(Disclaimer: This project involves electronics, your computer, a soldering iron and other bits and pieces - all care is taken but no responsibility. If you end up doing anything bad to yourself or your system after following these instructions please don't blame us.)
Connecting a LCD display to the end of a printer-cable is a simple and effective way of providing additional functionality to your computer system. The printer-cable LCD can be mounted on your computer case or fitted inside a Jiffy-Box and displayed somewhere away from your case. I was very surprised at how quick, easy and cheap this little mod was to make. Even the fully functional software interface is simple to install and configure.
In this article I am going to use a non-backlit 16 x 2 character LCD, mainly because it is cheap and simple. Keeping the costs fairly minimal meant that our printer-cable LCD project ended up costing under A$25. LCD displays can be purchased from mostly any electronics store, but I purchased this one from Oatley Electronics - because it was the cheapest I could find, and they are only just up the road from me.
You'll probably have to buy only three things to make your own printer-cable LCD: an 'Industry standard' LCD display with a Hitachi HD44780 (or compatible) chipset (cost A$16). A standard printer cable (cost A$6 here or pretty much everywhere) to take the data output from your PC's printer port to the LCD screen and a Molex (HDD power cable) splitter/connector to power the LCD from your computer's power supply. Some wire and soldering tools are also required, but if you are attempting this project, I'd hope you already have those. While the soldering is a little delicate, it is not difficult. I will be the first to admit that my soldering skills are quite poor and if I can do this, then almost anyone can. A little practice and some patience is all it takes. If you want to practise soldering on something simpler but still useful, try this project.
Note: I was using a 16x2 (Non Backlit) LCD (PowerTip, made by Actron as per here and found as Item DL9 here), however this method should work with any HD44780 compatible LCD. But you may need to take careful note of the pin-out layout if your LCD is different.
The first step - as you can see in the picture above - was to remove the plug-covers on the ends of the printer cable. Each cover had four little clips that could be opened to split the cover into two separate pieces. This allows you to easily identify which wires match the pins on the parallel port end of the cable.
A little bit of preparation can help quite a bit in a project like this one, and the next step was to 'tin' the contact points on the LCD's PCB. Simply place the bare solder wire over the contact, quickly push down with a hot soldering iron, and you should get a nice little blob of solder coating the contact.
Next cut two pieces of wire - about 20cm longer than your printer cable - to take the power from the Molex connector to the LCD. Then cut three smaller lengths of black (or whatever colour your negative will be) wire and solder them onto the end of the longer piece - there are three negative/earthed contacts required, and only one positive (5V) connection. Use some heat-shrink-tubing to cover and protect the joints - not only is this functional, but it hides your dodgy soldering joints as well.
After soldering the wires together it should look something like the image above. Except if you do it right (like I didn't the first time when I took this picture) there should be 3 black/earth wires and only 1 positive wire required.
Now the fun part begins - soldering
the wires up the LCD - take it slowly and carefully and things
should not be too hard. The following table defines the power/parallel
pin-to-LCD connections required. The LCD display's PCB should
have the connection points clearly marked, and these connections
should work for any HD44780 compatible LCD. (Backlit LCD's will
have two additional connections to power the backlight (these
are the two terminals to the left of contact '1' in the picture
above) also larger 40 x 4 LCD's may have a second Enable line.)
NOTE: The colours below are for EXAMPLE ONLY. The colours in your cable may differ. What's important is the pin numbers, not the colours.
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Earth (Black wire on Molex connector) | Earth |
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+5V (Red wire on Molex connector) | Power |
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Earth (Black wire on Molex connector) |
This adjusts the contrast of the
LCD digits. Earthing pin 3 provides maximum contrast. A 10k pot can be used to provide a variable value if required. |
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Parallel Port pin 16 (Green/white wire) | Register Select |
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Earth (Black wire on Molex connector) |
Read/Write. Since we are not going
to read data from the LCD, we can ground pin 5; to set it permanently in write mode. |
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Parallel Port pin 1 (Pink wire) | Enable - Strobe |
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Parallel Port pin 2 (Red wire) | Data bit 0 |
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Parallel Port pin 3 (Yellow wire) | Data bit 1 |
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Parallel Port pin 4 (Green wire) | Data bit 2 |
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Parallel Port pin 5 (White wire) | Data bit 3 |
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Parallel Port pin 6 (Blue wire) | Data bit 4 |
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Parallel Port pin 7 (Purple wire) | Data bit 5 |
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Parallel Port pin 8 (Pink wire) | Data bit 6 |
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Parallel Port pin 9 (Grey wire) | Data bit 7 |
It's a good idea to start with the four power cables, because they will be easier to attach. Use a little more heat-shrink around the ends of the wires, to ensure there is no shorting between the earth and positive wires. After the 5V line and three earth cables are connected, it's time to start to wire up the printer cable.
Work with the Centronics (printer) end of the printer cable - because you'll want to plug the parallel port end into the back of your computer. Applying the soldering iron to the wires where they are soldered to the Centronics connector and pulling gently should be enough to undo the wires from the unwanted connector. Check the parallel port end of the cable to ensure that you are attaching the appropriate coloured wire to the LCD's PCB (the wire colours listed in the table above are correct for the printer cable I was using.)
When you have finished wiring the nine printer cable wires to the PCB, it's just a matter of tidying up. First cut off the Centronics connector - the remaining wires are not required. Cut off any extra plastic bits and tidy the end of the cable up with some electrical tape and cable-ties. I also wound the two long power cables around the length of the printer cable and secured them with a fair number cable ties.
Connect the other end of the two power cables to your Molex adaptor - I cut off the 'floppy drive' connector and soldered the power wires into place. Make sure you use the red 5V line and not the yellow 12V line when wiring your LCD to the Molex connector. (Test the voltage of each of the rails with a multi-meter if you are unsure which to use). More heat shrink and some cable-ties to neaten everything up and you should have finished the hard work.
Now that the cable is finished, it can be attached to the printer port on the back of your computer, and powered via a Molex connector from the inside of your computer case. When the computer is powered up, the LCD screen should - if working correctly - display a number of solid blocks. You can mount it inside your computer case, or in a Jiffy-box or something similar and display it on your desk.
