode) {

  uint32_t period = tcode == 1 ? T1_US : tcode == 2 ? T2_US : tcode == 3 ? T3_US : tcode == 4 ? T4_US : T5_US;

  digitalWrite(TX_PIN, HIGH); // transistor on -> bus pulled low

  delayMicroseconds(STROBE_LOW_US);

  digitalWrite(TX_PIN, LOW);  // transistor off -> bus released, pull-up brings it back high

  delayMicroseconds(period - STROBE_LOW_US);

}

// which timing symbol encodes bit given the previous bit (differential code)

uint8_t encodeBit(int lastBit, int bit) {

  if (lastBit == 0) return bit ? 3 : 2;

  return bit ? 2 : 1;

}

void sendInit() {

  Serial.println("-> sending init");

  int lastBit = 1; // Start acts as an implicit "1" reference

  pulse(1); // AGC

  pulse(1); // AGC

  pulse(5); // Start

  for (size_t i = 0; INIT_BITS[i] != '\0'; i++) {

    int bit = INIT_BITS[i] - '0';

    pulse(encodeBit(lastBit, bit));

    lastBit = bit;

  }

  pulse(4); // Stop

  pulse(1); // MKII trailing pulse

}

void setup() {

  Serial.begin(115200);

  delay(500);

  Serial.println("BeoLab3500_MKII-Powerlink");

  pinMode(TX_PIN, OUTPUT);

 digitalWrite(TX_PIN, LOW); // idle: transistor off, bus released

  pinMode(TRIGGER_PIN, INPUT_PULLDOWN);

}

void loop() {

  static bool wasHigh = false;

  bool nowHigh = digitalRead(TRIGGER_PIN) == HIGH;

  if (nowHigh && !wasHigh)

       sendInit(); // edge-triggered - one send per LOW->HIGH transition, not a flood while held high

  wasHigh = nowHigh;

}