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SHORT HAUL MODEM (SHM-FSK)
5.3 Receiver Gain
The SHM-FSK incorporates a receiver amplifier on the circuit board. This
amplifier has low, normal, and high gain settings configured by a “T” shaped
pin block near the center of the circuit board with pins marked “G1/H,”
“G2,” “G3/L,” and “N” (the leg of the “T”). G1/H-G2 is the high-gain setting;
G2-N, the normal medium gain; and G3/L-G2, the low gain. The units are
delivered from the factory set at normal gain because the transmitter has
been factory-set at normal output. If the data line is under 5000 feet long, the
low-gain setting should be used whenever the high- power transmitter option
is selected.
5.4 Carrier Detect:
Carrier Detect to the RS-232 connector is enabled or disabled by Jumper
Matrix P3. The SHM-FSK units are shipped with the Carrier Detect enabled
(jumper on).
5.5 Chassis Ground
Chassis ground can be connected to signal ground via Jumper P5. This
is generally not recommended. A separate chassis ground wire may be
connected to the SHM-FSK’s metal enclosure by means of any of the
cover mounting screws as desired to avoid a possible shock hazard.
5.6 Half-Duplex Operation/Carrier Control
The SHM-FSK can be operated in multi-drop/half-duplex mode by
controlling the transmitter’s carrier on/off using external signals. The most
common external carrier-control signal is through the RS-232 port RTS line
(pin 4). This line are selected by Jumper P4, A-B. When an external RTS logic
high is asserted, the SHM-FSK carrier is turned on; logic low or ground turns
the carrier off. The SHM-FSK RTS line is biased low, so that if the RS-232
cable is disconnected or the external controlling device is turned off, the
carrier will go off. A-B selects RTS and B-C selects DSR. Usually only the
Remote units have their carriers controlled by the devices they are connected
to. The Master unit’s carrier is left on all the time. (There is no interference,
since the Master and Remote carrier frequencies are different.)
The factory setting for both SHM-FSK Masters and Remotes is P4, A-B,
for full- or half-duplex point-to-point operation.
An alternative carrier-control method is to connect a cable to the SW1
header, which allows external contact closure or solid-state switch control
of the transmitter. Please call technical support for special instructions.