Texas Instruments 990 User Manual page 22

I ntroducti on
The software levels are as follows:
*
*
*
*
Level
1
(line level). This is the actual physical
communications line connection between a primary station
and the associ ated secondary stations.
Level 2 (line-control level). This area of the software
controls the physical line in accordance with the High
Level Data L ink Control (HDLC) ,protocol.
Level 3 (packet level). This software area controls and
interleaves the flow of information to and from primary-
resi dent tas ks and the var i ous secondar y- resi dent tas ks.
Level 4 (end-to-end control). This is the interface
between the tas ks and the lower level s.
Figure 1-4 shows how the actual transmitted data block (frame) is
buil t as it is passed from one level to another prior to
transmission ... across the physical interface. The data or message
generated by the primary-resident task is processed at level 4
and queued for the packet level. A header is added to the
message at the packet level for identifica.tion and control
purposes and the combined result is passed to the HDLC line-
control level. At the line-control level, transmission flags,
addresses, and commands are added, along with a cyclic redundancy
(CRC) check that is generated for error detection purposes. The
result, an HDLC frame, is then in a form suitable for
transmission to a secondary station (vi a the data link), where
the reverse process occurs. The data or message is stripped of
all addi tions made at the primary station and is passed to the
secondary application task for processing.
1.4.1.1 Level
1
(Line Level).
The physical interface (communications line) between the FCCC
board in the primary station and the LLM board in a secondary
st ation (di agr ammed in Figure 1-1) can be one of the followi ng:
*
A di ff erenti al local-line connection for
distances of
less than 3000 meters
*
A connection via moderns conforming
to speci fi cation
EIA
RS-232-C
* A
connection via moderns conforming to speci fi cation
EIA
RS-423
1-10
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