Radio Shack TRS-80 model III Owner's Manual page 103

TRS-80
MODEL
III
DISK
SYSTEM
100
PRINT A$(WD)
110 NEXT ND
CMD"P"
Check
Printer Status
mDF\$tatus
status
is
a string variable
cmd"P" makes
it
possible for
Disk basic
to
check
the status
of
the
printer.
Unlike the video display, the printer
is
not always
available.
It
may
be
disconnected, offline,
out of paper,
etc.
In
such
cases,
when you
try to
output
information
to
the
printer,
the
Computer
will
wait
until
the printer
becomes
available.
It
will
appear
to
"hang
up."
To
regain
keyboard
control
(and cancel
the printer operation), press (BREAK)
.
Suppose you have
a
program which
uses printer output.
If
a printer
is
not
available,
you
don't
want
the
Computer
to
stop
and wait
for
it
to
become
available. Instead,
you
may
want
to print a
message
such
as
PRINTER
UNAVAILABLE
and go on
to
some
other operation.
To accomplish
this,
you need
to
check
the printer status,
cmd^p"
can be used
to
check
the printer's status at
any
time.
It
returns the contents as an
Ascn-coded
decimal number. The
specific
value of
this
number
depends
upon
the
type of
printer
you
are using as
well
as
its
status
at
any particular time.
The
value
may
then be printed or
examined by
the
program.
Only
the four
most
significant
bits
are
used
in this "status byte." In binary,
these
must
be:
"0011"
or else the print
operation will not
be
attempted.
To
check
for this
"go"
condition,
and
the status
byte with
240 and compare
the
result
with 48.
The meaning
of
each
status bit
depends on which
printer
you
use.
See
the printer
owner's
manual
for bit
designations.
Sample
Program
10
CMD"P"
#X*
20
SJ1
=
UAL<><$)
AND 240
30
IF
STZ <> 48
THEN PRINT "PRINTER UNAVAILABLE"
:
STOP
40
PRINT "PRINTER AVAILABLE"
50
REM PROGRAM MAY NOW CONTINUE
102