MDB USART mystery

hello!

i have connected uno r3 circuit desribed in nama mdb 4.2 standard can "talk" vending machine.

code: [select]

void init_mdb() {
  // set baud rate
  ubrr0h = ubrrh_value;
  ubrr0l = ubrrl_value;
  // disable usart rate doubler (arduino bootloader leaves enabled...)
  ucsr0a &= ~(1 << u2x0);

  // set asynchronous
  ucsr0c = (0<<umsel01)|(0<<umsel00);
  // set 1 stop bit
  ucsr0c |= (0<<usbs0);
  // set no parity
  ucsr0c |= (0<<upm01)|(0<<upm00);
  // set 9bit
  ucsr0c |=(1<<ucsz01)|(1<<ucsz00);
  ucsr0b |= (1<<ucsz02);

  ucsr0b |= (1<<rxen0);  // enable rx
  //ucsr0b |= (1<<txen0);  // enable tx
}
i can receive data fine.  when try enable transmission

code: [select]

  ucsr0b |= (1<<txen0);  // enable tx
the vending machine goes "out of service" mode.  not sending anything.  thing i've done enable bit.

is possible arduino transmitting , don't know it?  maybe tx set high or something?

should disable tx while don't need it?

will toggling @ run time introduce noise network , screw things up?

thank you.

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