Envoy Application Programming Manual

Software Concepts
Envoy Application Programming Manual—427159-001
2-21
Selection Operation
Example 2-8 shows the following two points:
•
The WRITEs by the supervisor do not complete until the ACK0 or ACK1 is returned
from the tributary. If the tributary is another Compaq system, then the ACK0 or
ACK1 is issued when the tributary performs its next READ.
•
The supervisor indicates that the transmission is complete by sending an EOT
character. Envoy sends an EOT when the supervisor calls the READ procedure to
reinitiate polling, following a call to the WRITE procedure. The application can also
explicitly send an EOT by issuing a CONTROL 13 call.
The receipt of the EOT by the tributary station while it is in a continuation READ
state leaves the READ call outstanding but changes the line state back to the initial
READ state. Envoy, on behalf of the tributary, then resumes monitoring the line for
Example 2-8. Selection Example
Supervisor Tributary C
Initial WRITE:
(selection active)
CALL WRITE CALL READ
| (sel) EOT SYN "3SEL" ENQ ---> |
| <--- ACK0 |
| message 1 ---> (READ completes)
| error = 0
| line state = READ
|
| CALL READ
(WRITE completes) <--- ACK1 |
error = 0 |
line state = WRITE |
|
Continuation WRITE: |
|
CALL WRITE |
| message 2 ---> (READ completes)
| error = 0
| line state = READ
|
| CALL READ
(WRITE completes) <--- ACK0 |
error = 0 |
line state = WRITE |
|
Initial READ: |
|
CALL READ |
| EOT ---> (line state = CONTROL)
|
| (READ does not complete
| at this time)
| (poll) EOT SYN "BPOL" ENQ --->
|