Radio Shack TRS-80 Model II Reference Manual page 243

Technical Information
Using the Serial Communications SVC's
The Model II contains two serial channels,
A
and
B.
If you are going to use only one
channel, the other channel must be connected to the Serial Dummy Terminator
described on page
4/77.
Initialization
Before performing any serial
liD,
you must initialize the desired channel with either
the library command
SETCOM
or the
SVC RS232C.
Input/Output
All serial
110
is character-oriented, rather like keyboard input and video output. The
major difference is that your program must check the communications status at
various times to ensure that the communications link is in place and that data is not
being lost for one reason or another.
After any attempted serial
liD,
the communications status is returned to your
program in
Z-80
register
A.
The bits in the register are used individually to show the
on/off status of various conditions. Certain bits apply to transmit operations; others
to receive operations; others to all serial
liD.
In the following tables, bit
7
refers to the most significant bit in a byte; bit
0,
to the
least significant.
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