Radio Shack TRS-80 Model II Reference Manual page 244

Modell! TRSDOS
Status byte in register
A
after serial output
(A/BTX):
BIT MEANING WHEN SET
0
Clear to Send (ers) was not detected.
1
Not used.
2 Transmitter is busy.
3 Modem carrier was lost.
4 Not used
5 Not used
6 Not used
7
Not used
Status byte in register A after serial input
(AlB
ReV):
BIT MEANING WHEN SET
0 Not used
1 Not used
2 Not used
3 Modem carrier was lost
4 Parity error occured on character found in
register B.
5 Data lost-more than 16 characters
received between sve's. B. contains last
character received.
6
Framing error occurred on last character
received.
7 A "break sequence" (extended null
character) was received. B will have a X'OO' in it.
It is possible to obtain the status of a serial channel without attemptmg I/O, using
A/BCTRL. These SVC's return the following information in register B.
BIT MEANING WHEN SET
0 Clear to Send is not present,
1 Not used.
2 Transmitter is busy.
3
Modem carrier is not present.
4
Parity error is occurring on character
currently being received.
5 Data is being lost due to overflow at
hardware level (a rare occurrence)
6 Framing error is occurring on the
character now being received.
7 A break sequence (extended null
character) is now being received.
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