Radio Shack TRS-80 Modem II Operation Manuals page 25

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If
your
Computer
requires carrier detect
before data can be
transmitted, the
Computer
software
must be
written so that instead
of
polling carrier detect,
it
must
rely
on
a
scheme
independent of
carrier detect.
This
is
most
easily
solved by
the
remote sending
a carriage return or
some
other
code
to
tell
it
when
it
has connected. This
requires the
remote
user
to
begin
communications.
In
fact,
there
is little
software difference
to polling the
UART
status register as
opposed
to
polling
the
UART
holding
buffer.
Once
a remote connection has been detected
when
the
Modem
is in
the
AUTO/ANSwer Mode
with
carrier
forced by
the
FORCE DTR
Switch,
it
is
necessary
to detect a
disconnect
as well.
The Computer
software must
rely
on
the
remote
user
to
type
a sign-off
code
(such as
GOODBYE,
etc.)
since loss
of
carrier can't
be
used.
This
would
tell
the
Computer
software user
to
drop
the carrier. If he/she
does
so, the
Modem
II
will detect
loss
of user carrier
and drop
the
phone
line
connection. After a
three
second
delay, the
Modem
will
go back
to
the
state set
by
the
front
panel
switches.
If
the
remote user
is
using a
Modem
II
and
types a sign-off
message
but leaves the
Modem
on,
you
can
turn his
Modem
off
through
the
use of a long space
disconnect.
To
do
this,
simply
instmct
the
UART
to
send
a
SPACE
disconnect (no
start
or stop
bits)
and
the
following
will
happen:
• The
host
Modem
II
will
drop
carrier after
receiving
a 1.5
second
SPACE
and drop
the
phone
line.
• The
remote
Modem
II
will
drop
the
phone
line
when
it
sees loss
of
host
carrier.
This technique
will also
work
if
the host
is
a
Modem
I.
In this case, the
remote
Modem
II
will
drop
the line after
reception of a
SPACE
of three seconds.
Modem
II
Timing
Protocol
The
Modem
II
has
several
software timers
implemented
in
its
program. These
timers are
used
for
various reasons and
are fairly accurate^ since the
microprocessor's clock
is
crystal controlled.
The
Operational Flowchart
(see
Appendix
C) should be
studied
if
the
Modem
is
to
be used
for
unattended
operation.
Table
3 describes the
Modem
II
timers:
Modem
II
Software
Timers
Time
Function
6 seconds
The
6 second
timer
is
used
for a
"warm
start" condition.
It is
invoked
when power
is
applied.
3
seconds
The
3
second
timer
is
used
for a
disconnect
timer. After
receiving a disconnect, the
Modem
drops
the
phone
line
and
delays three
seconds before executing
the front panel switches
2 seconds
The
2
second
timer
is
the Bell 103J standard
The
delay
is set
from
the
time
the
Modem
seizes the
phone
line to
the
time
it
transmits
carrier.
This
is
used
in
all
modes.
12 seconds There
is
a
12 second delay from
when
the
Modem
transmits carrier to
disconnect
if
remote
carrier
is
not detected.
33 ms. This
is
the
time
to transmit a character to or
from
the
Modem
II
(10
bits at
300
baud). Also,
the
delay time
for
the
echo hand-shaking scheme
used
to
program
the
Modem.
Table
3
19