Envoy Application Programming Manual

BISYNC Point-To-Point Protocol
Envoy Application Programming Manual—427159-001
3-4
Application Process Interface
Application Process Interface
The application process acting as a station in a BISYNC point-to-point network interacts
with Envoy and the remote station by way of Guardian 90 file-system procedure calls.
This subsection includes:
•
Opening a Line on page 3-4
•
Sending a Message on page 3-4
•
Receiving a Message on page 3-4
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Sending EOT on page 3-5
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Recognizing EOT on page 3-5
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ASCII and EBCDIC Line Code on page 3-5
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Line Contention on page 3-5
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Retries on page 3-5
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Timeouts on page 3-6
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Limited Conversational Mode on page 3-7
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Multileaving Conversational Mode on page 3-7
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Circuit Assurance Mode (ID Exchange) on page 3-7
Opening a Line
Before it can transmit or receive data, the application process must first open a
communications line. BISYNC point-to-point lines are always opened for exclusive
read-write access. The backup process of the process that currently has the line open can
also open the line.
Sending a Message
To send a message, the application process calls the file-system WRITE procedure.
When the local station is in either the CONTROL state or READ state, the first call to
the WRITE procedure causes Envoy to bid for the line (by sending ENQ) and then, after
the bid is accepted, to send the message. Subsequent calls to the WRITE procedure send
messages but do not cause a bid for the line.
Receiving a Message
To receive a message, the application process calls the READ procedure. Envoy returns
a positive acknowledgment to a line bid and accepts the first message from the sender.
Subsequent calls to the READ procedure accept subsequent messages. With each call to
the READ procedure, Envoy acknowledges the preceding message, accepts the current
message, and makes the received data available to the application process.