I also agree on short IRQ routines. And why bother displaying all characters on the LCD. You won't be able to read them yourself at a rate of 240 characters per second! Make a selection of what you want to display, and refresh the display at a slow pace.

Jacky, I still don't see the reason why you store 0 in a memory location first, before storing the Accu in it. You don't have to clear a RAM location before writing something to it.

The transmission rate can be calculated like this:

One bit time at 2400 baud takes 1/2400th part of a second, i.e. 0.000417 seconds. One byte is 8 bits long, but one start and one stop bit is added to that to synchronise. So to transmit 1 byte it takes 0.00417 seconds.