Radio Shack TRS-80 Owner's Manual page 120

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RS-80
MODEL
111
DISK
SYSTEM
100
PRINT A*(WD)
110
NEXT
WD
CMD"P"
Check
Printer Status
cmd'
p:
status
status
is
a siring variable
cmd"P" makes
it
possible for
Disk basic
to
check the
status
of
the printer.
Unlike
the
video display, the printer
is
not
always
available.
It
may
be
disconnected,
offline,
out of paper,
etc.
In such cases,
when
you
try
to
output
information
to
the printer,
the
Computer
will
wait
until the printer
becomes
available.
It
will
appear
to
"hang
up."
To
regain
keyboard
control
(and cancel
the printer operation),
press
(BREAK)
.
Suppose you have
a
program which
uses
printer
output.
If a
printer
is
not
available,
you don't
want
the
Computer
to
stop
and wait
for
it
to
become
available. Instead,
you
may
want
to
print a
message
such
as
PRINTER
UNAVAILABLE
and
go on
to
some
other operation.
To accomplish
this,
you need
to
check
the printer
status.
cmd"P"
can be used
to
check
the printer's status at
any
time.
It
returns the
contents
as
an Ascii-coded
decimal number.
The
specific
value of
this
number
depends
upon
the type of
printer
you
are using as
well
as
its
status at
any
particular time.
The
value
may
then
be
printed or
examined by
the
program.
Only
the four
most
significant
bits
are
used
in this
"status byte." In binary,
these
must
be:
"0011"
or else the print
operation will not
be attempted.
To
check
for
this
"go"
condition,
and
the status
byte with
240 and compare
the
result
with 48.
The meaning
of each
status bit
depends on which
printer
you
use.
See the printer
owner's
manual
for bit
designations.
Sample Program
10
CMD"P"
,){%
20
ST%
=
VAL
<>($}
AND 250
30 IF
SU
<> 38
THEN PRINT "PRINTER UNAVAILABLE": STOP
40
PRINT "PRINTER AVAILABLE"
50
REM PROGRAM MAY NOW CONTINUE
102