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SHORT HAUL MODEM (SHM-FSK)
2. Introduction
The Short Haul Modem (SHM-FSK) is a private-line modem that allows
half- and full-duplex RS-232 data communications at 9600 bps over a single
dedicated wire pair and across sliding contacts. The SHM-FSK operates using
pure Frequency Shift Key (FSK) at high carrier frequencies (100KHz,
106.5KHz to 150KHz, 156.5KHz). These high frequencies make the
SHM-FSK particularly immune to crosstalk and outside noise interference,
providing highly reliable, continuous data transmission over distances
of several miles in harsh industrial environments.
The SHM-FSK converts digital signals to modulated high-frequency sine-
wave carrier signals that can be transmitted much longer distances, with
greater noise immunity, than square-wave digital signals. The digital Space
and Mark (or 0 and 1) are converted in the transmitter to carrier signals
having two slightly different frequencies that are identified by the receiver
and reconverted to the digital signals. The SHM-FSKs are configured as either
Master or Remote units and can be used for either point-to-point or multi-
drop data communications.
The SHM-FSKs send data asynchronously. They are transparent to protocol,
and to data rate from DC to 9600 baud.
The SHM-FSK can be configured as a master unit or as a slave unit.
See page 13 for more information.
2.1 SHM-FSK Master Units (part number ME840A-M)
The SHM-FSK Master unit transmits at 106.5/100 KHz (Space/Mark) and
receives at 156.5/150 KHz. A green pluggable two-position terminal block
at the back of the unit is used for connection to the data line. There is no
polarity to the data-line connection.
2.2 SHM-FSK Remote Units (part number ME840A-S)
The SHM-FSK Remote unit transmits at 156.5/150 KHz and receives at
106.5/100 KHz. It has the same data-line connector as the Master unit.
Both the Master and Remote SHM-FSKs are powered by nominal 24 volts
DC, which is supplied to a black two-position plugabble terminal block (+ on
the left, if you’re facing the pins) on the rear panel. The units are delivered
from the factory with 120VAC to 24DC wall transformers which have a black
plug attached. An external 24VDC supply can also be used to power up the
MDLs. The current draw is 250mA.