diff -r fc55f6eaa8bc -r c146d19101a3 src/hp34comm.cpp
--- a/src/hp34comm.cpp	Sat Nov 07 19:24:11 2020 +0100
+++ b/src/hp34comm.cpp	Mon Nov 09 23:05:24 2020 +0100
@@ -32,6 +32,7 @@
   serial.baud(187500);
   serial.format(8, BufferedSerial::Even, 1);
   cur_state = STATE_IDLE;
+  send_thread.start(callback(this, &HPSerial::send_pending_key));
   serial.attach(callback(this, &HPSerial::rx_irq), SerialBase::RxIrq);
 }
 
@@ -69,6 +70,118 @@
     sendbuf.push(keycode);
 }
 
+
+bool HPSerial::wait_for(uint8_t value)
+{
+  char c;
+
+  for(uint8_t i=0; i<2; i++)
+  {
+    while(!serial.readable())
+		wait_us(10);
+	//ThisThread::sleep_for(0.1ms);
+	serial.read(&c, 1);
+	if (value == c)
+		return true;
+  }
+  return false;
+}
+
+void HPSerial::send_pending_key() {
+  uint8_t c;
+
+  while(true)
+  {
+	  if (!sendbuf.empty())
+	  {
+		  if (cur_gstate == GSTATE_IDLE)
+		  {
+			  serial.attach(0, SerialBase::RxIrq);
+			  cur_gstate = GSTATE_TX;
+
+			  c = 0x66;
+			  serial.write(&c, 1);
+			  if (!wait_for(0x99)) {}
+			  // break; // XXX what to do?
+
+			  sendbuf.pop(c);
+			  serial.write(&c, 1);
+			  if (!wait_for(0x00)) {}
+
+			  c = 0x55;
+			  serial.write(&c, 1);
+			  cur_gstate = GSTATE_IDLE;
+			  serial.attach(callback(this, &HPSerial::rx_irq), SerialBase::RxIrq);
+		  }
+	  }
+	  //else // prevent from flooding the main unit
+	  ThisThread::sleep_for(5ms);
+  }
+}
+
+void HPSerial::send_startup_seq(uint8_t keycode) {
+  uint8_t c;
+
+  while (cur_gstate != GSTATE_IDLE) {
+	  ThisThread::sleep_for(1ms);
+  }
+
+  serial.attach(0, SerialBase::RxIrq);
+  cur_gstate = GSTATE_TX;
+
+  // Send the init seq 0x33 0x02 0xFF <keycode> 0x55
+  c = 0x33;
+  serial.write(&c, 1);
+  if (!wait_for(0xCC)) {}
+
+  c = 0x02;
+  serial.write(&c, 1);
+  if (!wait_for(0x00)) {}
+
+  c = 0xFF;
+  serial.write(&c, 1);
+  if (!wait_for(0x00)) {}
+
+  c = keycode;
+  serial.write(&c, 1);
+  if (!wait_for(0x00)) {}
+
+  c = 0x55;
+  serial.write(&c, 1);
+  cur_gstate = GSTATE_IDLE;
+  serial.attach(callback(this, &HPSerial::rx_irq), SerialBase::RxIrq);
+
+}
+
+void HPSerial::send_startup_seq() {
+  uint8_t c;
+
+  while (cur_gstate != GSTATE_IDLE) {
+	  ThisThread::sleep_for(1ms);
+  }
+
+  serial.attach(0, SerialBase::RxIrq);
+  cur_gstate = GSTATE_TX;
+
+  // Send the init seq 0x33 0x02 0x00 0x55
+  c = 0x33;
+  serial.write(&c, 1);
+  if (!wait_for(0xCC)) {}
+
+  c = 0x02;
+  serial.write(&c, 1);
+  if (!wait_for(0x00)) {}
+
+  c = 0x00;
+  serial.write(&c, 1);
+  if (!wait_for(0x00)) {}
+
+  c = 0x55;
+  serial.write(&c, 1);
+  cur_gstate = GSTATE_IDLE;
+  serial.attach(callback(this, &HPSerial::rx_irq), SerialBase::RxIrq);
+}
+
 void HPSerial::send_key_when_idle() {
   if (!sendbuf.empty() && cur_gstate == GSTATE_IDLE)
   {