SHORT HAUL MODEM (SHM-FSK)
7.General Operating Notes
7.1Tuned 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.
