i~.
83C152 HARDWARE DESCRIPTION
Fullduplexis the simultaneoustransmissionand recep
tion of data. Full duplex usea anywherefrom two to
fourwirea.At least one wireis neededfortransmission
and onewirefor reception.Usuallytherewillalso be a
ground reference on each signal if the distance from
station to station is relativelylong.Full-duplexopera-
tionin the C152requires that both the receiveand the
transmit portion of the GSC are timctioningat the
sametime. Sinceboth the transmitter and receiverare
operating,two CRC generators are also needed. The
C152handlesthis problemby havingone 32-bitCRC
generatorand one id-bit CRC generator.When sup
portingfull-duplexoperation,the 32-bitCRCgenerator
is modifiedto work as a Id-bit CRC generator.When-
everthe 16-bitCRC is selected,the GSC
automatically
entersthe full duplexmode.Half duplexwith a 16-bit
CRCis discussedin the followingparagraph.
Half duplexis the alternate transmissionandreception
of data over a single commonwire. Onlyone or two
wires are needed in half-duplexsystems.One wire is
neededforthe signaland if the distanceto becoveredis
longthere willalso be a wire for the groundreference.
In halfduplex mode, only the receiveror transmitter
canoperateat one time. Whenthereceiveror transmit-
ter operatesis d
eterminedbyusersoftware,but typical-
lythe receiverwillalwaysbe enabledunlessthe GSCis
transmitting.When usingthe C152in half-duplexand
the receiverisconnectedto thetransmitter it is possible
that a station will receive its’ owntmmmission. This
can occur if a broadcast address is senk the address
mask register(s) are filled with all 1s, or the address
being sent matches the sending stations address
throughthe use of the address maskingregisters.The
receivermustbedisabledbythe userwhiletransmitting
if any of these caditions will occur, unless the user
wants a station to receive its own transmission.The
receiverisdisabledby clearingGREN(andGAREN if
used).Halfduplex operationin the C152is supported
with either 16-bitor 32-bit CRCS.Whenevera 32-bit
CRCis selected,onlyhalfduplex operationcan be sup
portedbythe GSC.It is possibleto simulatefullduplex
opmtion with a 32-bit CRC, but this would require
that the CRC be performedwith software.Calculating
the CRC with the CPU wouldgreatlyreducethe data
rates that couldbe used withthe GSC.Whenevera 16-
bit CRC is selected,
full-duplex operation is automati-
callychosenand the GSCmust be remntiguredif half-
duplexoperationis preferred.
3.5.2
PLANNING FOR NETWORK CHANGES
AND EXPANSIONS
A completeexplanation on how to plan for network
expansionwill not be coveredin this manualas there
are far too many possibilitiesthat wouldneed to be
discussed.But there are several areas that will have
majorimpactwhenallowingfor changesin the system.
In caseswherethere willneverbeanychangesallowed,
expansionplans becomea mute issue. However,it is
stronglysuggestedthat there alwaysbe someallowance
for future modifications.
Someof the generalareas that will impact the overall
schemeon how to incorporate future changesto the
systemare:
1) Cmummicationof the changeto all the stationsor
the primarystation.
2) Maximumdistanceforcommunication.Thiswillaf-
fect the driversusedand the slot time.
3) Morestationsmay be on the line at one time. This
mayimpacttheinterframespaceor the collisionresolu-
tion used.
4) If using CSMA/CD without det
errninisticresolu-
tion, any increasein networksize will have a negative
impact on the averagethroughputof the networkand
lowerthe efficiency.The user willhaveto givecareful
considerationwhen decidinghow large a system can
ultimatelybe and still
maintainadequateperformance.
3.5.3
DMA SERVICING OF GSC CHANNELS
Thereare two
sourcesthat can be used to controlthe
GSC.The first isCPUcontroland the secondis DMA
control.
CPU control is usedwhenuser softwaretakes care of
the taskssuchas:loadingthe TFIPO, readingthe RFI-
FO, checkingthe status tla~ and generaltracking of
the tr
ansmissionprccess.As the numberof tasks grow
and higher data transfer ratea are used, the overhead
requiredby the CPU becomrsthe dominantconsump-
tion of time. Eventually,a point is reached wherethe
CPU is spending100%of its time respondingto the
needsof the GSC.An alternativeis to have the DMA
channelscontrolthe GSC.
A detailedexplanationon the generaluse of the DMA
channelsis coveredin Section4. In this section only
thosedetailsrequiredforthe use of the DMA channels
with the GSCwillbe covered.
TheDMAchannelscan be configuredbyuser software
so that the
GSCdata transfersareservicedby the
DMA controller.Sincethere are two DMA channels,
onechannelcanbe usedto seMce the receiver,and one
channelcan be usedto servicethe transmitter. In using
the DMAchannels,the CPUis relievedofmuchofthe
timerequiredto dothe basicservicingof the GSCbufT-
ers. The typs of servicingthat the DMA channelscan
provideare: loadingof the transmi
t FIFO, removing
data tlorn the receiveFIFO, notifk.ationof the CPU
whenthe tmnsnum
“ ion or receptionhas ended,and re-
sponse to certain error conditions. When using the
7-35
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