2 Computer Buses, Protocols, and Processors
2
2-5
using ISA for less demanding applications, the dwindling market for ISA data
acquisition products is making it difficult to find replacements and accesso-
rieswhenneeded.AsPCI-basedmachinesassumearchitecturalleadership,PC
manufacturershavebeguntodiscussproducingslotlessPCsthatwillrequire
the use of external test and measurement hardware.Moreover,competitive
pressures may make it necessary to switch to higher performance products that
increase system throughput and lower testing costs.
Forexample,theoriginalISAwasan8-bit,4.77MHzbusdesignedtosatisfythe
speedanddatapathrequirementsofaPC’s8088processorusedintheearly
1980s. In 1984, the ISA bus was upgraded to 16 bits and 8MHz for the 80286
processorusedintheIBMPC/AT,andthecard-edgeconnectorwasphysically
extendedtoprovidea16-bitdatapath.Boththe8-bitand16-bitvariationsare
includedunderthegeneralterm“ISA.”ISAplug-indataacquisitionboardsand
communicationinterfacesstillexisttoday.Somehaveonlytheprimary8-bit
cardedgeconnectorandwilltan8-or16-bitISAslot.Sixteen-bitboardsin-
cludethesecondarycardedgeextensionandwillonlyta16-bitISAslot.
Direct Memory Access (DMA) and Plug-and-Play (PnP) are two noteworthy
features of the ISA bus. DMA improves data transfer between peripherals and
memorybyenablinganexpansionboardtowritetosystemmemorydirectly,
without intervention of the microprocessor. Strictly speaking, Plug-and-Play
was introduced on later ISA boards to coincide with the introduction of Win-
dows95.PnPenabledWindows95toidentifytheboardspresentinthePC’s
expansionslots.However,theconceptofautomaticboardidenticationwasa
featureofMCAandEISAarchitecturesyearsbeforeWindows95.Unfortunately,
theprevailingDOSoperatingsystemdidnotsupportPnP,sofewPCscould
takeadvantageofthisfeature.Plug-and-Playoperationrequiresacombination
ofPnP-compatibleperipherals,systemhardware,systemBIOS,andoperating
systemsoftware.Thesearestandardfeaturesoftoday’sPCI-basedsystems.
Plug-and-Playcapabilityallowsacomputertorecognizeanewinternalorexter-
nal peripheral plugged into the system automatically, and to configure system
resources(interrupts,memoryaddresses,etc.)tooperatethesedevices.Inthe
past(beforePnP),installinganISAboardtypicallyinvolvedsettingphysical
jumpers or switches, or configuring the board through software to establish
IRQ,I/Oaddresses,orDMAchannelsfortheperipheral.Inaheavilypopulated
system, users could find it difficult to install all the desired peripherals without
running out of resources or causing conflicts.
Nevertheless, ISA may be so well entrenched with some users that it may persist
forsometime,despitedwindlingsupportformIntel,Microsoft,andPCmanu-