First software testing of the Z80 Retro board.

It appears that the SIO /RTS pins don't quite work the way I expected when in Async mode.  I can't use them as a general-purpose output.  I should have used /DTRA and /DTRB for general purpose outputs.

This problem only affects the /PRN_LF signal if and when there is data being rendered by the printer and the console at the same time.

I have done much printing and have not seen any issues. But I should correct the mistake... on Rev 4?

A hand-wire fix looks easy enough to do on a Rev 3 board as well.