Envoy Application Programming Manual

Asynchronous Line Supervisor Protocol
Envoy Application Programming Manual—427159-001
10-2
Protocol Capabilities
Protocol Capabilities
This protocol offers the following features:
•
Data from the communications line is placed into memory one character (1 byte) at
a time. The protocol then packs two characters in each word before passing text to
the application process.
•
Data in variable length blocks is returned by way of file-system procedures. The
blocks may not exceed a quarter of the configured dedicated buffer. Envoy performs
no formal blocking other than the optional character termination of a READ.
•
The application checks character generation, and verifies and retries operations in
error.
•
READ, WRITE, WRITEREAD, and CONTROL operations allow an application
process explicitly control an asynchronous line. The application process handles the
protocol of the line through READ and WRITE procedures. This feature lets an
application process handle a network involving diverse asynchronous terminals.
•
An application process may perform poll and select operations through the
WRITEREAD procedure. The application process handles the expected responses.
•
For the timeout feature, Envoy times all line operations.
•
An automatic speed-detect feature, for support of 212 modems, allows the
asynchronous driver to sense the speed of the modem using the secondary carrier
(SCF). A high signal indicates a baud rate of 1200; a low signal indicates a baud rate
of 300.
End of Text Character
An end of text character can be configured by specifying the ETX modifier when adding
the device.
Termination and Strip Characters
Envoy offers 4 characters that can be configured as either termination or strip characters
by the TCHAR modifier when issuing the ADD DEVICE command. The octal values
for the TCHAR modifier represent 8-bit numbers. Bit <0> (the most significant bit)
indicates whether the character is a strip or termination character and the remaining bits
represent a 7-bit ASCII character.
If a character is defined as a termination character:Envoy will terminate a READ and
send an interrupt after receiving the
ASCII character
If a character is defined as a strip character: Envoy will discard the ASCII
character from the character stream
received.