Sunday, July 24, 2011

20" LCDs, Signaling, Inverters, and Pinouts

After getting some questions, I realized that it isn't very clear on my blog what makes the 20" iMac G4 different from the 15 and 17" models.  Simply put the difference is the inverter and powering the backlights on a 20"LCD.
Monitor Components

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.

JAE connectors
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" - IDTech M201Z2-M01
iMac G5 20" - LG LM201W01(A5)(K1, K2, or K3)
Aluminum Apple Cinema Display 20" - LG LM201W01(A6)(K1)

III) LCD Controllers
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.

IV) Inverters
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.

V) Solutions
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.


  1. JBerg,
    excellent conclusion on the LCD subject.

    Some Annotations: When you take a deeper look at the ACD Logic Board (LB), it turns out to be a mixture of VRM (voltage Regulation Module), USB Hub (with an additional USB device for the brightness control) and FireWire Hub. The display itself is connected to the LB via a 20(?) pin connector, which I did not inspect further.

    Generating 12V and 5V out of 25V should be easy. The display itself with its 12V power line does not consume a high current, neither does the so-called 3.3V line. So you might use two linear voltage regulators (Standard 3pin housing, 7812 - 12V, and 7805 - 5V) to generate the additional voltage. The 12V 7812 may become a bit warm and may require a small heat sink. That way you should only need one single power brick.

  2. JeanLuc, as always your input is greatly appreciated. while I know I have already asked your assistance in regard to the inverter, I am also going to pick your brain regarding power. As the new mini has lost its optical drive, placing one in the iMac G4 and/or G5 in my setup now will actually be functional.

    For the iMac G4 - this needs a tray loading 5.25" drive, the iMac G5 is a slim slot loader. I am gonna ditch the bluray as Apple will never natively support it for an internal DVD-RW / Superdrive. Then use a SATA to USB and connect either the the minis USB or to the Cinema Displays USB (which is then connected to the mini). My question is. can I leach power from the firewire ports? Or would you split the AC and use a small separate power source with its own on/off switch for the drive for either or both of these?

  3. As always - ite depends :-) Wikipedia says:
    "Typically a device can pull about 7 to 8 watts from the port; however, the voltage varies significantly from different devices. Voltage is specified as unregulated and should nominally be about 25 volts (range 24 to 30). Apple's implementation on laptops is typically related to battery power and can be as low as 9 V."

    In my understanding, 8W may be enough to run a Slim-line DVD drive (which usually requires about 800mA on a 5V USB line = 4W - in fact even this is out-of-spec for USB!), but you have to take care about this annotation about "unregulated" voltage. First, you have to find out the voltage on the FireWire-Port. If it is 12V and above, my proposal is to use one of these USB adapters for car cigarette lighters. Some of these allow a wide range input from 12-24V and they are very cheap on ebay. With this adapter you are usually able to pull more current than the usual 500mA, and you don't have to care about unregulated voltage. Maybe it even works with lower voltage.

    However, the situation is completely different for the tray loading 5,25" drives. These drives require much more power than the available 8W. In my opinion there is no way to connect these drives to a FireWire power source. Thinking about your BluRay drive (which is actually a 5,25" drive, right?) you should use a small separate power source.

    (Additionaly: In my own mod I decided to use a Slot-In drive within the iMac G4. It requires to remove the automatic-shut feature of the drive door, but placing the drive directly behind the door gives enough force to open it by the ejected CD. You have to close it manually of course. This also left more space for cooling th MiniITX board at the bottom of the dome)

  4. JL7, very creative solution with the slot eject. Did you simply remove the springs or did you alter the mechanism.

    I am probably going to put drives in both computers. I am going to try to utilize parts that I already have. I am probably going to split the incoming AC for the iMac G4 and utilize the power source for an external drive enclosure that I have. I'll change the rocker switch to a push button on/off so I can use the iMac G4's native button to turn on/off the drive. I'll use the USB on the Cinema Display as this will still give me an extra USB port, although I will probably forgo Firewire 400 on the G4. As I already have it I may as well put the Bluray drive in here.

    The iMac G5 I have a slim DVD burner that I'll use in similar fashion, but as the power source is external here, Ill probably use a DC-DC power supply (that can handle 24V input). I would actually love to find an SATA to firewire 400, but haven't been able to find one. I'm going to discuss this in my next post and appreciate any suggestions/advice. Thanks again, Jon

  5. Jberg,
    Slot eject: I just removed the springs and half of the internal structure. Just the drive door itself was left. Easy to open manually just by pressing against the door.

    Your ideas on the G5 sound very interesting. I am just modding a G5 myself, got me a housing with nothing it it.

  6. Hi, I have the iMac G5 20" - LG Philips LM201W01(A5)(K2) display, i think the pinouts are different for the g5 and g4, whats my best bet for connecting this to a DVI adapter? can i get a replacement cable from a G4 that would be easier to mod?


    1. Alan, I have hooked the iMac G4s ID tech using my pinout and it works. I have then swapped this monitor out with the cinema displays LG (A6)(K1) and it worked. I have hooked up the ID Tech to the apple cinema displays controller and it worked. I have hooked your monitor (A5)(K2) up to the apple cinema displays controller and it worked. Unfortunately, I have never done specifically what you are asking - my G4 pinout on the (A5)(K2). There are extra pins on the cinema display that I do not know the function of. However, it seems that the basic TMDS pins are in the same place on all of these LCDs. I am pretty confident that you can use my pinout. Also, the inverter is likely similar with 2 24V, 2 grounds, and an on/off. One problem may be working with the G5s cable, if I recall I think it may be a type of ribbon cable. You could use a G4 cable, but they go through the neck, I don't know where you would get a free G4 cable. The simplest solution would be to do what I did in my G5 mod.

      What I used was an apple cinema display (20" aluminum) with a broken screen. I used that inverter and it's LCD controller (which is very thin). The advantage of using this LCD controller is that it gives you touch control power and brightness as well as acting as a FireWire 400 and USB 2.0 hub. Using these 2 USB ports I hooked up the iSight and Mic up via the USB. You could even add a USB drive if you wanted. Just a thought. Let me know if I can help.

  7. Sounds promising, I just doubt I can find a Apple Cinema Display at a sensible price in the UK (even broken) yes the G5 uses a ribbon cable, Im tempted to check ebay for a G4 cable, if not then ill hunt for a Cinema Display, Thanks for your help

  8. Hi, your Pinout seems to match mine perfectly except on Pin 21 where you state its empty and I have a brown wire :/ i suppose I could just leave this bare lol

  9. The Apple Cinema Display has what I think is a ground at this position, it could also be some type of signaling wire (ex to the LED light or controller). Either way, I do not think leaving it unconnected will interfere at all with your image.

  10. LVDS cables
    Some words about LVDS interface to LCD TFT screens .
    For those who want to know better - please read this Wikipedia

    Almost all modern laptop's (and monitor's) screens are LVDS types, it is type of serial connection to data source .
    In short, LVDS is low cable count connection type, made as twisted pairs of data R G B (red, green, blue) plus clock data .
    In each LVDS pair exist Positive(+) and Negative(-) wire , and those Positive and Negative must match in LCD socket and controller socket for proper work.

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