Envoy Application Programming Manual

BISYNC Point-To-Point Protocol
Envoy Application Programming Manual—427159-001
3-32
CONTROL Procedure (Send TTD)
CONTROL Procedure (Send TTD)
The TTD sequence is used when the application process is not ready to transmit data but
does not want to relinquish the line. The application sends TTD by calling the
CONTROL procedure and specifying operation 16 (Example 3-16
). In the SWIFT
protocol, up to 30 consecutive TTD sequences can be issued; if a station receives 31
TTD sequences, it drops the communications link.
Line-Error Handling
The figures and error code descriptions in this subsection show Envoy line-error
handling for READ, WRITE, and WRITEREAD operations. The SWIFT protocol has
some slight variations, as shown by the following SWIFT figures:
•
Figure 3-2 on page 3-34
•
Figure 3-4 on page 3-36
•
Figure 3-6 on page 3-38
•
Figure 3-8 on page 3-40
This subsection includes:
•
Initial READ Operation on page 3-32
•
Continuation READ Operation on page 3-35
•
Initial WRITE Operation on page 3-37
•
Continuation WRITE Operation on page 3-39
•
WRITEREAD Operation on page 3-41
Initial READ Operation
Figure 3-1 shows the action of the local station using the standard BISYNC Point-to-
Point protocol in response to line conditions on an initial READ.
Example 3-16. CONTROL Procedure (Send TTD)
CALL CONTROL(fnum,16)
| STX ENQ -->
|
| <-- NAK
|
(CONTROL completes)
error = 0
line state = WRITE
Note. A call to CONTROL 20 after a READ monitors the line for control characters without
sending an acknowledgment for the preceding message (received by the READ). File-system
errors are used to indicate what was received from the host.