Custom PC - UK (2020-08)

FEATURE / ANALYSIS

10000000 inbinary?Howdoesthe
receiverknowhowtodistinguishbetween
eachzero,whichis justrepresentedasa
continuouslowvoltage?Theansweris that
thetransmitterreliesonthereceiverbeing
abletoaccuratelycounttoitselfsothatit
canworkouttheboundariesofeachbit.
Thisis whysuchserialcommunicationsare
consideredasynchronous,astheydon’thave
anaccompanyingclocksignal,unlikeparallel
communications.
Achievingthisindependenttimingisn’ttoo
difficultuptoa certainclockspeedandfora
briefperiodoftime– if thedataratewereslow
enough,youcouldeasilyachievethesame
ideamechanically– butkeepingthedata
flowinsyncathighspeedandforprolonged
periodsis considerablytrickier.Evena very
rudimentary1KHzserialline(alongwayoff
theMHzorevenGHzclockspeedsofmany
moderncomponents)wouldtransmitan8-bit
numberinunder100thofa second,sothat
timingcircuitryneedstobeveryprecise.
What’smore,westillneeda wayof
distinguishingbetweenthestartandendof
anygivenstringofdata.It’sallverywellbeing
insyncmostofthetime,butif youmissedthe
firstbit,orif therewasa smallslip-upalong
theway,howdoesthesystemknowwhere
onestringstartsandtheotherends?

Frame-basedtransmission
Togetroundtheproblemsofnothaving
anaccompanyingclocksignal,serial
transmissionsystemsrelyonhavingthedata
theysendsplitintostringsofonesandzeroes
ofa certainlength,whicharecalledframes.If
receiverknowstoexpectsomesortofgapor

otherformofsignaltodifferentiatebetween

eachframe,it canthenworkoutthestartand

endofeachframe,andalsore-syncitsown

clocksignaltomatchtheframeperiod.

Lookingata practical example,theRS232

serialstandardwasthedefactoconnection

formanyPCperipheralsformanyyears,and

it usesa verysimpleschemafordenoting

thestartandendofthedatastream.The

startofthetransmissionis simplydenoted

bythevoltageonthelinegoinglowfor1.5

clockcycles(thedefaultstateofthelineis

tobeheldhigh,toindicatethatthelineisn’t

damaged),whiletheendis signalledbythe

voltagegoinghigh.Theclockspeed– known

asthebaudrate– andtransmissionlength

arepresetatbothtransmitterandreceiver,so

if thereceiversamplesthevoltagewhereit

expectsthestopbittobelocated,andit’slow,

it knowsthere’sa synchronisationerror.

Thisframe-basedsystemworkswelland

allowsforallthekeyadvantagesofserial

communications,suchaslongercableswith

veryfewconductors.However,earlyversions

wereslow.ThatRS232standardtoppedout

at14.4KB/sec,whereasexternalparallel

cablescouldreachupto2.5MB/secand

internalparallelcablessuchasIDEharddrive

cablestoppedoutat133MB/sec.Similarly,

RAMaccessanddatatransferbetween

components,suchastheCPUandgraphics

card,werestillallperformedbyparallelPCB

tracesonthemotherboard,upuntila couple

ofdecadesago.

Serialvsparallel

Bothserialandparallelcommunications

havetheirissues,sowhyis parallel

communication– andthebulkycablesthat

camewithit – largelyconsignedtothepast?

Well,therearethreeprincipalfactors.The

firstis thatit becomesproportionallymore

difficulttoincreasetheclockspeedofa

parallelconnectionascomparedwitha serial

connection,simplydue tothenumberof

signalsthathavetoarriveperfectlyinsync.

Whileerrorscancertainlybeintroduced

toa serialconnection,it’smucheasierto

isolateandcorrectthem.Withparallel,it just

becomesa garbledmess.

Butif youcan’tincreasefrequency,youcan

stilltheoreticallyaddmoreconnections,can’t

you?Doubletheconnectionsandyoudouble

thedatarate!Unfortunately,therearepractical

limits.Whetherit'sspaceona motherboard

orthesheerbulkofcables,there’sonlyso

Clock start 8 data bits

Margin of error

for clock

Stopbit

Startbit

Signal sampling points

Asynchronous character: 8 data bits, one stop bit

Start and stop bits frame the eight bits of data that are being sent

USB WAS THE ORIGINAL

POSTER CHILD FOR MODERN

APPLICATIONS OF SERIAL

COMMUNICATIONS