Radio Shack TRS-80 Modem II - 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.
20