Line 19 The OPEN statement defines the serial port for input
as
file number 1 at 300
baud, 7 data bits, odd parity, 1 stop bit and
XON/XOFF
disabled.
Note: This configuration should parallel that
of
the peripheral device. Change the
configuration if
it
is
not the same.
Line
29 An interrupt subroutine beginning at line number 100 will be executed
if
a
character
is
received through the serial port.
Line
39 The serial port interrupt l:apability
is
turned on. You can prevent the serial
port from interrupting the program with the
COM
OFF
statement. Similarly,
if
you wish
to
delay interrupts, you can use the
COM
STOP
statement which defers the interrupt until a COM ON statement
is
executed.
Line
49 This loop prints numbers starting at zero and incrementing by one each time.
Since it
is
an infinite loop, you must press
~
to
terminate the program.
Line
199 When a character is received through the serial port, an interrupt
is
generated and execution jumps
to
this line. No interrupts can occur during an interrupt
subroutine.
If
a character
is
received during the interrupt subroutine,
it
will be
remembered, and another interrupt will occur when the current one
is
completed.
This line will display the received character and then return to the main program. The
INPUT$
(l,
l)
statement inputs one character from file number
1.
In general, the
function
INPUT$ (n,t)
will return
n characters from file numberf. In the case
of
the COM: device, the
program will wait until all
n characters are received before it returns.
Experiment
#8
Multiple
Character
Interrupt
While the procedure in the previous experiment works well with short strings, it will
not work if the peripheral device sends strings longer than around
20. characters at a
time. You can handle this situation by having the interrupt subroutine input all the
characters in the string before returning.
This experiment shows how
to
modify the program to accommodate longer strings
with interrupts. Change line
100
to
100
N$
= INPUT$ (1
d)
: PRINT N$;
and add two new lines
110
IF
N$<>CHR$(13)
GOTO
100
120
PRINT :
RETURN
173
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