The
Internal Modem Program
The Modem
IPs
internal
program is ROM based. (See the
Operational Flowchart
in
Appendix
C
for details on program execution.) The program is
based on receiving two elements:
•
The RS-232 control DTR.
•
ASCII text strings.
These text strings must be sent in a certain format when the
Modem is in programming mode.
*ru~ „4._:_.— —„.. u„ „ —* .,„: — r»
a
cir<
T7/"\n
r
Ti
me duiug& may uC acui using ijs\*jik*, iuimi
software which has RS-232
drivers/receivers.
me duiug& may uC acui using ijs\*jik*, i x/i\x
ry^-ii
1
*,
aaatuiuiy-iauguagc, v ilj lun., ui any uuiui
The
UART's of TRS-80 Computers and Data Terminals must
have
carrier detect
asserted
at
their
end to allow data transfer. However, the Modem does not usually assert
carrier detect until it
receives carrier from another modem. Therefore, the
Modem asserts carrier to the Computer at
selected times. If the FORCE DTR switch is set to ON,
carrier detect is always forced to the
Computer.
The
Computer software never knows when the Modem has been disconnected from
the
phone line
by
examination of the carrier detect bit in the UART status register.
If the Computer software requires carrier detect to signal loss of carrier, the FORCE DTR
Switch
must be set to OFF. However, you must realize that carrier detect is
controlled by
the
internal
microprocessor and close attention must be paid between the
Modem II forcing carrier (to allow
the text strings to be sent) and the actual carrier detect
resulting from another modem.
odem II Carrier Detect
Protocol
The
following discussion
refers to
AUTO/ORIGinate Mode only. (In the Manual Modes, the
microprocessor does not force carrier. Therefore, the carrier detect the
Computer
sees is
the true
carrier.)
In AUTO/ORIGinate Mode, the Modem forces carrier (not the true
carrier) when:
•
The Modem is in an idle state and AUTO/ORIG.
•
The Modem is in programming mode.
•
The
time
from
entry
into programming mode
until two seconds after the number has been
dialed. After the two second delay, the Modem releases the forced carrier
detect
and the
CD
line to the Computer is off. When the true carrier is detected, the
CD line will go on again.
•
During LOCal Test.
There are
several
considerations concerning this protocol.
For instance:
How long does it take the Modem to dial? For details, see
Appendix A.
What
if
another modem answers the phone
before
the two
seconds are up? If this happens, the
"other" modem is not conforming to Bell 103J standards for
auto-answer modems and we do
not support such cases.
The Modem II does not force carrier in the idle state of
the AUTO/ANSwer Mode.
However,
the
Modem
will dial
out and go to the Originate Mode if
properly programmed. This is called
"override" and needs to be explained
in
detail.
When the Modem II was designed, compromises were made
between how much functionality
could be packed into the ROM (it's only IK!) and ease of
programming. The
override
feature is
useful since you can have
one modem do the
job of two
—
the Modem II can be
switched in
software to AUTO/ORIGinate even through the switches are set to
AUTO/ANSwer and the
Modem may be 200 feet away in a locked closet.
18
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