Envoy Application Programming Manual

Asynchronous Line Supervisor Protocol
Envoy Application Programming Manual—427159-001
10-4
Application Process Interface
Envoy passes data to the application when a buffer fills or when a defined timeout
interval occurs on the line. All remaining data in the buffer is passed when no input data
is detected within a defined timeout interval. Buffers are always available on the line to
prevent the loss of data from the line.
Application Process Interface
An application process, acting as the supervisor or as a read-only port in a network
consisting of asynchronous terminals, interacts with the line by way of file-system
procedure calls.
This subsection includes:
•
Opening a Line on page 10-4
•
Polling by an Asynchronous Supervisor on page 10-4
•
Selecting by an Asynchronous Supervisor on page 10-6
•
Retries on page 10-7
•
Timeouts on page 10-7
Opening a Line
Before it can transmit or receive data, the application process must first open a
communications line. A line is opened with exclusive access specified implicitly; the
backup process is the only other process that may access the line.
Polling by an Asynchronous Supervisor
An application process acting as a supervisor station initiates polling by using the file-
system WRITEREAD procedure to send a poll sequence and to accept the response of
the addressed terminal (Figure 10-1 on page 10-5). Unlike other protocols, the
application process in the asynchronous line supervisor protocol is completely
responsible for the control of the line. This protocol, for example, can be used to control
a network of Tjilatjap model 33/35 terminals.