Driving a character type LCD from a PC printer port - Hantronix, Inc
Driving a character type LCD from a PC printer port - Hantronix, Inc
Driving a character type LCD from a PC printer port - Hantronix, Inc
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Application Note<br />
Crystal Clear and Visibly Superior <strong>LCD</strong> Modules<br />
sourceupto10mA, soitispossibletopowerthe<strong>LCD</strong><strong>from</strong> oneof theoutput lines. This doesn’t apply to the<br />
back light whichrequires <strong>from</strong> 50mA to300mA. Oneof thecontrollines canbeusedforthis purposeas shown<br />
C2 16<br />
Figure2<br />
1N4148<br />
+<br />
1uF<br />
V DD<br />
P<br />
A<br />
R<br />
A<br />
LL<br />
E<br />
L<br />
P<br />
O<br />
R<br />
T<br />
D0 2<br />
3<br />
4<br />
5<br />
6<br />
7<br />
8<br />
D7 9<br />
C0 1<br />
C3 17<br />
D0<br />
<strong>LCD</strong> Display Module<br />
D7 E RS R/W V SS<br />
V DD V DD<br />
10K<br />
10k 10k<br />
V L<br />
V DD<br />
V DD<br />
Schematic<br />
inFigure2. This alsohas theadvantageof being abletoshutoff thedisplay viasoftware. A1onthisline turns<br />
thedisplay onand a0turns it off.<br />
The<strong>type</strong> of cableusedtoconnectthe parallel <strong>port</strong> tothe<strong>LCD</strong>will determinethemaximumlengthofthe cable.<br />
Ribboncable, forinstance, shouldnot beusedinlengths over3’.Ashielded, twistedpaircablecanbeused<br />
upto50’. The quickest and most economical way ofbuildingashielded, twistedpairin smallquantities is to<br />
useacommercial <strong>printer</strong>cableof thedesiredlengthandcut offtheconnectorthatwould normallyconnect to<br />
the<strong>printer</strong>. The wiresarethenpreparedand connectedtothe<strong>LCD</strong>module.<br />
Software:<br />
Most contemporary <strong>PC</strong>s sup<strong>port</strong> 3parallel<strong>port</strong>s at addresses 278/378/3BC. All values areinhex. Usually only<br />
one <strong>port</strong>isphysically installed andinmostsystems it is at address 378 andis assignedtoLPT1. Anoutput<br />
instructiontothe base addressof the<strong>port</strong>,278/378/3BC, willlatchdatatothe data <strong>port</strong>of the<strong>LCD</strong>as shown<br />
below.<br />
BIT 7 6 5 4 3 2 1 0<br />
PIN 9 8 7 6 5 4 3 2<br />
<strong>LCD</strong> D7 D6 D5 D4 D3 D2 D1 D0<br />
Anoutputinstructiontothebaseaddress ofthe<strong>port</strong> +2, 27A/37A/3BE, willlatchthelower 4bits of thedata<br />
bus tothecontrolpins of the<strong>LCD</strong>. Onlytwoof thesesignals areneededtocontrolmost <strong>LCD</strong><strong>character</strong><br />
modules.Theexceptionis the40<strong>character</strong>by4linemodules. Thesemodules havetwocontrollers onthem<br />
and havean extraenableline.The<strong>port</strong> pinC1canbeusedforthis 2ndenableonthe40x4modules.Thebits<br />
areassigned tothe<strong>LCD</strong>as shownbelow.<br />
BIT 7 6 5 4 3 2 1 0<br />
PIN - - - - 17 16 14 1<br />
<strong>LCD</strong> RS E2* E *=40x4modules only.<br />
Page2<br />
Copyright 2000, <strong>Hantronix</strong>,<strong>Inc</strong>.Allrights reserved
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