OK ...
I have been doing a little digging and I now know what I need to do but not how to do it.
I need to use USART_TX_vect which is hardware serial transmission complete NOT buffer empty wthat occurs before the last bit is sent.
I am using a Mega so I have 4 to chose from with slightly different names USARTn_TX_vect where n is 0-3
Is it as simple as adding code like this:-
The mode of MyEnablePin1 would have been set in the constructor.
It would have been set high when the First character was placed in the transmit buffer.
ISR(USART1_TX_vect)
{
digitalWrite(MyEnablePin1, LOW);
}
Or do I need to be doing something else to enable that particular interrupt and if so, what.