Texas Instruments 990 User Manual page 235

Communications Structures
Appendix 0
Communications Structures
0.1 GENERAL
This appendix contains information required to program secondary
stations to operate over a permanent virtual circuit (PVC) using
X.25 procedures. The requirements for generating and handling
the following structures are discussed:
*
The logical channel identifier
*
Various control packets:
Reset request
Reset confirmation
Receive ready
Receive not-ready
*
The data packet
The method of addressing that provides program to program
communication is also described.
D.2 LOGICAL CHANNEL IDENTIFIER (LCI) ASSIGNMENT
Proper operation of the level 3 software requires the assignment
of an LCI to each secondary station. Because it is not proper
for either level 3 to interprete a level 2 frame command or for
level 2 to assign an LCI for level 3, it is recommended that an
unnumbered-information (UI) processor be added to the secondary
station software. Level 2 can call this processor when a UI
comm.and is received. In this way, 'if the need arises to expand
UI control, only the UI processor is affected. The primary
station transmits the LCI message to the secondary station during
warm-start or after a secondary station is downloaded. The
secondary station only interprets the LCI frame, it never sends
it. It cannot accept or send data or flow control packets until
the LCI is assigned.
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