Driving a character type LCD from a PC printer port - Hantronix, Inc

Application Note
Crystal Clear and Visibly Superior LCD Modules
Driving acharacter type LCD from aPC printer port
General:
Character(alphanumeric)typeLCDmodules are most commonlydrivenfromanembeddedmicrocontroller. It
is sometimes desirabletobeabletodrivethis typeof moduledirectly from aPC. This wouldallowthemodule
tobetestedorademonstration orsimulationtobeset upquicklyandwithaminimum of engineering. This
applicationnotedescribesamethodof drivinganLCDcharactermodulefromthe printerport of aPCwith
minimalexternalhardware.
The printer port of aPC:
Thereareat least three versions of printerportsavailablenowon PCs.
! SPP (StandardParallel Port)
! EPP (EnhancedParallel Port)
! ECP (Extended Capability Port)
Tomakethis designasuniversal as possible itwill bebasedontheleast capableandoldest specification,
SPP.
S4 S5 S7 S6
D7 D6 D5 D4 D3 D2 D1 D0 C0
Figure1illustrates thepinoutof theparallelport andthe
registerthat controls eachpin. Theconnectordrawingis
thefemaleconnectoronthePC.
Figure1
13 12 11 10 9 8 7 6 5 4 3 2 1
25 24 23 22 21 20 19 18 17 16 15 14
GND
C3 C2 S3 C1
Tomaintain compatibility withallversions of parallel
portsnowavailable onPCs onlytheoutput capabilities
are used inthis example. TheLCDmoduleprovidesa
busy flagthat canbereadto control thetimingof data
andcommand writes. Thetiming of alldataand
commandtransfers is knowandthis is usedtotimethe
transferof dataandcommands tothe LCDinsoftware.
Theparallel port has 12bufferedTTLoutput pins which arelatchedandcanbewrittenunder
program control usingtheprocessor’s Out instruction. The portalsohas input pins but they arenotusedin this
example.
Hardware Description:
Thesuggestedcircuit is quitesimple. Thedisplay,dataandcontrollines areconnectedtotheprinterport lines
as showninTable 1andtheschematic.Theread/write(RW)lineis tiedlowtofixthedisplayin thewrite mode.
TheEnable (E)andtheRegisterSelect (RS)lines are tiedtotwoof theparallelport controllines. Mostparallel
ports haveactiveorpassivepull-ups ontheselines, butnot all.Tobeuniversal 10k pull-upresistors areshown
onthesetwolines. Contrast controlis provided bya10k pot. DCpowerfortheLCDand theback light,if
installed, must beprovidedexternally.
LCDs requirevery littlepowertooperate, typically less than5mA. The outputs ontheparallelportareableto
Page1
Copyright 2000, Hantronix,Inc.Allrights reserved

Application Note
Crystal Clear and Visibly Superior LCD Modules
sourceupto10mA, soitispossibletopowertheLCDfrom oneof theoutput lines. This doesn’t apply to the
back light whichrequires from 50mA to300mA. Oneof thecontrollines canbeusedforthis purposeas shown
C2 16
Figure2
1N4148
+
1uF
V DD
P
A
R
A
LL
E
L
P
O
R
T
D0 2
3
4
5
6
7
8
D7 9
C0 1
C3 17
D0
LCD Display Module
D7 E RS R/W V SS
V DD V DD
10K
10k 10k
V L
V DD
V DD
Schematic
inFigure2. This alsohas theadvantageof being abletoshutoff thedisplay viasoftware. A1onthisline turns
thedisplay onand a0turns it off.
Thetype of cableusedtoconnectthe parallel port totheLCDwill determinethemaximumlengthofthe cable.
Ribboncable, forinstance, shouldnot beusedinlengths over3’.Ashielded, twistedpaircablecanbeused
upto50’. The quickest and most economical way ofbuildingashielded, twistedpairin smallquantities is to
useacommercial printercableof thedesiredlengthandcut offtheconnectorthatwould normallyconnect to
theprinter. The wiresarethenpreparedand connectedtotheLCDmodule.
Software:
Most contemporary PCs support 3parallelports at addresses 278/378/3BC. All values areinhex. Usually only
one portisphysically installed andinmostsystems it is at address 378 andis assignedtoLPT1. Anoutput
instructiontothe base addressof theport,278/378/3BC, willlatchdatatothe data portof theLCDas shown
below.
BIT 7 6 5 4 3 2 1 0
PIN 9 8 7 6 5 4 3 2
LCD D7 D6 D5 D4 D3 D2 D1 D0
Anoutputinstructiontothebaseaddress oftheport +2, 27A/37A/3BE, willlatchthelower 4bits of thedata
bus tothecontrolpins of theLCD. Onlytwoof thesesignals areneededtocontrolmost LCDcharacter
modules.Theexceptionis the40characterby4linemodules. Thesemodules havetwocontrollers onthem
and havean extraenableline.Theport pinC1canbeusedforthis 2ndenableonthe40x4modules.Thebits
areassigned totheLCDas shownbelow.
BIT 7 6 5 4 3 2 1 0
PIN - - - - 17 16 14 1
LCD RS E2* E *=40x4modules only.
Page2
Copyright 2000, Hantronix,Inc.Allrights reserved

Application Note
Crystal Clear and Visibly Superior LCD Modules
Program Example:
Thefollowingsample
code is writteninMicrosoftCandwilldisplayamessageonatwolineby 16character
LCDmodule. It can beusedasaguide inworkingwithlargerorsmallermodules as it has allof theelements
needed. Atotal of 80characters is writentothedisplay whichis themaximum any LCDcharacterdisplay with
one controllercan store. This program will thuswork fora1, 2or4linedisplay althoughonlytwolines will
display characters ona4linemodule.
Amoresophisticatedwayof writing characters is toissueaSETDDRAMADDRESS commandforeachline
tobewritten. Check withthedatasheet fortheparticularmoduleyouareusingto set theDDRAM address
and linelengths.
Theprogram first initializes thedisplay andthensends thetwolines ofdata. Thedisplayedmessageshould
read:
>> HANTRONIX > HANTRONIX

Application Note
Crystal Clear and Visibly Superior LCD Modules
_out(CONTROL, _inp(CONTROL)|0x08); /*Set RS */
/* Initializationroutine */
for(count=0;count