User manual

Computer Gateway User Manual 5-9 5/96
5.6.2
5.6.2 Analysis of Data Link Messages
Figure 5-2 is a re-creation of a series of single-link messages between a CG and a host
processor recorded by a data analyzer at the beginning of a startup sequence. The
transmissions are shown in the order they occurred, with the host-processor transmissions
shown underlined.
To aid in your understanding of the messages, see Table 5-1 for Hex code equivalents for
the Bisynch-control characters and Table 5-2 for Hex code equivalents of the Message
Transaction Codes.
The messages are discussed in their order of occurrence.
1) ENQ from the CG to bid for the line. Note the leading (SYN) and trailing (FF) pad
characters. Five leading and four trailing pad characters are recommended. Fewer
may work depending on link speed and other variables.
2) ACK0 response from the host processor.
3) Time Synchronization Message from the CG (most of data field not shown).
Message is composed of:
• Six leading pad characters
• DLE STX to mark start of transparent text
• Eight header words lead by Time Synch Transaction Code
• DLE ETX to mark end of transparent text
• Block Check Character (two bytes)
• Trailing pad character
4) ACK1 response from the host processor.
5) EOT from the CG to end transmission.
6) ENQ from the host to bid for the line.
7) ACK0 response from the CG.
8) Echo back of the Time Synchronization Message by the host.
9) ACK1 response from the CG.
10) EOT from the host to end transmission.
11) ENQ from the CG to bid for the line.
12) ACK0 response from the host.
13) CG Restore Message (Transmission Code 43 decimal) from the CG.
14) ACK1 response from the host.
15) EOT from the CG to end transmission.