IBM RT Series Hardware Reference Manual page 437

Port 2
The data format will be as follows:
DO 01 02 03 04 05 06 07
+ + + + + + + +
Marking
I s~~:t I I I I I I I I I
Parity
Bit
Stop
Bit
Data bit 0 is the first bit to be sent or received. The controller
automatically inserts the start bit, the correct parity bit (if
programmed to do so), and the stop bit (1, 1.5, or 2, depending
on the command in the line-control register).
Controller Specifications
The following describes the function of controller input/ output
signals.
Serial/Parallel Adapter 3