Radio Shack TRS-80 Modem II Operation Manuals page 26

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The
Modem
II
can be used
in
two
general
ways:
•
Attended
use,
in
which
a
Modern
II is
connected
to
a
Computer which
is
constantly
being used
by
a
person.
•
Unattended
use, in
which
the
Modem
II is
connected
to
a
Computer which
is
maintained
by
software
and
an operator
is
not present.
A
good example
of attended use
is
a
TRS-80 Model
111
running
VIDTEX
software
to
access an
information service (such as
CompuServe
or
Dow
Jones). In
this
instance,
you
type
on
the
keyboard and
VIDTEX
transmits, receives,
and echoes
the characters to
and from
the
Modem.
When
the
session
is
over,
you
turn the
Modem
off.
An
example of unattended use might be an auto parts
dealer
who
writes
an assembly-language
program
for his
TRS-80 Model
16.
The
program might
dial the
home
office
computer
at
2:00
A.M. when
the
phone
rates are
cheaper. Daily
reports
could then be
downloaded.
As
far as the
Modem
II is
concerned, there are
no
real
differences
between attended
and
unattended
use.
However, attended use tends
to
mask
subtle timing restraints
(humans
are
slow)
so timing
is
important
to
correctly
program
the
Modem.
For unattended use,
VIDTEX
is
unsuitable.
This
is
because
there
is
not
an operator present
to
type
on
the
keyboard and no one
to
read
the
Screen
for
information
that
must be responded
to.
Therefore,
you must write different routines
to
program
the
Modem
to dial
and
to
turn the
Modem
off
when
the session
is
over.
The
first
thing to
do
is
to
place the
Modem
in
programming mode. This can be
done two ways,
depending on
the setting
of the
FORCE DTR
Switch:
•
If
the
Switch
is
ON,
an asterisk (*)
must be
sent.
•If
the
Switch
is
OFF,
either
send an
asterisk (*)
or
drop
DTR.
If
the
Modem
is
in
programming
mode
(remember,
it
must
be
in
an
idle state first),
the
TR
Light
will
go
off.
For attended
use, this signals
you
that all
entries
typed
from
the
Computer
will
be
interpreted as
Modem
programming commands.
Since you
will
not be there for
unattended
use,
there
must be another
way
to
signify
programming mode.
This
is
done by
character
echo
hand-shaking.
The Modem's
microprocessor constantly
monitors
the
incoming
data
from
the
Computer.
When
the
Modem
is
in
programming mode,
this
data
is
examined
for syntax errors. If
an
error
occurs,
the
Modem
transmits a question
mark
(?).
If
the data
is
a valid
command
or data string,
it
is
transmitted
back
to
the
Computer
at
300 baud.
Therefore,
to
program
the
Modem
II,
follow these
steps:
1.
Enter
the
programming mode.
If
an
*
is
used, the
Modem
may
not accept the
first
time because of
interrupt
handling.
When
the
programming
mode
is
entered, the
* will
be
sent to the
Computer. Therefore,
the
Computer program must
be
set
up
to
send an
* until
there
is
an
echo back from
the
Modem.
2.
Send one character (such as a character in a
phone number)
to
the
Modem
at a
time.
This
means you must
wait until
a character
is
echoed back before sending the next character.
This
is
necessary because
the
Modem's
interrupt stack
would
overflow causing
loss
of
data.
3.
There
is
a
time delay required
between
the
time the character
echo
is
received and
the
next
character
is
sent.
This time
must
be
a
minimum
of 33 ms.
4.
After
all
characters
have been
sent,
the
Modem
will
wait
until
DTR
is
asserted before
it
will
execute
the
program
and
dial
the
number.
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