Connecting arduino programming and networking with the ethernet shield

ConnectingArduino:ProgrammingandNetworkingwiththeEthernetShield
Copyright©2014BobHammell.
EBooksarenottransferable.Allrightsreserved.Nopartofthispublicationmaybe
reproduced,distributed,ortransmittedinanyformorbyanymeans,including
photocopying,recording,orotherelectronicormechanicalmethods,withoutthe
priorwrittenpermissionofthepublisher,exceptinthecaseofbriefquotations
embodiedincriticalreviewsandcertainothernon-commercialusespermittedby
copyrightlaw.
Trademarkednames,logos,andimagesmayappearinthisbook.Ratherthan
useatrademarksymbolwitheveryoccurrenceofatrademarkedname,logo,or
image,thenames,logos,andimagesareusedonlyinaneditorialfashionandto
thebenefitofthetrademarkowner,withnointentionofinfringementofthe
trademark.
TheinformationinthispublicationisprovidedbyBobHammellonan“ASIS”
basis.BobHammellmakesnowarranties,expressorimplied,regardinguseof
theinformationaloneorincombinationwithyourproducts.Neithertheauthornor
theeditorsnorthepublishercanacceptanylegalresponsibilityforanyerrorsor
omissionsthatmaybemade.
PublishedintheUnitedStatesofAmericabyBobHammell.
ISBN-10(Print):1-500-74567-7
ISBN-13(Print):978-1-500-74567-7
ISBN-13(ePub):978-1-312-41034-3
Anysourcecodeorsupplementarymaterialsreferencedbytheauthorinthistext
areavailableforreadersatwww.connectingarduino.com.



TableofContents
Preface

GettingStarted
ConnectingtheEthernetShield•Establishinganetworkconnection•
Testingconnections
UsingSDCards
FormattingandinitializingSDcards•ReadingandwritingfromSD
cards•Creatingandremovingdirectories
ArduinoasaWebClient
MakingHTTPGETandPOSTrequests•Scrapingwebpages•
Handlingtimeouts•Sendingtweets
ArduinoasaWebServer
UsingastaticIPaddress•PortforwardinganddynamicDNS•
AcceptingincomingHTTPconnections•ServingfilesfromtheSD
card•Creatingaweb-basedUI
UsingUDPandSocketProgramming
CommunicatingoverUDP•BuildingaDNSserver•Implementinga
customapplicationprotocol
AppendixA–HypertextTransferProtocol–HTTP/1.0
AppendixB–DNS–ImplementationandSpecification



Preface
Atthispoint,theArduinohardlyneedsanyintroduction.It’sbecomeaforceof
nature–inspiring,initsshortlifetime,millionsofpeoplefromallwalksoflifeand
withvaryinglevelsofpriorexperienceinelectronicsandcomputerprogramming.
There’smuchyoucandowiththisflexibledevelopmentplatform,andsomuch
amazingworkhasalreadybeendone.Butwherethingsreallygetinteresting,
reallygetuseful,iswhenyoumakeprojectsthattalktoeachotherandtotherest
oftheworld.
Thathobbyistandbeginnerelectronicshackersand“makers”cancreate

standalonedeviceswhichcommunicatewithothermachinesonthelocalnetwork
andacrosstheInternet,andusingthesameInternetprotocolsasusedby
desktopPCs,servers,andmobiledevices,iscertainlynotinsignificant.
Despitetheemergenceofnewdevelopmentboards,shields,andmodules,the
EthernetShieldremainsapopularchoiceforArduinoprojects.Andit’seasyto
seewhy–thesectionofthisbookthatcoversgettingtheshieldupandrunningis
verythin.Unfortunately,makingfulluseofthisingeniousdeviceisalittlemore
difficultthanthefirststepssuggest…Andthatbringsmeneatlytothesubjectof
protocolsandthereasonwhyIwrotethisbook.


What’sinthisBook?
Internetandnetworkcommunicationismadeupofmanylayers–startingwith
low-levelprotocolsandtechniquesusedtohandlecommunicationwithhardware
devices,suchasnetworkcards,modems,andWi-Fidongles.Ontopofthislayer,
theInternetprotocol(IP)isresponsibleforthedeliveryofmessagefragments(or
packets)totheintendedrecipient.Then,runningoverIP,youhavethetransport
layerwheretransportcontrolprotocol(TCP)addserror-checkingandstreaming
capabilities.Theapplicationlayerconsistsofprotocolssuchashypertexttransfer
protocol(HTTP),domainnamesystem(DNS),andsimplemailtransferprotocol
(SMTP).Theseapplicationprotocolsdefinehowdataisencodedandexchanged
foraspecificpurpose.Youcouldalsosaythatweb-basedapplication
programminginterfaces(APIs)andwebserviceswhichrunoverHTTPadda
fourthlayertothissystem.
TheEthernetShield,inpartnershipwiththeEthernetlibrarythatcomeswiththe
ArduinoIDE,doesanexcellentjobofencapsulatingthecomplexitiesofTCP/IP
andtalkingtotheWiznetW5100integratedcircuitontheshield.Butitshelponly
goesasfarasthetransportlayer;you’reonyourownwhenitcomestoHTTPand
theapplicationlayer.Atfirst,workingwiththeseprotocolsseemsadauntingtask
–onethatappearsthatonlyaccomplishedandexperiencedprogrammershave

theskillsorknowledgetoattempt–andthissuggestionisreinforcedbythe
relativelysmallnumberofexamplesandguidesthatreallytrytoexplainthe
detailsofapplicationprotocolstoArduinoprogrammers.Soifyoulearnonlyone
thingfromthisbookthenIhopeitisthis:workingwithapplicationprotocolsis
nothingtobeafraidof.
I’vewrittenConnectingArduinotoshowyou,inquitealotofdetail,howtouse
applicationprotocolsinyourArduinosketchesandgetthemostoutofthe
EthernetShield.Themajorityoftheinformationisorganizedintoeight“projects”–
andIusethattermloosely.Thegoalwasnottogiveyouarecipebook,ora
collectionofplansforArduinoprojects.Instead,Iwanttowalkyouthroughthe
backgroundinformation,libraryclassesandmethods,andprogramming
techniquesthatyoucanuseinyourownprojects.But,critically,Iwantedtogive
eachitemenoughcontextualinformationsothatit’seasyforyoutoseeits
relevancetothetaskathand.Thechaptersdivideintothemes,andeachproject
buildsontheknowledgeandinformationpresentedinthepreviousproject.As
such,youmayfinditbeneficialtoreadthroughthebookinorder,evenifyoudo
notactuallybuildandcompleteeachproject.
Myprojectsmightseembasic,buttheonesyoudevelopyourselfafterwardswill
bemuchmoreinteresting.AndIhopeyouletmeknowaboutthecoolthingsyou
build–orbetterstill,havethedevicescontactmethemselves.


WhoShouldReadthisBook?
Unfortunately,Ican’tteachyoueverythingabouttheArduinointhespaceofone
book.There’stoomuchaboutelectronics,C,andprogrammingingeneralto
cover.Ihavetoassumethatyou’realreadycompetentatprogrammingthe
Arduinoandbuildingsimplecircuits.Withthissaid,ifyoucanconnectalightemittingdiode(LED)totheArduino,throughanappropriateresistor,andwritea
sketchthatturnstheLEDonandsendsamessagetotheArduino’sserialport
thenyou’lleasilyunderstand90%ofthecodeandcircuitryinthisbook.
Forsomeoftheprojects,basicfamiliaritywithhypertextmark-uplanguage

(HTML)wouldbeuseful.Buildingwebpagesandweb-baseduserinterfacesis
definitelyaskill,butitisonethatyoucanlearnasyougo,andthereisno
shortageofexcellenttutorialsavailableonlinetohelpyou.


OnlineResources
ConnectingArduino.comisthecompanionwebsiteforthisbook;youcancontact
methereifthere’sanythingIcanhelpyouwithorifyouwanttoshowoffyour
work.I’vealsoputalloftheprojectsketchesuptheresothatyoucandownload
them,insteadoftypingthemin.It’llbeworthyourwhiletovisitthesiteregularly–
anynews,updates,andaddendumswillbepostedtherefirst.
Tothebestofmyability,Ihaveverifiedtheaccuracyofalloftheinformationinthis
book,andtriedtoensurethatthecodesamplesarerobustenoughforyoutouse
(whilenotbeingsofullofoptimizedprogrammingcodeanderror-checkingasto
makethecodedifficulttounderstand).However,thingschangeandmistakesdo
happen.Youcanhelpmetoimprovefutureeditions,forthebenefitofother
Arduinoenthusiasts,bycontactingmeatthewebsiteifyoufindanyerrors,
inaccuracies,orplaceswhereinformationisconfusing.


ConventionsUsedinthisBook
Thefollowingtabledescribesthetextconventionsusedinthisbook.
Convention

Meaning

Italic

Textthatappearsinitalicsreferstofilenames,variableand
functionnames,orothercodethatexistsintheprojectsketch

orArduinolibraries.Withinthecontextofgivinginstruction,
italictextshouldbetypedexactlyasshown.

Bold

Withinthecontextofgivinginstruction,itemsinboldtextare
userinterfaceelements,suchaskeystrokes,menuitems,or
buttonlabels.Inothercontexts,wordsmaybeemboldenedfor
emphasis.

Monospace
font

AmonospacefontisusedforArduinoC,Processing,
JavaScript,andHTMLcodethatshouldbetypedinyour
project.

Coloredtext

Itemsshownwithcoloredtextarelinkstootherpagesinthis
book.



GettingStarted
TheArduinoEthernetShieldisanadditionalcircuitboardthatfitsontopofyour
Arduino.ItextendstheArduino’scapabilitieswithcircuitrytoconnecttoanetwork
router,usingacommonly-availableRJ45Ethernetcable.YourArduinoprojects
cancommunicatewiththeworldthroughthisconnection–everythingfrom
fetchinginformationfromtheInternetanddisplayingitonaliquidcrystaldisplay

(LCD),toprovidingpublically-accessible,web-basedtoolsthatcancontrolmotors
andotherhardware.
Morethanjustahardwaredevicethatcanconsumecontentandaccept
messages,theEthernetShieldisyourentrypointintobuildingthingsforthe
InternetofThings–devicesthattakeanactiveroleintalkingtohumansandother
machinesoverInternetprotocols.
InThisChapter
ConnectingtheShield
EstablishingaNetworkConnection
IntroducingWebClientsandWebServers


ConnectingtheShield
TheArduinoEthernetShieldR3mountsontopofArduinodevicesusinglong
wire-wrappedheadersthatextendthroughtheshieldandintotheheadersofthe
Arduinobelow.Itonlyfitsinonedirection.
ToconnecttheshieldtoanArduinoUnoR3orArduinoLeonardo:
1. DisconnecttheArduinofromallpowersources,andremoveanywires
connectedtoit.
2. Lineuptheshield’sheaderswiththoseoftheArduino.
3. Applygentlepressureuntiltheshieldslotssecurelyintoplace.

Figure1.ConnectingtheshieldtoanArduinoUno

ToconnecttheshieldtoanArduinoMega2560:
1. DisconnecttheArduinofromanysourceofpowerandremoveanywires
connectedtoit.
2. Lineuptheshield’sheaderswiththoseoftheArduino.Theshieldslotsinto
thetwoleft-mostgroupsofheaders–uptoRX0onthetoprowandA5onthe
bottomrow.

3. Applygentlepressureuntiltheshieldslotssecurelyintoplace.


Figure2.ConnectingtheshieldtoanArduinoMega2560

TheEthernetShieldR3canalsobeusedwithearlierUnodevicesandthe
Duemilanove.However,whenusingolderArduinos,fouroftheshield’sheader
pinsareleftunconnected.Youmustensurethatnoneofthesepinsareallowedto
makecontactwithanyoftheArduino’scomponents,oreachother.
Suitableoptionsforthisare:
Wrapthetwoleft-mostpinsonthetoprow,andthetwoleft-mostpinsonthe
bottomrow,ininsulatingtape.
Bendthetwoleft-mostpinsonthetoprow,andthetwoleft-mostpinsonthe
bottomrow,awayfromcontactwiththeArduino.
OncetheshieldisfittedsecurelyontheArduino,youcanreconnectthepower.
Caution:Itisusuallysafetoconnectanddisconnectcablesandwires
fromtheEthernetshieldwhiletheArduinoisconnectedtoitspower
supply.However,toavoidanyaccidentaldamagetoelectronic
components,itispreferabletodisconnectthepowerbeforedoingso.
TheconnectorsandkeycomponentsoftheArduinoEthernetShieldR3are
shownbelow:


Figure3.TheArduinoEthernetShield

ItispossibletostackothershieldsontopoftheEthernetShield,andtousemost
oftheArduino’spinsasusual.However,theArduinotalkstotheEthernetShield
overSPIandwhenactuallyusingtheEthernetShield,thefollowingpinsare
unavailableforanyotherpurpose:
ArduinoUno

Pin

ArduinoMega
Pin

Function

D4

D4

SS–whenusingtheSDcard.

D10

D10

SS–whenusingtheEthernetShield’sSPI
interface.

D11

D50

MOSI

D12

D51

MISO

D13

D52

SCK

–

D53

Notused.Butmustnotbesettoaninput.

ThereareseveralmethodsofcablingtheEthernetShieldtoyournetwork.This
choicemakesnodifferencetohowArduinosketchesareprogrammed,andyou
shouldsimplychoosetheonethatismostconvenientforyouandyour
workspace.

ConnectingtheEthernetShieldtoaRouter
ToconnecttheEthernetshieldtoarouter:


1. PlugoneendofaCAT5orCAT6EthernetcablewithRJ45connectorsinto
thesocketontheEthernetShield.
2. PlugtheotherendofthecableintoanavailableEthernetportonyourrouter.
3. PlugtheArduinointoasuitablepowersupply(ifitisnotconnectedalready).

UsingPowerLineAdapters
UnlessyouhaveaverylongEthernetcable,itmaynotbeconvenienttocable

yourEthernetShielddirectlytoyourrouter.
PowerLineadaptersaredevicesthatplugintoelectricalsocketsandsend
computersignalsalongthepowerlinesinyourhome.Theyaresoldinpairs:one
istobepositionedneartotherouterandoneistobeusedwhereneeded.
TheseadaptersrequirenoconfigurationandworkwellwiththeArduinoEthernet
Shield.

ConnectingtheEthernetShieldthroughaBridged
Connection
UsingastandardEthernetcable(oracrossovercable,ifyouhaveareallyold
PC),youcanconnecttheEthernetShieldtoyourPCandshareitsnetwork
connection.
1. PlugoneendofaCAT5orCAT6EthernetcablewithRJ45connectorsinto
thesocketontheEthernetShield.
2. PlugtheotherendofthecableintoafreeEthernetportonyourPC.
3. PlugtheArduinointoasuitablepowersupply(ifitisnotconnectedalready).
Fortheshieldtobeabletoconnecttothenetwork,youmust“bridge”the
connectionthatyourPCusestoconnecttothenetworkwiththeconnectionthatis
madetotheEthernetShield.Theprocessfordoingthismaybedifferent
dependingontheoperatingsystemthatyouarerunning.
OnWindows8/7/Vista/XP:
1. PresstheWindowslogokey+R.
2. Typencpa.cplandpressEnter.
3. HolddowntheCtrlkeyandclickboththenetworkconnectionthatisusedby
yourPCtoconnecttothenetwork,andthenetworkconnectionthatis
connectedtotheEthernetShield.
4. Right-clickoneoftheselectedconnections,thenclickBridgeConnections.
OnMacOSXyoucanshareyourMac’sWi-FiconnectionwiththeArduino
EthernetShield:

1. OntheApplemenu,clickSystemPreferences,andthenclickSharing.
2. Onthesidebar,clickInternetSharing1 ,andchoosetheInternetconnection
youwanttosharefromthe“Shareyourconnectionfrom”menu.
3. Selectthecheckboxlabelled“Built-inEthernet”.
4. ClickStart.
IfyourPC’soperatingsystemwarnsyouthatithasdetectedanIPaddress
conflict,youmayhavetoconnecteitheryourPCortheArduinotothenetwork
usingastaticIPaddress.


EstablishingaNetworkConnection
TheRJ45socketontheEthernetShieldcontainstwolight-emittingdiodes(LEDs).
TheleftLEDisthelinkindicatorandglowsorblinksgreenifasuccessfullinkhas
beenmadetotherouter.YoucanalsofindthissameindicatorasasurfacemountedLEDjustabovetheRJ45socketontheshield.IfthelinkLEDisnotlitat
all,checkyourconnectionsandreplacetheEthernetcableifpossible.Insome
circumstances,itmayalsobeusefultorestartyourrouter.
AgreenLEDdoesnotmeanthattheArduinoisnowconnectedtothenetwork,
onlythatcommunicationbetweentheshieldandtherouterisworking.Toactually
makeafullconnectiontothenetwork,theArduinomustbeprogrammedwitha
sketchthatusestheEthernetlibrarytosetseveralconfigurationoptions.
Inthissectionyouwillseehowtocreateabasicsketchthatconnectstoyour
networkoverdynamichostconfigurationprotocol(DHCP),andhowtotestthat
yourArduinoisproperlyconnected.

StartingaNewSketch
IntheArduinointegrateddevelopmentenvironment(IDE),startanewsketch.
TheArduinotalkstotheEthernetShieldoverserialperipheralinterface(SPI),and
sotoworkwiththeshieldyoumustincludeboththeEthernetandSPIlibrariesin
yourproject.Todothis,addthefollowingtwolinestothetopofthesketch:

#include<SPI.h>
#include<Ethernet.h>

SpecifyingaMACAddress
Thenextpieceofinformationthatisusuallydefinedinthesketchisthemedia
accesscontrol(MAC)address.AMACaddressisa48-bitnumber(usually
expressedas6bytes)thatuniquelyidentifiesadeviceonalocalareanetwork.
Thesenumbersareusuallybuilt-intothedeviceandneverchange.Youcan
generallyfindtheMACaddressforyourArduinoEthernetShieldprintedona
stickerontheundersideoftheshield,orontheboxthattheshieldcamein.Butif
youdonothaveone,itisusuallyfinetomakeupsixrandomnumbersfrom0
through255.Itishighlyunlikelythatyouwillrandomlychooseanaddressthatis
currentlybeingusedbyanotherdeviceonyournetwork.
MACaddressescanalsobeboughtfromtheIEEERegistrationAuthority,oryou
canbuyaread-onlymemory(ROM)chipthatispre-programmedwithaunique
address.MicrochipTechnologyInc.andMaximIntegratedInc.offerarangeof
low-costchipsofthistype.


TheMACaddress(eitherrandomlygeneratedorpurchased)istypicallyincluded
intheArduinosketchasaglobalarrayofbytes:
bytemac[]={0x00,0xC3,0xA2,0xE6,0x3D,0x57};

Asitisunlikelythatyouwilleverneedtomodifythisaddresswhilethesketchis
running,youcanalsodefinetheMACaddressusingaconstantarray:
constbytemac[]={0x00,0xC3,0xA2,0xE6,0x3D,0x57};

CompletingtheSketch
ThefullArduinosketchisshownbelow.Thisexampleconnectstothenetwork
usingDHCP,duringwhichtherouterassignsconnectionpropertiestothe

EthernetShielddynamically,andthenthesketchsendstheconnectiondetailsto
theserialport.Youcanviewthisinformationintheserialportmonitorinthe
ArduinoIDE.
#include<SPI.h>
#include<Ethernet.h>
bytemac[]={0x00,0xC3,0xA2,0xE6,0x3D,0x57};
voidsetup(){
//D53onanArduinoMegamustbeanoutput.
pinMode(53,OUTPUT);

Serial.begin(9600);
while(!Serial);

Serial.print("Establishingnetworkconnection…");

if(Ethernet.begin(mac)==0){
Serial.println("FAILED!");
}
else{
Serial.println("OK!");

Serial.print("IPAddress:");
Serial.println(Ethernet.localIP());

Serial.print("DefaultGateway:");
Serial.println(Ethernet.gatewayIP());

Serial.print("SubnetMask:");
Serial.println(Ethernet.subnetMask());

Serial.print("DNSServer:");
Serial.println(Ethernet.dnsServerIP());
}
}
voidloop(){
}

Themethodbegin()intheEthernetlibrary’sEthernetclassattemptstoconnectto
thenetworkusingthedetailspassedintoitasarguments.
Thereareactuallyfourformsofthismethodthatyoucanuse,dependingonhow
muchinformationyouwanttospecify:
voidbegin(uint8_t*mac,IPAddressip)
voidbegin(uint8_t*mac,IPAddressip,IPAddressdns)
voidbegin(uint8_t*mac,IPAddressip,IPAddressdns,IPAddressgateway)


voidbegin(uint8_t*mac,IPAddressip,IPAddressdns,IPAddressgateway,IPAddresssubnet)

Atabareminimum,youmustcallbegin()andpassaMACaddressasanarrayof
bytes.IfyoudeclaretheMACaddresswiththekeywordconst,youwillneedto
castittoapointerofuint8_tvalues.Forexample:
Ethernet.begin((uint8_t*)mac);

IfyoupassanIPaddressthentheEthernetShieldwillmakeanetwork
connectionusingastaticIPaddress.IfyoudonotdefineanIPaddressthenthe
shieldwillobtainonefromtherouterusingDHCP.Formoreinformationabout
staticIPaddresses,seeUsingaStaticIPAddress.
Theremainingtwo,optionalparametersareusuallynotneededexceptwhen
workingwithcomplicatednetworks.Ifyouhavetospecifythegatewayaddress
thenyoumustuseastaticIPaddress.Ifyouneedtospecifythesubnetaddress

thenyoumustprovideallthreeoftheotherarguments,andconnecttothe
networkusingastaticIPaddress.
begin()willreturnthevalue1ifitconnectedsuccessfully,and0iftheconnection
failed.
Ifthesketchfailstoestablishaconnectionthenthereareafewthingstotry:
ChangethecodetouseadifferentMACaddress.
Checkthatthelinkindicator(asdescribedabove)issolidgreenorblinking.If
thereisnolightthenthisindicatesaproblemwithyourwiring.
Checkyourwiringcarefully.ReplacetheEthernetcable(ifpossible),andtrya
differentconnectionmethod–suchasdirectlytoyourrouter.
Whenthesketchsuccessfullyestablishesanetworkconnection,itcallsfour
methodsoftheEthernetclasstoretrievetheconfigurationsettingsthatwere
givenbytherouter.
Method

Description

dnsServerIP()

ReturnstheIPaddressoftheprimaryDNSserverusedto
lookupdomainnamestofindtheirIPaddresses.

gatewayIP()

ReturnstheIPaddressoftherouterdevicethatprovidesthe
otherswithnetworkaccess.

localIP()

ReturnstheIPaddressoftheEthernetShieldonthenetwork.

subnetMask()

Returnsthesubnetmaskusedbythenetworktosegregate
devicesintologicalgroups.

TheDNSserver,gateway,andsubnetmaskdetailssenttotheserialportshould
matchthoseusedbyotherdevicesonyournetwork.


TolearnhowtosettheIPaddressandothernetworkconfigurationparameters
manually,andrelatedtopicssuchasportforwardinganddynamicDNS,see
ArduinoasaWebServer.

TestingtheConnection
TocheckthattheArduinoisabletorespondtonetworktraffic,youcanpingit.
OnWindows8/7/Vista/XP:
1. PresstheWindowslogokey+R.
2. Typecmd,thenpressEnter.
3. Typeping,followedbyaspace,andthentheIPAddressdisplayedinthe
serialportmonitoroftheArduinoIDE.
4. PressEnter.

Figure4.AsuccessfulpingonWindows

OnMacOSX:
1. Onthedock,clickFinder.
2. Onthesidebar,clickApplications.
3. ClickUtilities,thendouble-clickNetworkUtility.
4. OnthePingtab,intheboxlabelled“Enterthenetworkaddresstoping”,type

theIPAddressdisplayedintheserialportmonitoroftheArduinoIDE.
5. ClickthePingbutton.


IntroducingWebClientsandWebServers
Oncomputernetworks,suchastheInternet,machinesanddevicescanbe
definedintermsoftherolestheyplaywhenexchanginginformation.Thereare
twodifferentroles:clientsandservers.
Clientsstartconnectionswithothermachinesinordertoaccesstheinformation
thatiscontainedonthem.Forexample,awebbrowserisaclientthatconnectsto
othermachinestorequestwebpagesorfilesfromthem.
Themachinethatstoresand“serves”theinformationthatisrequestedisknown
asaserver.Serverssitandwaituntilaclientstartsaconversationwiththem,and
theyaretypicallycapableoftalkingtomanythousandsofclientsatthesame
time.Iftheserveris“offline”,oritisinanywayinaccessible,thentheinformation
thatneedstobesharedwithclientsisunavailable.
Inmostmoderncomputersystems,amachinecanactasbothaserveranda
clientatthesametime.Itsroleintheexchangeofinformationdependsonthe
softwareitisrunning,anditcanrunbothclientsoftwareandserversoftwareif
youneeditto.ButtheArduino’srelativelylowhardwarespecifications,andits
lackofamulti-taskingoperatingsystem,largelypreventitfrombeingbotha
serverandaclientatthesametime.
WhenwritingArduinosketchesthatusetheEthernetShield,youwilloftenneedto
decidebetweenprogrammingaclientandprogrammingaserver.Thequestions
toaskare:
1. Doyouneedtosendrequeststoothersystems,forexampleTwitter,eBay,
Facebook,andotherstoaccomplishthetask?Ifyoudo,youwillcreatea
clientbecausethosesystemsareservers.
2. DoestheArduinogatherorholdinformationthatmultipleothermachines
needtoaccess?Ifso,youwillcreateaserver.

3. DoestheArduinostartconnections,orshoulditsitandwaitforothersto
connecttoit?Createaclientwhenyouneedtostarttheconnections,anda
serverwhenyouneedtowaitforincomingconnectionstobecreatedby
anotherdevice.
ForinformationaboutcreatingclientsusingtheArduinoEthernetShield,see
ArduinoasaWebClient.
ForinformationaboutcreatingserverprojectsusingtheArduinoEthernetShield,
seeArduinoasaWebServer.
1

InearlyversionsofMacOSX,clicktheInternettab.



UsingSDCards
TheArduinohasquiteasmallamountofon-boardstorageandmemory.Onits
own,itcannotstoreenoughinformationtoservealargeweb-basedinterfaceor
sendmanyfilestoconnectedclients.Andwhenactingasaclientitself,manyof
thefilesanArduinoprojectneedstodownloadaretoobigtobeheldinmemory.
TheArduinoEthernetShieldcomeswithabuilt-inSecureDigital(SD)cardsocket
thatyoucanaccessusingtheSDlibrary,SD.h.Thislibraryissuppliedwiththe
Arduinointegrateddevelopmentenvironment(IDE),andsupportsFAT16and
FAT32filesystemsonstandardSDcardsandhigh-capacitySDHCcards.
SD.hisawrapperthatsimplifiesaccesstotheSDcard.Itusesanotherlibrary,
SDFat.h,whichisnotcoveredinthisbook.SDFat.hismuchmorecomplicated,
containingmanymethodsanddatastructuresforworkingwiththeSDcardata
low-level,anditmaybeinterestingtoreaderswhoarealreadyexperiencedwith
SDcards.
InThisChapter
FormattingandInitializingSDCards

ReadingfromSDCards
WritingtoSDCards