Radio Shack TRS-80 Model II Reference Manual page 251

Technical Information
Exit Conditions
NZ
flag No character sent. Check register A for communications status.
C flag Modem carrier not present when SVC was entered.
A Communications status (bit
7
is msb; 0 is Isb):
BIT MEANING WHEN SET
a
Clearto Send (CTS) not present.
1 Not used.
2 Transmitter bUsy.
3 Modem carrier not present.
4 Parity error occurred on last character received. *
5 Data lost due to hardware-level overflow.*
6 Framing error occurred on last character received.*
7 A break sequence (extended null character) has been received.
*
*
Bits 4 through 7 are only used when the character was not sent.
ACTRL
BCTRL
Control Channel A/B (function codes 100/101)
These routines control the RS-232-C interface, Channels A and B.
Entry Conditions
A
=
100 for ACTRL, 101 for BCTRL.
B
=
Option switch
Contents of
Register B Result
0
Get current status of serial
110
Channel into register B
1
Get current character count in the serial receive buffer into B
2 Tum on Request To Send. On when channel is initialized.
3 TumoffRTS
4*
Start transmission of a break-sequence.
S
Stop transmission of break-sequence.
6
Reset serial receive buffer count to zero
7
Reset SIO error condition
*
Before starting to transmit a break sequence, make sure the transmit buffer is
empty.
It
is the programmer's responsibility to send the break sequence for the
appropriate time period, as required by the application or host computer.
287