Hello,

Thanks for all the answers.It's much appriciated

Kai:

When I I placed data in my TX buffer which was empty I generate a TX interrupt. Then on every TX interrupt (caused by the byte been send) I place the next byte in SBUF. I do this untill the TX buffer is empty again.
When I place data in the TX buffer which wasn't empty I do nothing. The new data is send anyway.

Philip:

I wanted to an RX and TX buffer because in he future there will be some processing of the received data before it's send. To my knowledge that's not possible with only one buffer.

Michael:

Thanks for the sampleprogram but I can't understand assembler that well. Isn't there a simple flowchart of what I want to do? Two circular buffers, int driven and a managing routine?

I think my main problem is the fact that a RX interrupt can arrive at the same time as a TX interrupt.
I'm going to check that.