<sect1 id="ppttrouble">
<title>Parallel Port Troubleshooting</title>

<para>
Unfortunately attaching an LCD module to a parallel port is not
trivial.
</para>
<para>
In most cases it requires soldering abilities and basic knowledge
of electronics.
</para>
<para>
The following hints might be helpful:
</para>

<sect2 id="ppt-check-wiring">
<title>Check The Wiring</title>

<para>
Wiring errors can easily be made.
If you are inexperienced with the soldering iron better have someone solder
it for you.
Display modules are sensitive to electrostatic discharges, so touch an
earthed surface (computer case, water pipes...) before you handle these.
</para>
</sect2>

<sect2 id="ppt-power">
<title>Power Source Unregulated / Noisy</title>

<para>
Make sure your power supply delivers steady 5 Volts without noise or
interruptions.
The bare wall plug-in transformer is often not suitable, though you can
make it stabilized by adding an 7805 and a few capacitors.
Some noise induced in the supply lines my be tricky to track, even if
you have an oscilloscope.
</para>
</sect2>

<sect2 id="ppt-GND-lift">
<title>Ground Lift</title>

<para>
The power supply wires and especially the GND wires should be a little thicker
than the other wires.
If GND is not thick enough (or not existent, see 1) the resistance of the
wire may cause differing GND potentials in the circuit.
This may lead to strange display behaviour. It may also be wise to solder
a 100nF capacitor directly to the GND and V<subscript>CC</subscript> pins
of the display.
</para>
</sect2>

<sect2 id="ppt-latchup">
<title>Latchup</title>

<para>
Never let the supply voltage get much below the io signal voltage.
It may lead to a latchup condition which will destroy the controller
chip on the display.
</para>
</sect2>

<sect2 id="ppt-contrast">
<title>Contrast</title>

<para>
If you don't see anything on your display it may be that your contrast voltage
is set wrong. Turn your contrast potentiometer all the way to the end connected
to GND.
Contrast is highest then.
</para>

<note>
<title>Beware</title>
<para>
The module you got so ultra cheap may be an enhanced temperature model which
needs a negative contrast voltage for sufficient contrast - see chapter
99 on how to make negative voltage.
</para>
</note>
</sect2>

<sect2 id="ppt-voltage">
<title>Parallel Port Voltage</title>

<para>
Many modern mainboards and especially notebooks will not nearly output 5V for
a logic H as the older parallel ports did, because the operating voltage of
computers is lower than 5V these days.
I have measured voltages between 2.5V and 4V for logic H, which is barely
within specification of the HD44780.
If you account R<subscript>CL</subscript> of your cable, this may not be
enough and can cause unreliable operation.
</para>
</sect2>

<sect2 id="ppt-signal-rise-timing">
<title>Enable Signal Rise Time</title>

<para>
If you ever read the HD44780 datasheet you will notice that somewhere in the
'signal timing' table is written: 'Enable Signal Rise Time  max. 20nS'.
That means the Voltage on the HD44780 pin called 'Enable' has to rise
from 0 Volts to 5 Volts within 20 Nanoseconds and the other way round.
They should better print that in big fat red letters, because most HD44780s
are really picky about the enable signal rise time.
</para>

<para>
That is a Problem: If you count together the bad driving characteristics
of the parallel port combined with the capacitance of flat ribbon cable
you may easily get an order of magnitude slower rise time.
Therefore you should only use really short cable (shorter than 50cm)
for connecting the display to the parallel port.
It may also be useful to use pull-up resistors on the display module
or a schmitt-trigger.
</para>

<note>
<para>
The rise time of a digital output can (usually) not be altered by software.
</para>
</note>
</sect2>

<sect2 id="ppt-emi">
<title>EMI</title>

<para>
The cable from the parallel port to the display may be sensible to
electromagnetic interference and may emit electromagnetic radiation.
If you place your cellphone near the cable, you may get unexpected display
readings, on the other hand your house neighbour may not be able to listen
to his/her favourite radio station any more - so better use shielded cable
and put the display in a metallic case, perhaps a computer case.
</para>
</sect2>

<sect2 id="ppt-black-lines">
<title>One or Two Black Lines</title>

<para>
If you see one or two black lines on the display it means nothing more than
that the display is powered and contrast voltage is present.
If one or two black lines appear the controller has not been reset properly
by the on chip power on reset generator. No need to worry - it will be reset by
the <application>LCDd</application> software.
But if the black line will not disappear although the wiring is working,
the controller on the display may be defective.
</para>
</sect2>

<sect2 id="ppt-too-fast">
<title>Software Too Fast</title>

<para>
If you have a super GHz computer it may happen that the signal timing generated
by <application>LCDd</application> is too fast.
Adjust DELAYMULT in the source file to a bigger value.
Parallel port wirings usually don't permit to read back the busy flag of the
controller chip, so timing must be adjust so that the controller never is busy.
</para>
</sect2>

<sect2 id="ppt-backlight">
<title>LED Backlight</title>

<para>
Check whether you need a resistor for your LED Backlight and which value
it should have.
If you forget the required resistor the backlighting LEDs might become hot
and draw excessive current.
</para>
</sect2>

<sect2 id="ppt-hd44780-compatible">
<title>HD44780 Compatible</title>

<para>
The original HD44780 controller that we advertise to support has become the
industry standard for alphanumeric character displays.
The original HD44780 is out of production. It has many successors from many
manufactures, which sometimes won't tell you that their chips are 'compatible'.
</para>
<para>
To name a few: KS0066, KS0070, KS0076, LC7985, NT3881, SED1278, ST7066 ...
</para>
</sect2>

<sect2 id="ppt-misc">
<title>Miscellanea</title>
<para>
This text has originally been taken from a <ulink url="http://lists.omnipotent.net/pipermail/lcdproc/2002-May/005832.html">message</ulink> by
Robin Adams <email>robin@adams-online.de</email>
</para>
<para>
Converted to Docbook and slightly modified May 2002, Rene Wagner <email>reenoo@gmx.de</email>
</para>
</sect2>

</sect1>