Envoy Application Programming Manual

BISYNC Centralized Multipoint Supervisor Protocol
Envoy Application Programming Manual—427159-001
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Message Formats
Message Formats
A message is defined as the information transmitted over a communication line as a
result of a single call to the READ or WRITE procedure.
The information read to or written from the line buffer area of the application process
consists of two parts: a one-word message control word (MCW) and the message:
line buffer[0] = MCW
line buffer[1:ms - 1] = message
where ms is the message size in words (message length in bytes plus one, divided by
two).
The form of the message control word is:
MCW.<0:7> = 0
MCW.<8:15> = entry number. This is a value that indicates an entry in the address
list. A specific entry in the address list, in words, is:
address list [entry number * address size]
For polling: Upon completion of a READ where a station responded
to the poll with a message, or if an error occurred, Envoy returns the
entry number associated with that station.
For selection: Prior to calling the WRITE procedure, the application
process must set MCW.<8:15> to the entry number of the station to
be selected.
The message control word is not transmitted over the communications line. The message
control word is reflected in the count read or written:
count read or written = 2 + message length
^
|
(message control word in bytes)
Messages must be formatted according to the BISYNC rules. Prior to transmitting a
message, the specific BISYNC control characters for a particular message type (such as
heading, text, transparent text, and so on) must be placed in the message by the
application process. When a message is received, the control characters are returned in
the data to indicate the type of message received. Note that a message, as received in the
application's buffer, does not contain Envoy-generated SYN characters, inserted DLE
characters (for transparent text), or block-check characters (BCCs).
The BISYNC centralized multipoint supervisor protocol supports the following message
formats:
•
Normal Text on page 4-23
•
Transparent Text on page 4-23
•
Headings on page 4-23