Sunday, July 24, 2011

20" iMac G4 Inverter Pinout (Not tested)

Based on the voltages from my current setup with the Apple Cinema Display as the power source.  You should be able to hook this up to a native working iMac G4 20" power source, but as I do not have a working one I can not confirm this.

iMac G4 20" Inverter Pinout
To be honest I am not sure exactly what the yellow wire does and I can not guarantee that there is not something else missing.  I believe its also involved in on/off control.  I am not sure about what resistor as I do not know the current, but it would be high - at least 1Kohm.

The TMDS to DVI for the ID Tech panel can be found here.  Again this is based on ID Techs own specifications, so I am very confident about in this pinout, but have not tested it myself.

If anyone does have a power source and gives this a shot please let me know the results.  Good luck.

11 comments:

  1. Since it's summer i'm finding things to do with my spare time. one thing I would like to do is to modify my imac g5 ( which is absolutely useless despite running in it's full potential) as a primary monitor for a PC, good to see people are still interested in recovering these kind of things. seems that your post for the DVI pinout for the g4 is very helpful.

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  2. JBerg,
    I am currently working on 20" Acylic Apple Cinema Displays, which look very similar to the one you own. There we find the following pinout:
    1 - Brightness (bright=0..4,8V=dark)
    2,6 - Backlight Control - here the Inverter reports normal operating backlights with 0V (ground). If you use the ACD logic board, these wires coming from there have to be connected to ground to report a normal behaviour.
    3 - +5V low power supply
    4,7,8 - Ground
    5 - Display on (3.5V)/off (0V)
    9,10 - +25V power supply

    Could be that your yellow cable is the equivalence to the backlight countrol. It is an output pin, so you won't measure anything there. For using the inverter it is not necessary to use a pull-up resistor there. But with a resistor above of 1 kOhm it's harmless.

    On/off should be 0 or higher than 3.3V; I could also use +5V without causing damage.

    The ACD Inverters are a bit wider, but in my case the intention is to completely remove the Inverter/CCFL and replace them with LEDs and a PWM controller. It actually works - I modded 2 ACDs at this time; on one I also cut off that proprietary ACD plug and replaced it with standard USB, DVI and power supply.

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  3. Reu, thank you very much. I agree, I love being able to "recycle" and "re-use" these old machines. Even by todays standards, the LCD screens are high quality. Having the iMac G4 and G5 on my desk the past few weeks has been a blast, hopefully my new mini will be delivered soon to utilize both of them.

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  4. Jean-Luc - thank you for the reply. I figured if anyone would know it would be you. When I look back and think about the endless combinations I tried when trying to get the backlights of the native 20" iMac to turn on, I have no doubt that I had everything in place except for that yellow wire. I agree with you that this wire must be the backlight control. But there is something else I have to tell you. There is an 8pin plug that goes into the 20" iMacs native inverter, but only 7 pins are filled in the iMac, there is a gap in the middle. The aluminum apple cinema display uses the same connector except all 8 pins are filled. Initially I thought using only the 7 pins of the native inverter cable (which goes through the neck) worked perfectly. As I began to use the mod more, I noticed oddities. During sleep the backlights weren't entirely off, there was a faint glow. Also, if you changed source during operation, you would have to unplug and replug in the monitor to get the image back. I was able to correct this simply by using an extra wire as that "8th wire". Now I can clearly hear the backlights click on and off from sleep mode. This reads 0V and it is likely a ground. But remember when the inverter was hooked up to the G4's motherboard there was no wire at this location and it obviously functioned fine.

    If I was to guess it appears that the yellow wire and this pin are analogous to Pins 2 and 6 (Backlight controls) in your acrylic ACD. However, somehow the iMac G4 was able to control the backlights via the one yellow wire only. Regardless, even if this can't be reproduced exactly, I now know that adding an additional wire may replicate the effect. My question is Does This Theory Make Sense? And How Would You Hook up this Yellow Wire?

    I also do not want to experiment on this particular mod. Except for adding ports and a decision about what optical drive to use, it is near perfect. But, now I have this "mystery" that is going to drive me crazy. Any advice is appreciated. Thanks in advance. - Jon

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  5. I hope, I have understood everything. First, let's start with the pinout for the iMac 20" inverter.

    (1) There's that purple wire (on/off). This is 3.5V for "on" and 0V for "off". During sleep, the iMac sets the input to 0V which causes in switching off the backlights completely. If you let it stay at 3.3V, you will always experience that faint glow. To solve this, you might use the +5V line from the DVI cable (which should be actually switched off when the graphics card is shut down during sleep).

    (2) You left the orange Wire for the dimmer unconnected. I propose to set it to ground, which should set the display to the highest brightness. Use a 1kOhm pull-down resistor for the connection. I don't konw if this is really necessary, but I would feel better having an input cable connected to a defined voltage.

    (3) The backlight control cable is only an output line from the inverter. This cable could be left unconnected as it only reports a malfunction to the ACDs logic board. On the other side, in the Acrylic Displays the backlight control warning only results in a special blinking code for the power led (short-short-long) - nothing else.

    Thus I was able to use my "new" LED-Acrylic ACD without taking further notice of the backlight control. However, I found out that connecting the backlight control wires on the logic board to ground results in a normal power LED state. But it is only decoration, not core functionality.

    I also did not care about this wire when I did my first LED conversion on an iMac G4 15". I only had to use the on/off wire to switch off the display with the system preference of "having the monitor switched off at 15mins but the computer goes into standby at 1h" (example).

    The G4's use a different method for switching the backlight off and on when going to standby: They completely switch off the power source. The ACDs do require another way to switch off the display because of their separate power supply (which is always connected to the mains), and that is in my opinion the reason why they use this 8th wire.

    You did not mention where you connected this 8th wire to. In my understanding, it's another input wire and so you won't measure anything there because it's a high-resistance input.

    Hope that helps to solve the mistery a bit.
    JL7

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  6. JL - I just realized I never answered your question. You are 100% correct and your reasoning makes perfect sense. I did not sense any voltage - so initially I was going to just ground the wire, however, I did not know if this changed in sleep/standby mode. Luckily (since I used an additional grey cable from a different iMac neck) I had 2 small extra wires that run though the neck. I simply added the missing pin connected to the extra wire than runs through the neck and plugged it in to the empty slot on the ACD, where the original "8th wire" came from. Then it worked like a charm.

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