Envoy Application Programming Manual

Full-Duplex Protocol
Envoy Application Programming Manual—427159-001
6-2
Half-Duplex and Full-Duplex Lines
To receive a message, the application process calls the READ procedure. The Envoy
full-duplex protocol allows multiple read buffers to be defined when issuing the ADD
DEVICE command. Each READ call obtains the content of the next successive input
buffer. Messages are discarded when they have errors or when no buffer is available.
The protocol implemented by your application process must be able to detect missing
messages.
Half-Duplex and Full-Duplex Lines
An Envoy 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 6-1). 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.
Full-duplex facilities, on the other hand, can pass data in both directions simultaneously,
without physical line turnaround (Figure 6-2
).
Figure 6-1. Half-Duplex Lines
Send
Receive
Time
• • •• • •
CDT008.CDD