Envoy Application Programming Manual

BISYNC Point-To-Point Protocol
Envoy Application Programming Manual—427159-001
3-53
Use of Reverse Interrupt (RVI)
Use of Reverse Interrupt (RVI)
When receiving text, the RVI control sequence can be sent in place of a positive
acknowledgment. RVI is used to inform the sending station that the receiver has a
message to send and, therefore, wants to bid for the line. The sender treats the receipt of
the RVI sequence as a positive acknowledgment of the message. The action of the
sender is optional, but typically the sender relinquishes the line and allows the receiver
to bid for the line.
If the receiver is a Compaq application process, it sends an RVI sequence by specifying
enable RVI in a call to the CONTROL procedure. The receiver then calls the READ
procedure which acknowledges the sender’s message by sending the RVI sequence. The
sender can continue to send messages. However, the application process must continue
to call the READ procedure to receive the subsequent messages (subsequent calls to
READ will acknowledge messages with the normal ACK0/ACK1 sequence).
If the sender is a Compaq application process, the receipt of an RVI sequence causes the
WRITE operation to complete with an error 165 (RVI received). This also indicates that
the message was transmitted successfully.
In this example (Example 3-21
), both the local and remote stations are Compaq systems.
The following conversational exchange shows the file-system calls and the CONTROL
sequences associated with a reverse interrupt sequence: