M
IDIAN
E
LECTRONICS
I
NCORPORATED
P
AGE
7
TDM-1 v 1.0
5-12-00
To disable the addressability feature, make certain the first
character of all four decode addresses is a dash (-) character
(the characters following the dash are ignored).
3. TECHNICAL INFORMATION
3.1
RS-232 H
ANDSHAKING
The CTS line is used by the modem during key-up delay to
prevent the DTE it is connected to from sending more
characters. The modem will buffer up to 63 characters during
the key-up delay, which allows for 500 ms of key-up delay
even if the CTS signal is ignored by the device connected to
the RS-232 port.
If the CTS signal is ignored or not connected, and the key-up
delay exceeds 500 ms, data will be lost: Each new character
after the 63
rd
will cause the oldest character in the buffer to be
discarded.
Normally, the CTS signal will be dropped as soon as the 61
st
character is received, with the expectation being that the
connected device will buffer characters itself until CTS is once
again asserted (after front porch key-up delay period is
complete).
There is no buffering or handshaking in the opposite direction.
The DTE must be able to accept data coming in from the
modem at the same rate as the data is being received off the
air.
3.2
M
ODEM TO
R
ADIO
I
NTERFACE
As soon as data is received from the DTE, PTT to the radio
will be asserted (except when busy lockout is active). The
modem
carrier
tone
is
also
turned
on.
Actual
data
transmission on the TX out wire begins after the front porch
key-up delay.
Audio recieved from the radio is fed to the RX in wire and
translated to data and sent to the DTE’s RS-232 port. In order
to prevent noise on the air being translated as garbage
characters, COR is used. If COR is not active the modem
ignores incoming noise, voice, or data. If no COR wire is
used, data will pass all of the time since COR is pulled high
and programmed for positive polarity (by default). In this
configuration, the COR wire may be tied to PTT (of negative
polarity) to enforce simplex operation.
This will also keep
noise or voice on the RX path from being interpreted as
garbage data while transmitting.
3.3
T
ROUBLESHOOTING
Q & A
Is there any way I can test my TDM-1 to be sure it is okay?
Yes, you can perform a loop-back self-test. The TDM-1, as
shipped from the factory, may be self-tested using a dumb
terminal or dumb terminal program. Configure the terminal
settings for 1200 baud, 8 data bits, 1 stop bit, and no
parity.
Also
configure
the
terminal
for
FULL
DUPLEX operation. Connect the blue and yellow
wires of the TDM-1 together, connect the TDM-1 to
the computer (or terminal), and apply power to the
TDM-1. Characters typed at the terminal should be
echoed back to the terminal by the modem. Note
that there will be a 0.4 second delay from the time
the 1
st
character is typed, to the time it is echoed
back. This is due to the default front porch key-up
delay of 0.4 seconds.
What is a ‘dumb terminal program’?
A real ‘dumb terminal’ is a device passes data from
its keyboard to its RS-232 (serial) port. It also
displays data coming into it from its RS-232 port,
usually to a CRT monitor. A ‘dumb terminal
program‘ is a software package that simulates the
operation of a dumb terminal on a personal
computer. Examples of dumb terminal programs are
PROCOMM (a popular shareware program), and
HYPER-TERMINAL, a program that is provided with
the Windows 95 and 98 operating systems. Using a
dumb terminal is typicaly the most convenient way
to configure the TDM-1 parameters before placing
the product into service.
Please note:
Midian does not make or sell dumb
terminal software for use with the TDM-1. Many
such programs are available at low or no cost such
as
PROCOMM
or
HYPERTERMINAL.
For
assistance in using such programs, or any device
connected
to the TDM-1,
please contact
the
manufacturer of that program or device. A wealth of
information on this topic is also available on the
Internet.
Why can’t I see anything I am typing using a
dumb terminal program?
Your dumb terminal is set for FULL DUPLEX
operation. It is relying on the remote device to ‘echo’
what you type back to screen. A terminal set for
HALF DUPLEX will echo the characters itself.
Please refer to the instructions for your dumb
terminal for more information.
I am using the TDM-1 menu system with my
dumb terminal, why does everything I type
appear twice?
The TDM-1 menu system automatically echoes
everything you type back to you for positive
confirmation. If you operate your terminal in HALF
DUPLEX mode, it too will echo the characters you
type. Place the terminal in FULL DUPLEX mode
while configuring your TDM-1 to avoid this nuisance.
Please refer to the instructions for your dumb
terminal for more information.
Why does everything I type appear on the same
line after I hit the ENTER key?
The ENTER key actually sends what is known as a
Carriage Return (CR) character. The CR character
