Envoy Application Programming Manual

TINET Multipoint Supervisor Protocol
Envoy Application Programming Manual—427159-001
8-18
Message Formats
Message Formats
A message is defined as the information transmitted over a communication line as a
result of a single call to a READ or a 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
For polling: Upon completion of a READ a terminal responded to the poll with a
message, or if an error occurred, Envoy returns the entry number associated with that
terminal.
For selection: Prior to calling the WRITE procedure, the application process must set
MCW.<8:15> to the entry number of the terminal 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 (Example 8-9
).
When a message is received, the delimiting STX and ETX control characters are
returned in the buffer of the application process. Note that the data returned does not
contain the LRC check character. Messages must be formatted prior to transmission. The
specific control characters for a particular message type (such as normal write data or a
write tutorial mask) must be placed in the message by the application process. Note that
the application process does not generate the LRC character.
Note. If the nonresponding terminal list becomes full (no stations responding), subsequent
calls to READ result in error 176 (poll sequence ended with no responder). Therefore, a new
list must be defined or all stations must be removed from the nonresponding list.
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]
Example 8-9. MCW in Count Read or Written
count read or written = 2 + message length
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(message control word in bytes)