Comparison of Byte-Wide Communications Ports, ACIAs, UARTs, etc.
       ================================================================

               Data     TxStatus
            Read/Write  RxStatus  TxRdy  RxRdy  RxControl  RTS=Stop  RTS=Cont
------------------------------------------------------------------------------
2681 DUART    base+3     base+1    &08    &02    base+2
6402 UART     base+0       -        -      -       -
6551 ACIA     base+0     base+1    &10    &08    base+2   %xxxx00x0  %xxxx10x1
6850 ACIA     base+1     base+0    &02    &01    base+0   %x1xxxxxx  %x0xxxxxx
6854 ADLC     base+2     base+0    &40    &01    base+1   %?xxxxxxx  %?xxxxxxx
8251 UART     base+0     base+1    &01    &02    base+1   %xx?xxxxx  %xx?xxxxx
8279          base+0     base+1
9914 IEEE     base+7     base+3    &10    &20      -
Tube          base+1     base+0    &40    &80      -      Write Data Read Data
USB           base+0     base+1   &40=0  &80=0     -
Z80 SIO       base+0     base+1    &04    &01    base+1   %xxxxxx?x  %xxxxxx?x
ASCII KBD     base+x     base+y           &80

VIA
PIO





ASCII KBD:
LDA STATUS
AND #&80
BEQ LOOP
LDA DATA

ASCII KBD via User Port:
LOOP:
LDA VIA+13
AND #&10
BEQ LOOP
LDA VIA+0