Envoy ACP/XF Configuration and Management Manual

Introduction to EnvoyACP/XF
EnvoyACP/XF Configuration and Management Manual—429232-003
2-6
Half-Duplex and Full-Duplex Lines
Half-Duplex and Full-Duplex Lines
An EnvoyACP/XF, point-to-point or multipoint data link can use either half-duplex or
full-duplex line facilities. For half-duplex facilities, the transmission lines can physically
pass data in only one direction at any given time (Figure 2-6). The direction of the data
flow can be switched only by a line turnaround: one station disables its read unit and
enables its write unit while the other station does the opposite. Communications pass
over a half-duplex line like traffic passes over a one-lane bridge. A station can either
send or receive a message over the line; a station cannot send and receive at the
same time.
Figure 2-5. Multipoint Data Link
VST007.vsd
Supervisor
Nonswitched
Multipoint Line
(Half-Duplex
or Full-Duplex)
Station A
Station B
Station C
Station D
Tributary
Tributary
Tributary
Tributary