Changes

=== Control Register (W) ===

The first write after internal or external Reset is the "Mode Instruction":

0-1 Baud Rate Divider (0=Sync Mode, 1=Div1, 2=Div16, 3=Div64)

2-3 Character Length (0=5bit, 1=6bit, 2=7bit, 3=8bit)

3rd write - Sync Character 2

All further writes to the Control Register are "Command Instructions":

0 Transmit Enable (0=Disable, 1=Enable)

1 Data Terminal Ready (0=No, 1=Yes) (DTR Pin)

=== Status Register (R) ===

0 Transmit Ready (0=Busy, 1=Ready; "DB Buffer Empty")

1 Receive Ready

=== Data Register (R/W) ===

0-7 Data

== 8251 vs 8251A ==