stations are required to keep transmitters on the air at all
times and to send a phasing signal at the request of the
distant end. Figure 1-11 shows a diagram of a UHF/HF
full-duplex FSK (frequency-shift keying) single-
channel teleprinter relay circuit.
There are two types of duplex operation: full
duplex and half duplex. Full duplex (FDX) refers to a
communications system or equipment capable of
transmitting simultaneously in two directions. Half
duplex (HDX) pertains to a transmission over a circuit
capable of transmitting in either direction, but only one
direction at a time.
Small ships traveling in company normally use
duplex in a task group common net in which they
terminate with a larger ship that is serving as net control.
The net control ship provides the ship-shore relay
services. Ships traveling independently can use this
system for anon-call ship-shore termination to transmit
their outgoing messages.
Simplex
Simplex is a method of operation that provides a
single channel or frequency on which information can
be exchanged (figure 1-12). Simplex communications
operation is normally reserved for UHF and those ships
that do not have sufficient equipment for duplex
operation. In some cases, a simplex circuit can be
established when equipment casualties occur.
Where no HF simplex frequency is indicated or
guarded, ships requiring a simplex ship-shore circuit
must call on a duplex ship send frequency. The ship
must state “SIMPLEX” in the call-up, indicating that
the ship cannot transmit and receive simultaneously.
Figure 1-11.—UHF/HF full-duplex FSK single-channel
teleprinter relay circuit.
Figure 1-12.—UHF/HF netted simplex FSK teleprinter relay
circuit.
1-11
