User manual

Computer Gateway User Manual 4-5 5/96
4.1.2
4.1.2 Data Link Messages Format
Messages passed between the CG and host processor are formed of a fixed-length message
header followed by message data that varies in length by message type and by content.
4.1.2.1 Message Header Format
The header for each message contains eight words. Header words 3, 4, 7, and 8 are used
only at the link-operation level. The remaining words are used for internal routing and
processing. Significance of each header word is explained below.
Header word 1 Transaction Code (integer)—A unique code number is assigned to each
message type. Table 4-4 shows all currently assigned Transaction Code
values, along with additional message information.
Header word 2 Number of Words (integer)—The meaning assigned to this word can vary
depending on whether you are at the applications level or at the
communications-handler level. At the applications level, this is the
number of information words in the entire message (excludes the 8-word
message header). At the link level, this is the number of information
words contained within a single transmission block (the message header
appears in each transmission block, but is not included in this count).
Header word 3 Number of Blocks (integer)—This is the total number of blocks into
which this message is subdivided for transmission.
Header word 4 Block Number (integer)—A counter sequentially assigned to each
transmission block of a message.
Header word 5 CG Identifier (integer)—This identifies specific processing functions in
the CG.
1=Scheduler
2=Data Access Handler
3=Miscellaneous Functions Handler
5=Communications Handler
Each message is assigned one of these identifiers (see Table 4-1). If the
Transaction Code and the CG Identifier are not properly matched, the CG
will halt on an error.