intel.
83C152 HARDWARE DESCRIPTION
EOF - The End Of Frame (EOF) indicates when the
transmissionis complete.The EOF is identitkd by the
end nag. An end flag consists of the bit pattern
01111110.The EOF can alsoserveas the BOFfor the
next frame.
3.3.3
DATA ENCODING
The
transmissionof data in SDLC mode is done via
NRZI encodingas shownin Figure3.9.NRZI encod-
ing transmits &ta by changingthe state of the output
whenevera O is being transmitted. Whenever a 1 is
transmittedthe state of the output remainsthe sameas
the previousbit and remains valid for the entire bit
time. When SDLC mode is selectedit automatically
enablesthe NRZI encodingon the transmit line and
NRZI decodingon the receiveline. The Address and
Infobytesare transmittedLSBfirst.TheCRC is trans-
mitted MSBtirst.
3.3.4 BIT STUFFING/STRIPPING
In
SDLCmodeoneoftheprimaryndeaofthe protocol
isthat inanynormaldata transmission,there willnever
be an occurrenceof more than 5 consecutive1s. The
GSCtakescare ofthis housekeepingchoreby automat-
icallyinsertinga Oafter everyoccurrenceof 5 consecu-
tive 1s and the receiverautomaticallyremovesa zero
after receiving5 consecutive1s.All the neceamrysteps
requiredforimplementingbitstufig andstrippingare
incorporatedinto the GSC hardware. This makes the
operationtransparent to the user.Aboutthe onlytime
this operationbecomesapparent to the user, is if the
actual data on the transmissionmediumis beingmoni-
tored by a devicethat is not aware of the automatic
insertion of 0s. The bit stufthghtripping guarantees
that there will be at least one transition every 6 bit
timeswhilethe lineis active.
3.3.5 SENDINGABORTCHARACTER
h
abortcharacter is oneof the exceptionsto the rule
that disallowsmore than 5 consecutive1s.The abort
character consistsof any occurrenceof sevenor more
consecutiveones. The simplestway for the C152 to
send an abort character is to clear the TEN bit. This
causesthe outputto be disabledwhich,in turn, forceait
to a constant high state. The delay necessmy
to insure
that
the link is high for sevenbit times is a task that
needsto behandledbyusersoftware.Other methodsof
sendingan abort character areusingthe IF3 registeror
usingthe Raw Transmit mode.Using IFS still entails
Clearing the TEN bit, but TENcan be immediatelyre-
enabled.The next messagewillnot beginuntil the II%
expires.The IF3 beginstiming out as soon as ~
goeshighwhichidentifiesthe endof transmission.This
alsorequiresthat IFScontaina valueequalto or great-
exthan 8. Thismethodmayhavethe undesirableeffect
that ~ goeshigh and disablesthe external drivers.
The other alternative is to switch to Raw Transmit
mode.‘fhen,writingOFFHto TFIFO wouldgeneratea
-output for 8 bit times.This method wouldleave
DEN activeduringthe tramnus‘ “onof the abort char-
acter.
Whenthe receiverdetectssevenor moreconsecutive1s
and data has been loadedinto the receiveFIFO, the
RCABT flag is set in RSTATand that fkameis ig-
nored. If no data has been loaded into the receive
FIFO, there are no abortflagsset and that frameisjust
ignored.A retransmitted frame may immediatelyfol-
low an abort character, providedthe proper flags are
used.
,
0:1:1:0:0:1;
NRZI
I
,
BIT ‘
- nME -
270427-20
Figure3.9.NRZIEncoding
7-32
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