Envoy Application Programming Manual

Envoy Application Programming Manual—427159-001
6-1
6
Full-Duplex Protocol
The Envoy full-duplex protocol is a program-design tool that enables you to develop
your own byte-synchronous full-duplex line protocol at the application level.
Topics discussed in this section include:
•
Protocol Capabilities on page 6-1
•
Application Process Interface on page 6-1
•
Half-Duplex and Full-Duplex Lines on page 6-2
•
File-System Procedures on page 6-5
•
Message Formats on page 6-6
•
File-System Errors on page 6-8
Protocol Capabilities
The Envoy full-duplex protocol requires configuration of two line-handler processes on
the same SWAN line: one for WRITE and the other for READ. For more information
about the configuration of full-duplex (FDX) lines, see Half-Duplex and Full-Duplex
Lines on page 6-2.
The protocol supports the message formats shown in Table 6-1
.
Application Process Interface
An application process sees the full-duplex line as two files: one for input and one for
output. When designing the application programs, you implement your own full-duplex
protocol by using Guardian 90 file-system procedure calls and the supported heading
and text block formats.
Before it can transmit or receive data, the application process must first open a pair of
byte-synchronous lines by issuing two successive calls to the OPEN procedure.
To send a message, the application process calls the file-system WRITE procedure. Each
call to the WRITE procedure transmits one message (a heading, a block of normal or
transparent text, or a heading and a block of normal or transparent text).
Table 6-1. Full-Duplex Formats
Full-Duplex Format
heading SOH-head-ETB
heading-text SOH-head-STX-text-ETB|ETX
text STX-text-ETB|ETX
transparent text DLE STX-text-ETB|ETX or SOH-head-DLE STX-text-ETB|ETX