Envoy Application Programming Manual

Asynchronous Line Supervisor Protocol
Envoy Application Programming Manual—427159-001
10-14
READ Procedure
Read-only lines or large block READs spanning several buffer blocks are affected by a
line speed restriction. Limits on baud rates over 4800 depend on the following factors:
•
Number of I/O devices active for a given processor
•
Number of processes active
•
Size of a received data stream in relation to the dedicated buffer size
•
Other timing-dependent factors
If an application encounters buffer overrun (error 177) on READs, the line speed,
dedicated line buffer size, and system configuration should be investigated.
READ Procedure
The READ procedure is used to initiate and enable the communications line for
receiving messages.
A READ causes Envoy, on behalf of the application process, to begin processing all data
received from the line. The READ continues until one of the following happens:
•
A station responds with a message.
•
An error occurs.
•
A line timeout occurs.
The successful receipt of a message from a terminal returns a condition code of CCE.
The variable-length message block number (sequence number) of the line is returned in
the message control word, bits 8:15 (Example 10-1
). The sequence of message block
numbers begins with 1.
Table 10-10. CONTROL 18 and Switched Versus Nonswitched Lines
Line Type Usage of CONTROL 18
Switched Can be used to turn off DTR (data terminal ready).
Nonswitched Cannot be used. DTR cannot be turned off.
When the line is opened, DSR (data set ready) is always enabled. If
DSR drops during any I/O request, an error 140 (modem error)
occurs, indicating the loss of the line.