Custom PC - UK (2020-08)

FEATURE / ANALYSIS

Ofcourse,anyonewhohasbeeninthe
PCgamelongenoughwillrememberwhen
USBfirststartedtakinghold,withsoftware
oftenneedingtobeinstalledbeforea USB
devicewaspluggedintoa PC,anddevices
regularlyjustdisappearingfromyoursystem.
It wasalldowntothefactthatextrasoftware
supportwasrequiredinorderforthesystem
tocorrectlyrecognisea deviceandroutedata
toandfromit. Withserial,thesystemhad
fixedportsandit justfiredthedatastraight
intowhateverwasconnectedtotheport,for
betterorworse.
WiththearrivalofUSB3,andthedesirefor
evergreaterdatatransferrates,extrawires
wereaddedtothestandard,withanother
twopairsofdatalinesadded.Theseallowed
fora muchfasterSuperSpeedtransfermode
(againenabledbyfurtherenhancementsin
chipdesignandsignalhandling)whilethe
existinglinescouldstillbeusedforlegacy
compatibility.That’swhyUSB3 B ports
havethatfunnydouble-deckershapeand
extracontacts.
ThemostrecentadditionhasbeenUSB
Type-C;strictlyspeaking,thisisn’ta USB
standardatall,butjusta physicalconnection
standardthatcanbeusedtotransmitUSB
data.Insidethattinyrotationallysymmetrical
connectorare24-pinsthatallowthe
connectortobeinsertedeitherwayround
andstillmaintainitsfull12-pinconnectionfor

conveyingthemajorityofhigh-speedUSB

standards.Infact,thelatestUSB3.2standard

evenusesthosespare 12 pinstodoublethe

datarate,soyouessentiallyhavea cable

that’srunningfourserialdatalinestogetherin

parallel.Themorethingschange...

Memory

AttheotherendofthescaletoUSB,system

RAM(andmostotherformsofRAM,suchas

VRAM)is oneofthelastholdoutsoflargely

conventionalparallelcommunicationinour

PCs.Thesheervolumeofdatathatneeds

tobesentbackandforthbetweentheCPU

andsystemmemorystilllendsitselfto

thisefficientmethod.Moreover,because

memoryis basicallya singular,dedicated

componentincloseproximitytotheCPU,it’s

possibletoengineerourwayaroundthecore

shortcomingsofparallelcommunication.

PCBtracescanbelaidouttoensuredata

pathwaysareconsistentandcrosstalk

is minimal,whiletheextraoverheadof

serialisingthedatatransferwouldonly

servetoholdupa lotofoperations.

Assuch,modernmemorycommunicates

witha CPU– whichthesedayshousesthe

memorycontroller– viaverywideparallel

interfaces.DDR4usesa 64-bitmemory

interfacewitha whopping260-pinphysical

connectiontothemotherboard.Internally,

theRAMtakesadvantageofdataparallelism

too,spreadingdatatransferrequestsacross

multiplebanksofintegratedcircuitson

eachmemorymoduleandacrossmultiple

memorymodules.Thisallowsthememory

tokeepthatincrediblyfastinterfacefedwith

data,whichwouldn’tbepossibleif it were

sendingallitsdatarequeststoa singlechip.

TheinterfaceonDDR4usesa clock

speedthat’stypicallyintherangeof1200-

2000MHz;thankstodata beingtransferred

onboththerisingandfallingedgesof

theclocksignal,thisresultsinthetypical

2400MHz-4000MHzdataratesweseefor

modernDDR4memory.

Dozens of data lines connect the RAM slots to the

CPU. Note the zigzag pattern in some, used to

ensure all the traces are the same length

The use of multiple serial lanes enables PCI-E
to scale its connection speed according to the
connected device