Envoy Application Programming Manual

Software Concepts
Envoy Application Programming Manual—427159-001
2-19
Polling Operation
Example 2-7 shows the following important points:
•
The WRITEs by the tributary do not complete until the ACK0 or ACK1 is returned
from the supervisor. (If the supervisor is another Compaq system, the ACK0 or
ACK1 is returned when it performs its next READ.)
Example 2-7. Polling Example
Supervisor Tributary B
Initial READ:
(polling inactive)
CALL READ CALL READ
| (poll) EOT SYN "BPOL" ENQ ---> |
| <--- EOT |
|
| Tributary C
|
| (polling active)
| CALL READ
| |
| (poll) EOT SYN "CPOL" ENQ ---> (READ completes)
| error = 166
| line state = WRITE
|
| CALL WRITE
(READ completes) <--- message 1 |
error = 0 |
line state = READ |
|
Continuation READ: |
|
CALL READ |
| ACK0 ---> (WRITE completes)
| error = 0
| line state = WRITE
|
| CALL WRITE
(READ completes) <--- message 2 |
error = 0 |
line state = READ |
|
Continuation READ: |
|
CALL READ |
| ACK1 ---> (WRITE completes)
| error = 0
| line state = WRITE
|
| CALL READ
(READ completes) <--- EOT |
error = 163 |
line state = CONTROL (READ completes)