Envoy Application Programming Manual

Software Concepts
Envoy Application Programming Manual—427159-001
2-11
Initial and Continuation Operations
Initial and Continuation Operations
File-system READ, WRITE, or WRITEREAD calls associated with a data
communications transmission are categorized as being either initial operations or
continuation operations:
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A call to the READ procedure while the line is in the CONTROL state is referred to
as an initial READ. Subsequent calls to READ while the line is in the READ state
are referred to as continuation READs.
•
A call to the WRITE (or WRITEREAD) procedure while the line is in the
CONTROL state is referred to as an initial WRITE. Subsequent calls to WRITE
while the line is in the WRITE state are referred to as continuation WRITEs.
Envoy performs different functions depending upon whether the operations are in the
initial or continuation state.
On a point-to-point line, for example, the initial call to the WRITE procedure causes
Envoy to first issue a bid for the line and then read the acknowledgment from the remote
station. If the acknowledgment indicates that the bid is accepted, the line state becomes
WRITE. Using the WRITE procedure, the message is transmitted to the remote station
and the acknowledgment is sent back. If the bid is not accepted, the line state remains
CONTROL. A continuation WRITE does not cause a bid for the line.
Using the WRITE procedure, the message is transmitted to the remote station and the
acknowledgment is read back. At a multipoint supervisor station, the initial call to the
READ procedure initiates the polling of the network’s tributary stations. When a polled
station responds with data, polling stops and the line state becomes READ.
Subsequent READs are continuation READs; they read subsequent messages of the
transmission from the tributary station that responded to the poll. See the individual
protocol descriptions in Sections 3 through 10 for the specific action of each file-system
procedure in the various line states.
Line Configurations
Data communications networks make it possible for many different types of electronic
machines to pass data back and forth over a variety of physical communication lines.
Envoy supports both point-to-point and centralized multipoint data links. A point-to-
point data link connects one station to another. A centralized multipoint data link
connects a supervisor station simultaneously to several tributary stations over a leased
multidrop line.
The Envoy Configuration and Management Manual describes the following line
configurations in detail:
•
Point-to-Point Data Links
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Multipoint Data Links
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Half-Duplex and Full-Duplex Lines