POKE
59411,53 pokes
"53"
into memory address location 59411
which
con-
trois the cassette motor. Value 53 turns
on
the
motor
to advance the tape. Once
the
motor
is
on, a
wait
loop lets the tape advance for a few jiffies. The
wait
loop
will
be
discussed shortly.
To
stop the tape, a
POKE
59411,61 turns off the cassette
motor. The length
of
the
wait
loop may
be
varied or altered, but these
two
POKEs
are
absolutely necessary to turn the cassette
motor
on and off.
Following
is
a sample
wait
loop inserted between the
two
POKE
statements:
1000
POKE
59411.53
101E:1
T=Tl
1020
IF
(TI-T><10
GOTO
1020
1030
POKE
59411.61
1040
G!T=0
1
E:15E:1
F.:ETUF.:t·i
REM
~8TART
TAPE
MOTOR~
:REM
~WAIT
10
JIFFIE8~
REM
~8TOP
TAPE
MOT
OR'
Lines 1010 to 1020 make up the
wait
loop. Line 1010 sets variable T to the
current value
of
TI.
TI
is
the number of jiffies since the
PET
was powered up or the
clock was zeroed.
(A
jiffy
is
1/60
of a second.)
TI
is
incremented once every jiffy,
or 60 times a second.
By
subtracting T from
TI.
the elapsed time
is
calculated. The
program must
wait
until ten jiffies
(1/60
of a second) has elapsed before the pro-
gram can continue. While
TI
increments, until the difference between
TI
and T
equals ten jiffies the program
is
stalled, letting the cassette tape
ad
vance. This
blank space
on
the tape
is
the interrecord gap. Once
(TI-
T)
equals ten, the next
statement turns off the cassette
motor
with
a
POKE
59411,61.
The routine calculates the space between each record. The tape
is
advanced
exactly the same amount between each physical record because the time be-
tween POKEing on and off the cassette
motor
will
always
be
ten jiffies:
The length of the
wait
loop may
be
adjusted by changing the constant of the
condition expression:
TI-T
< X
The larger the value
of
X,
the larger the interrecord gap
will
be.
If you're
unsure
how
long the interrecord gap should
be,
keep the
wait
loop between 5 and
30 jiffies. It
is
always better
to
have the interrecord gap too long than too short.
406
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