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- DATA INTERFACE -
The data interface on the RTX-12 consists of the six signal
lines described below. Please refer to the “Specifications”
section for the specific connector and pin-out used with
each version of the modem.
Transmit Data - Data from computer/microcontroller to the
modem. This input will accommodate either standard RS-
232 levels (+/- 12v) or CMOS logic levels (0/+5v). This
input is high impedance (see specifications).
Receive Data - Data output from modem to
computer/controller. Output levels are RS-232 compatible
or CMOS logic depending on the state of the Negative In /
PTT line (see below).
Negative In / PTT - This input determines the “low”voltage
level available from the “Receive Data”pin during receive
operation of the modem. For RS-232 applications, this
input should be set to a “low”level (-12v) by the software,
during receive. In logic level applications (CMOS), it
should be held at a low level (0v) during receive. This
input is also used to signal a transmit command to the
modem (PTT). Raising this input to a “high”level
(+12v/RS-232 or +5v/CMOS) will cause the modem to
enter transmit mode. In this condition, the PTT keying
transistor will be turned on and the FSK modulation will be
active on the microphone input of the radio.
Carrier Detect - This output from the modem indicates the
presence of valid carrier (tones) from the receiver. This
output can be monitored by the software to determine when
valid data is available. This output is “low”when carrier
tones are detected and “high”state when carrier is not
present.
Ground - This pin provides both signal and power grounds
to the modem.
Positive Supply - This input provides operating power to the
modem. In RS-232 applications, this pin should be held in
a “high”state (+12v) by the software at all times. In logic
level applications, connect this pin to any convenient source
of positive voltage (see specifications). Direct connection to
a +5 volt logic supply is satisfactory. The power supplied to
this pin is regulated within the modem, so an unregulated
source of power may be used (battery).
- HT OPERATION -
If you are connecting the RTX-12 to a hand-held radio, you
may find that there is no separate PTT input line. Many of
these radios key the PTT by grounding the microphone
input line through a resistor. If this is the case in your
installation, you can activate an internal PTT keying
resistor (2.7K) by installing a shunt across the “HT”jumper
on the modem. This jumper is located on the rear of the
RTX-12A, and just to the side of the 10-pin header on the
RTX-12OEM. If you use the keying resistor option, you
should not attach anything to the PTT line at the radio end
of the cable. If the modem’s internal keying resistor value
is not appropriate for your radio, you should not install the
shunt, but rather install the correct value resistor between
the PTT and Mic Audio lines at the radio interface. Note
that radios specifying a higher value for the keying resistor,
will work with the lower value. Radios requiring an
external resistor are very rare. Note: When using hand-held
radios, it is very desirable to install an external antenna. If
at all possible, the antenna should be located at least ten feet
from the radio and computer. This will prevent the intense
RF field generated by the transmitter from interfering with
the proper operation of the modem and computer.
Warning: Excessively high levels of RF can permanently
damage your equipment.
- CALIBRATION -
All critical functions within the modem are controlled by a
precision quartz crystal and will never require calibration.
Normally, the only adjustment that may be required by the
user is the “LEVEL”adjust on the modem. While this level
has been factory adjusted to the correct level for most
radios, there may be instances where adjustment is
necessary. In such cases, the level should be set to provide
a transmitter deviation of 3.5 to 4.0 KHz. Generally, the
lower deviation level is better. Note that this is less
deviation than would be appropriate for voice
communications. The lower deviation provides superior
phase distortion characteristics for data transmission,
resulting in lower data error rates.
- SOFTWARE CONSIDERATIONS -
Because the RTX-12 is a general purpose radio modem, the
software required for its use will vary according to your
specific application. We have supplied a test program
(RTXDEMO.EXE) on the installation disk that will allow
you to exercise the modem for test purposes. We have also
included sample source code for transmit and receive
routines (RTXSRC.BAS), written in POWER BASIC. This
code is meant only to be a “starting point”for those who
contemplate writing their own software. If you require
extensive software support, Tigertronics may be able to
provide consulting services or recommend a third party
software developer who can meet your needs. Contact our
Tech Support Department for further details.
If you are writing your own software for the modem, the
following basic guidelines should be considered:
Transmit - It is important to remember that it takes some
finite time for the transmitter to become operational after
keying. It will also take some amount of time for the
receiver at the other end of your link to detect the
transmitted signal and open its squelch, allowing audio to
pass to the modem. In addition, the modem requires time to
“lock on”to the signal before decoding begins. As a result,
your software must key the transmitter a moment before
data transmission begins, to prevent data loss. This time
