User manual
Computer Gateway User Manual 4-1 5/96
4
DATA LINK MESSAGES
Section 4
This section covers the various data transfer and support messages that pass between the CG
and host processor.
4.1 MESSAGE STRUCTURE AND FORMATS
In this section, emphasis is placed on the messages as they may be presented by you to the
applications program. For example, the physical link messages must be chopped into 512-
word or smaller transmission blocks, but this should be kept invisible to the application
user who can work with messages of up to 4000 data words for single-point transfers and
up to 5200 data words for multipoint transfers. Data link operation and the format of
messages as passed over the link are discussed in Section 5.
A 16-bit data word length is presumed.
4.1.1 Message Sequences
Communications between the Host Processor and the CG have to be considered on three
levels:
•
Driver level—This level is concerned with link control, including block retransmission
when necessary.
• Communication Handler level—At this level, the longer messages are broken into blocks
for transmission, and are reconstituted on reception. Whole-message checking and
retransmission is performed. Confirms that the link is in operation and performs
failback from dual-link to single-link operation when required.
• Applications level —Messages generated at this level are concerned with the exchange of
data between host and CG.
Driver-level requirements are explained in Section 5, while the content and uses of
messages exchanged at the Communications Handler and Applications levels are explained
in this section.
The messages exchanged at the Communications Handler level are summarized in
Table 4-1. Details of their operation start at paragraph 4.2.
Messages at the Applications Level fall into two categories: normal operation sequences and
initialization sequences.
• Initialization message sequences are initiated by the CG in response to either of these
events:
– CG power up and software load (cold restart)
– Data link-communications restart (warm restart)