Back from Holidays

Back from Holidays, a wonderful week in Sardinia with sun, good food and fantastic sea.
I've tried doing a small schematic with gEDA but in the end I went for "Paintbrush", sorry if my skills are not "excellent". When I use linux I'll try to use Dia or a similar for later postings.

I've decided to use the SID and SOD lines for auxiliary input (RDR: - reader in CP/M 2.2) and auxiliary output (PUN: - punch in CP/M 2.2).

I used RST5.5 for direct serial input, the inverter (74LS04) is needed for generating a interrupt (5V) at the start bit (GND), then interrupts are temporarily disabled during reception. A timing loop waits for the middle of the start bit and checks if it is still low (i.e. the RST5.5 pin is at 5V as it is inverted), if not it was a false start bit and exits the interrupt service routine (ISR). From then on reception depends on a software timing loop, inversion of the sensed interrupt line, and shifting the bit in.
The received character is placed in a buffer and a buffer full flag byte is set. The code is posted below as a picture. I'm still not very "experienced" with blogging and placing code snippets online, so I made a "png" with yellow background, it looked better than any other alternative...