204 lines
6.3 KiB
Plaintext
204 lines
6.3 KiB
Plaintext
<sect1 id="ppttrouble">
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<title>Parallel Port Troubleshooting</title>
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<para>
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Unfortunately attaching an LCD module to a parallel port is not
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trivial.
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</para>
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<para>
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In most cases it requires soldering abilities and basic knowledge
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of electronics.
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</para>
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<para>
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The following hints might be helpful:
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</para>
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<sect2 id="ppt-check-wiring">
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<title>Check The Wiring</title>
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<para>
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Wiring errors can easily be made.
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If you are inexperienced with the soldering iron better have someone solder
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it for you.
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Display modules are sensitive to electrostatic discharges, so touch an
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earthed surface (computer case, water pipes...) before you handle these.
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</para>
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</sect2>
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<sect2 id="ppt-power">
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<title>Power Source Unregulated / Noisy</title>
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<para>
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Make sure your power supply delivers steady 5 Volts without noise or
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interruptions.
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The bare wall plug-in transformer is often not suitable, though you can
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make it stabilized by adding an 7805 and a few capacitors.
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Some noise induced in the supply lines my be tricky to track, even if
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you have an oscilloscope.
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</para>
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</sect2>
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<sect2 id="ppt-GND-lift">
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<title>Ground Lift</title>
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<para>
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The power supply wires and especially the GND wires should be a little thicker
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than the other wires.
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If GND is not thick enough (or not existent, see 1) the resistance of the
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wire may cause differing GND potentials in the circuit.
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This may lead to strange display behaviour. It may also be wise to solder
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a 100nF capacitor directly to the GND and V<subscript>CC</subscript> pins
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of the display.
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</para>
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</sect2>
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<sect2 id="ppt-latchup">
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<title>Latchup</title>
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<para>
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Never let the supply voltage get much below the io signal voltage.
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It may lead to a latchup condition which will destroy the controller
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chip on the display.
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</para>
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</sect2>
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<sect2 id="ppt-contrast">
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<title>Contrast</title>
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<para>
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If you don't see anything on your display it may be that your contrast voltage
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is set wrong. Turn your contrast potentiometer all the way to the end connected
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to GND.
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Contrast is highest then.
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</para>
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<note>
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<title>Beware</title>
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<para>
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The module you got so ultra cheap may be an enhanced temperature model which
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needs a negative contrast voltage for sufficient contrast - see chapter
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99 on how to make negative voltage.
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</para>
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</note>
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</sect2>
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<sect2 id="ppt-voltage">
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<title>Parallel Port Voltage</title>
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<para>
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Many modern mainboards and especially notebooks will not nearly output 5V for
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a logic H as the older parallel ports did, because the operating voltage of
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computers is lower than 5V these days.
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I have measured voltages between 2.5V and 4V for logic H, which is barely
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within specification of the HD44780.
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If you account R<subscript>CL</subscript> of your cable, this may not be
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enough and can cause unreliable operation.
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</para>
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</sect2>
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<sect2 id="ppt-signal-rise-timing">
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<title>Enable Signal Rise Time</title>
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<para>
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If you ever read the HD44780 datasheet you will notice that somewhere in the
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'signal timing' table is written: 'Enable Signal Rise Time max. 20nS'.
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That means the Voltage on the HD44780 pin called 'Enable' has to rise
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from 0 Volts to 5 Volts within 20 Nanoseconds and the other way round.
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They should better print that in big fat red letters, because most HD44780s
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are really picky about the enable signal rise time.
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</para>
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<para>
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That is a Problem: If you count together the bad driving characteristics
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of the parallel port combined with the capacitance of flat ribbon cable
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you may easily get an order of magnitude slower rise time.
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Therefore you should only use really short cable (shorter than 50cm)
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for connecting the display to the parallel port.
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It may also be useful to use pull-up resistors on the display module
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or a schmitt-trigger.
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</para>
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<note>
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<para>
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The rise time of a digital output can (usually) not be altered by software.
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</para>
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</note>
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</sect2>
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<sect2 id="ppt-emi">
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<title>EMI</title>
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<para>
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The cable from the parallel port to the display may be sensible to
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electromagnetic interference and may emit electromagnetic radiation.
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If you place your cellphone near the cable, you may get unexpected display
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readings, on the other hand your house neighbour may not be able to listen
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to his/her favourite radio station any more - so better use shielded cable
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and put the display in a metallic case, perhaps a computer case.
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</para>
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</sect2>
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<sect2 id="ppt-black-lines">
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<title>One or Two Black Lines</title>
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<para>
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If you see one or two black lines on the display it means nothing more than
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that the display is powered and contrast voltage is present.
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If one or two black lines appear the controller has not been reset properly
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by the on chip power on reset generator. No need to worry - it will be reset by
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the <application>LCDd</application> software.
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But if the black line will not disappear although the wiring is working,
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the controller on the display may be defective.
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</para>
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</sect2>
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<sect2 id="ppt-too-fast">
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<title>Software Too Fast</title>
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<para>
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If you have a super GHz computer it may happen that the signal timing generated
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by <application>LCDd</application> is too fast.
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Adjust DELAYMULT in the source file to a bigger value.
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Parallel port wirings usually don't permit to read back the busy flag of the
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controller chip, so timing must be adjust so that the controller never is busy.
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</para>
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</sect2>
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<sect2 id="ppt-backlight">
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<title>LED Backlight</title>
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<para>
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Check whether you need a resistor for your LED Backlight and which value
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it should have.
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If you forget the required resistor the backlighting LEDs might become hot
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and draw excessive current.
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</para>
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</sect2>
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<sect2 id="ppt-hd44780-compatible">
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<title>HD44780 Compatible</title>
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<para>
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The original HD44780 controller that we advertise to support has become the
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industry standard for alphanumeric character displays.
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The original HD44780 is out of production. It has many successors from many
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manufactures, which sometimes won't tell you that their chips are 'compatible'.
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</para>
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<para>
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To name a few: KS0066, KS0070, KS0076, LC7985, NT3881, SED1278, ST7066 ...
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</para>
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</sect2>
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<sect2 id="ppt-misc">
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<title>Miscellanea</title>
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<para>
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This text has originally been taken from a <ulink url="http://lists.omnipotent.net/pipermail/lcdproc/2002-May/005832.html">message</ulink> by
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Robin Adams <email>robin@adams-online.de</email>
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</para>
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<para>
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Converted to Docbook and slightly modified May 2002, Rene Wagner <email>reenoo@gmx.de</email>
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</para>
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</sect2>
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</sect1>
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