5.3. Parallel Port Troubleshooting
Unfortunately attaching an LCD module to a parallel port is not trivial.
In most cases it requires soldering abilities and basic knowledge of electronics.
The following hints might be helpful:
5.3.1. Check The Wiring
Wiring errors can easily be made. If you are unexperienced with the soldering iron better have someone solder it for you. Display modules are sensitive to electro static discharges, so touch a grounded (earthed) surface (metal computer case, water pipes...) before you handle these. You may also want to buy a static ground bracelet that you connect to your wrist to a ground. They are usually sold for a couple dollars (or the price of a couple beers in case the dollar is not a valid currency where you live 8) in electronics stores.
5.3.2. Power Source Unregulated / Noisy
Make sure your power supply delivers steady 5 Volts without noise or interruptions. The bare wall plug in transformer is 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.
5.3.3. Ground Lift
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 VDD pins of the display.
5.3.4. Latchup
Never let the supply voltage get much below the IO signal voltage. It may lead to a latchup condition (the LCD will try to get current from the IO ports) which will destroy the controller chip on the display.
5.3.5. Contrast
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.
 | Beware |
|---|---|
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. |
5.3.6. Parallel Port Voltage
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 RCL of your cable, this may not be enough and can cause unreliable operation.
5.3.7. Enable Signal Rise Time
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.
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.
| The rise time of a digital output can (usually) not be altered by Software. |
5.3.8. EMI
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.
5.3.9. One or Two Black Lines
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 LCDd software. But if the black line will not disappear although the wiring is working, the controller on the display may be defective.
5.3.10. Software Too Fast
If you have a super GHz computer it may happen that the signal timing generated by LCDd 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.
5.3.11. LED Backlight
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.
5.3.12. HD44780 Compatible
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'.
To name a few: KS 0066, KS 0070, KS 0076, LC 7985, NT 3881, SED 1278, ST 7066 ...
5.3.13. Miscellania
This text has originally been taken from a message by
Robin Adams <robin@adams-online.de>
Converted to docbook and slightly modified May 2002, Rene Wagner <reenoo@gmx.de>