I) Intro - LCDs
To explain just some basics about LCDs: (I am no expert so please forgive me if things are slightly wrong or grossly oversimplified).
Most monitors (that connect to an AC power source) contain the LCD itself and then two boards. (See image on the right)
1) Top of the image is the LCD controller board. This is where DVI/VGA plugs in and gets converted to a signal type, usually LVDS. The controller also connects to a power button and some other control buttons. Thus, the board can adjust aspects of the image such as brightness, contrast, position etc.
2) Bottom of the image is the Power Source/Inverter. AC Plugs in here and powers the backlights (connectors at the bottom). The backlights traditionally CCFL lights (now more LED) allow you to see the displayed image. The AC is also converted to DC and supplies power to the LCD controller and via the LCD controller the LCD panel itself.
II) Signaling Types
LVDS (Low Voltage Differential Signaling) along with power (usually 5 - 12V) and ground wires go from the controller to the LCD via a multi cable connector - usually a JAE connector.
Every Apple Product involving an LCD that I have worked on so far works somewhat differently. This includes all the iMac G4's, the 20" iMac G5, and the 20" aluminum Apple Cinema Display.
The major difference is that the Type of signaling used is TMDS (Transition Minimized Differential Signaling). While I do use the term "TMDS to DVI conversion" in my blog, this is actually a misnomer as there is no real conversion. TMDS is the type of signal that DVI and HDMI cables use. Nothing gets converted, its just that the wires get arranged in the right sequence and POWER is added. Inside the LCD casing itself there is a Texas Instruments chip that allows the monitor to utilize the TMDS signal. Therefore, ALL of the LCDs in the above computers can be connected directly to any TMDS signal - meaning any source that sends a signal over a DVI or HDMI cable.
Panel Names: (May not be complete) - All TMDS compliant
iMac G4 17" - LG LM171W02(A4)
iMac G4 20" -
|Inside the Cinema Display|
|DVI going directly to LCD|
Even though all the data goes directly to the panel, there are still LCD controllers. In the case of the iMac G4 and G5, the controller is in the motherboard. There is no conversion of signal, but power to the panel and brightness is controllable. There is an LCD controller in the cinema display. The actual TMDS data cables go directly to the LCD from the DVI cable. The ACD's LCD controller does add the power to the display and changes brightness, connects to a power button, and is also a USB/Firewire hub.
Again, for all these panels, you can connect DVI or HDMI wires directly to the LCD's JAE wires. As long as you also supply power for the panel (12V for the 20" panels and Grounds) as well as forgo hardware control of brightness - you will produce an image. But, you will not be able to see the image without backlights.
Like any monitor with CCFL backlights you need to AC power. The iMacs do not need a power source because the computer itself already has one. DC voltage goes to an inverter that turns the DC voltage into high AC voltage (>1.5 Kilovolts) allowing the bulbs to turn on. The Cinema Display actually works the same way. The external powerbrick is the power source, taking AC and putting out 24V DC. So even with a working image you need an inverter to turn on the backlights to see it.
For the 15" iMac G4 other modders were able to figure out what to feed the inverter to get it to turn on. They used an ATX power source. I determined the 17" iMac inverter pinouts because I had a working inverter, power source, and although it froze with booting - motherboard. So because there was an image I was able to determine what voltages went to what inverter cables to turn the backlights on. I then used an AC to molex adapter (Gives 12V, 5V, and Ground), approximated the voltages (I didn't have a 3.3V line), and turned on the backlights.
So here is the problem with the iMac G4 20". First, no other inverter which can power the backlights that I have found is thin enough to fit in the case. Trust me I've tried a lot - See this post for details. So while there was only 12V, 5V (3.3V recommended but not needed) for the 15" and 17", the 20" was not so simple. Remember all these voltages are easily available from any ATX power supply. The 20" inverter needs a 24V power line. In addition, while I can identify the power lines, grounds, on/off, and dimmer of the other inverters, there was an extra wire with the 20" that I'm not really sure what it does. So, you need a power source capable of 24V, 12V (to power the LCD panel), and 3.3V - any old ATX power supply will not do. The native iMac G4 20", G5 20" or 20" ACD all have power supplies that do have a 24V output. That was original reason i was using the Apple Cinema Display in the first place - I did not have a working iMac G4 20" power supply. What complicated things further was although I had a general idea, I could not determine voltages from a multimeter, thus it was guess and check. Unfortunately, when I burned out an inverter I realized that this method wasn't going to work.
Using the controller of the Apple Cinema Display solves the problem. The controller is really a series of resistors that takes the 24V DC and splits it up into the voltages needed to power the panel and the inverter. Plus it adds on/off, brightness control, and a USB/Firewire hub. But, this is essentially the same as the "TMDS to DVI conversion for the 20" that I posted here, as far as the TMDS data is concerned. Using this controller simplifies and stabilizes the system very well. This is obviously not the most cost efficient method. However, buying a working 24V power source can be expensive as well, so if you need to buy one anyway, I would recommend going for a cinema display with a broken screen.
But for those of you that either have a working iMac G4 20" power supply or some other 24V source there is obviously a way to get it working. Now that I actually have a set up that works, some may wonder if I can now determine what DC voltages are needed to "activate" the native inverter. The answer is yes and I have already done so - I will post the inverter voltages in my next post. Two things about this are unusual. For one, there is a yellow wire with an extremely low voltage, I believe its part of the on/off mechanism or backlight control. I am not sure if there is something else that I am missing with this wire. Second, I have given just the voltages, you will need a fairly high resistor for the yellow wire even if you connect it to the 3.3V, but I don't know the current of the power supply to calculate it.