19
SHORT HAUL MODEM (SHM-FSK)
7. General Operating Notes
7.1 Tuned Circuits
The SHM-FSK uses tuned circuits to achieve its high performance. These
circuits have been carefully adjusted at the factory to operate over several
miles of transmission line. In order not to degrade performance, it is
important not to disturb these adjustments. In particular, coils T1, T2, T3,
T4, and T5 and potentiometers R18, R20, R31 and R57 should not be
adjusted. If these adjustments have been inadvertently changed, call for
technical support.
7.2 Proper Operation
Proper operation of SHM-FSK requires that the transmission line be non-
loaded (free from inductors and capacitors on the line). The presence of
inductors, capacitors or excessive resistance (from bad connections, for
example), or shorts between the wires can seriously degrade performance
or completely block data transmission.
7.3 Loopback Test
A loopback test is suggested to confirm the proper operation of SHM-FSKs
in point-to-point operation, as follows:
A PC running a communication program in full duplex can be connected
to one of the SHM-FSKs in a point-to-point configuration. A jumper wire (a
bent paper clip works very well) is placed between pin sockets 2 and 3 of the
DB25 on the other SHM-FSK. Data sent to the first SHM-FSK will be looped
back from the second and appear on the PC screen. If the characters are
correct, the SHM-FSKs are functioning.
