^iwi^a^i^^ffg
i
;i
ffS^ffij
^a
p
'
»BW'm
i
ij
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
To Next Page
Loading...