Jameco Electronics 3000 User Manual

Figure 12-9 illustrates the standard asynchronous serial output patterns.

If only 7 data bits are being sent, sending an additional parity or signal bit is easily solved 
by sending 8 bits and always setting bit 7 (the eighth bit) of the byte to "1" or “0” depend-
ing on what is desired. No special precautions are needed if two stop bits are to be 
received. If parity is received with 7 data bits, receive the data as 8 bits, and the parity will 
be in the high bit of the byte.

In order to receive parity with 8 data bits, a check is made on each character for a 9th bit 
low. The 9th bit, or parity bit, is low if bit 6 of the serial port status register (SxSR) is set to 
a "1" after the character is received.  If the 9th bit is not a zero, then the serial port treats it 
as an extra stop bit. So if the 9th bit low flag is not set, it should be assumed that the parity 
bit is a "1."

Setting the 9th bit high or low can easily be done in the Rabbit 3000.  The 9th bit can be 
set low by a write to the Serial Port A-F Address Register (SxAR) and the 9th bit can be 
set high by a write to the Serial Port A-F Long Stop Register (SxLR).