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iMac ATX ConversionReturn to News Page

PowerComputing iMac: With Soft-Power On/Off
(or Making PS Connections for a Rev. A-D iMac Motherboard in ATX Cases)
by Mark Hooker
Published 5/30/2003

    Disclaimer/Warning: This page is for entertainment only and is not guaranteed accurate. Performing modifications or other work inside your Mac will void your Apple warranty, may cause damage to your computer or result in personal injury (CRT iMacs have hazardous voltages stored even with power off). Although Mark indicates he made these modifications successfully, neither the site publishers nor the author warrants any of the information listed here. The author and site publishers DO NOT recommend attempting any of the mods listed here. You assume all risk from the use of any of the information in this article.

The Story
Emboldened by the recent spate of articles on re-casing iMac motherboards from units with a failed video board (Apple should have done a better job of quality control with that video board), I looked around on eBay, found that the price was right, and decided to try it myself. The result is a PowerComputing iMac.

Front view

What makes this iMac resuscitation different from the others that are sprouting up like mushrooms all over the Web, is that Soft-Power On/Off works.

The Parts
The best deal I found on eBay for an iMac motherboard was from a Southern-California electronics scrap-yard that sells on eBay under the user name scrapmatic. The unit I got from them is a 233mhz board complete with 96mb of RAM and 6mb of VRAM in the original metal chassis. The board was untested, but it powered right up the first time and hasn't skipped a beat since.

Chassis

A PowerComputing PowerCenter PRO 210 DT Case (also from eBay) offered plenty of room for the installation. The case is 17X16X4 inches and has two 5.25-inch bays, plus a floppy bay that can be used for an additional 3.5-inch drive, since the iMac does not have a floppy.

Top view

I especially appreciated having an iMac motherboard unit with the board still mounted in its metal chassis, because that made installation much easier. The chassis consists of a cradle for the motherboard and a drive bay for the HDD and CR-ROM drive. To make the unit fit in the PowerCenter Pro case, I had to cut off the first 4 inches of the drive bay. That left enough of the drive bay so that I could use it to bolt the chassis directly to the bottom of the PowerCenter case. This is the most elegant solution for installing the motherboard, which has a lot of elements that hang off the bottom, giving it a lot more depth than other motherboards. The alternative is to use high risers, but I find them not quite as steady as I would like.

backplane

The backplane of the PowerCenter case was removed so that the iMac port panel could fit flush with the back of the case. The opening will eventually be covered with plastic to improve the air flow provided by the power-supply-mounted exhaust fan. Additional cooling is supplied by a thin-line fan mounted directly on the CPU cage. It is secured with cable ties rather than screws because the cable ties are non-conductive and won't interfere with the components underneath if they should touch. The CPU cooling fan is powered from the motherboard cable that originally was used to power the HDD. The first time I powered the unit up, seeing the fan spin gave me a warm and fuzzy feeling, because the fan starts spinning as soon as power goes on. I knew that I had not completely miss-wired the power-supply cable adapter. The other power-up indicators take a bit longer to show up as the unit goes through its power-up self tests.

fan

The current HDD is powered directly from one of the ATX power-supply tails. The HDD is a 4GB Quantum (another eBay purchase) with Apple ROM. The HDD was mounted under the CD-ROM drive in the lower 5.25-inch bay, using a set of adapter rails from my junk box. Mounting the drives one above the other means that they can be run from a single ATA cable, which was one of my goals, since I did not want to build an adapter cable for the second ATA cable (narrow 50-pin) that was originally used for the CD-ROM. (See this link for cable instructions. The original Conversion of an iMac Rev. B into an ATX Case has another approach to the CD-ROM cable.)

The new CD-ROM drive is also from my junk box. It is a 24X Matushita ATAPI drive with Apple ROM pulled from a Beige G3 DT. With the price of internal 5.25-inch CD-RW drives as low as it is, upgrading the optical drive will not be a problem, if I ever feel like it.

Power-Supply Adapter Cable
The diagrams below are based on the diagrams in the article Conversion of an iMac Rev. B into an ATX Case. (Be sure to read it for other handy hints.) The pin-outs for the ATX power-supply cable come from the ATX specs. The pin-outs for the iMac power-supply cable were taken from the power-supply board of an Rev. B iMac.

ATX Power-Supply Cable Connector Pin-Outs

Pin Signal Color Pin Signal Color
1 +3.3VDC Orange 11
[11] 
+3.3VDC
[+3.3 V default sense*]
Orange
[Brown*]
2 +3.3VDC Orange 12 -12VDC Blue
3 COM Black 13 COM Black
4 +5VDC Red 14 PS_ON# Green
5 COM Black 15 COM Black
6 +5VDC Red 16 COM Black
7 COM Black 17 COM Black
8 PWR_OK Gray 18 -5VDC White
9 +5VSB Purple 19 +5VDC Red
10 +12VDC Yellow 20 +5VDC Red

* - The signal identification and wire color shown in square brackets [ ] are options from the ATX spec.


 

iMac Power-Supply Cable Connector Pin-Outs

Pin # signal color   Pin # signal color
1 GND Black   2 GND Black
3 GND Black   4 GND Black
5 GND Black   6 GND Black
7 GND Black   8 3.3VR green [black]
9 5VR gray [black]   10 5V red
11 5V red   12 5V red
13 5V red   14 5VS white [red]
15 12V yellow   16 12V yellow
17 -12V dark blue   18 3.3V orange
19 3.3V orange   20 3.3V orange
21 3.3V orange   22 3.3VS light blue [orange]
23 PFW brown   24 T5V purple
    • Where more than one wire color is given, the first is from the Conversion of an iMac Rev. B into an ATX Case, the second is the actual color of the wire in the cable that I used. When in doubt, count. While the wires with 'dual' colors did not have a distinctive color on the cable I used, you could tell that there was something different about them, because the gauge of the wire was smaller than the wires adjacent.
ATX to iMac Power-Supply Adapter Cable Pin-Outs
This table shows the connections necessary to build the ATX to iMac Power-Supply Adapter Cable. The cable is built by splicing together an ATX Power-Supply Extension Cable (available on eBay or at your local parts supply house) and an iMac Power-Supply Cable. By building the Adapter into an ATX Extension Cable, the Adapter becomes portable and can be reused, if something were to happen to the power supply.

ATX Cable End iMac Cable End   ATX Cable End iMac Cable End
pin # pin #   pin # pin#
1 18+19   11  No Connection
2 20+21   12 17
3 (Black) 1 with a tap to IC pin #7   13 4
4 10   14 to IC pin #2 23 to IC pin #1
5 (Black) 2   15 5
6 11   16 6
7 (Black) 3   17 7
8  No Connection   18 (White)  No Connection
9 24  with a tap to IC pin #14   19 12
10 15+16   20 13

The references to IC pin numbers are to the Soft Power On/Off Sense Inverter IC. See below.

I plugged the ATX extension cable into an ATX power supply so that I could not cut off the wrong end of the cable (there are different connectors at each end). This also helps, in case the extension cable you buy has generic color coding (wires in blocks of 4 all the same color) instead of the color coding of the real power-supply cable. Getting a properly color coded extension cable is a big help and worth the extra effort. I cut the iMac cable in half (both connectors are the same) so that I could use the other half for another cable adapter later. The wires were spliced together, using twist and solder. The splice was covered with heat shrink tubing. (Be sure to put the heat shrink tubing on the wire before you twist and solder.)

To make the connections to the IC socket easier, I added a 'tap' or 'extension' to the splice for the wires corresponding to iMac cable end pins #1 (black), #23 (brown) and #24 (purple). (See the table above, and the narration below.) The green wire from ATX cable end pin #14 was soldered directly to IC socket pin #2.

When you are finished, some lines will not be connected:
On the iMac cable-end, it will be lines corresponding to pins:

  • #8 (green/black)
  • #9 (gray/black)
  • #14 (white/red) and
  • #22 (light blue/orange)
    • Where more than one wire color is given, the first is from the Conversion of an iMac Rev. B into an ATX Case, the second is the actual color of the wire in the cable that I used. When in doubt, count. While the wires with 'dual' colors did not have a distinctive color on the cable I used, you could tell that there was something different about them, because the gauge of the wire was smaller than the wires adjacent.

On the ATX cable-end, the lines originating at the following pins will not have a connection:

  • #8 (gray)
  • #11 (orange [brown]) and
  • #18 (white)
    • Where more than one color is given, the color in square brackets [ ] is an optional color in the ATX spec.

All the unconnected lines are sense lines. The iMac and the ATX supply work together just fine without them connected. I did not cut them off, I just left them dead-ended, in case someone ever figures out a use for them.

Soft Power On/Off Sense Inverter
A reasonably easy to fabricate soft-power enabled power-supply adapter to allow an iMac to be powered by an ATX power supplies has thus far been the Holy Grail of iMac conversions. The original iMac power supply is rather large and is naked (i.e. not in a box), so that it is hard to re-house. One
site asks $40 for the schematics to make soft-power work. A Japanese site has a schematic of a bread-board that does the task, but I found an article right here on xlr8yourmac that was within the level of my technical skills. The article is about using an ATX power supply with a PPC 8600 or 9600, but the principles of operation are the same.

Dick Moore, the author of the 8600 ATX article, uses a 74HCT04 Hex Inverter CMOS IC to translate the signal on the Power-on line from Mac power-supply talk to ATX power-supply talk. The Power-on signal for a Mac power supply goes high when the power button is pushed, while it goes low for an ATX power supply. The Hex Inverter turns the signal from the Mac up-side-down so that it will be at the correct level for the ATX power supply. By using the CMOS version of the 7404, you can wire the IC directly to the sense line without any extra parts. While I did not feel like trying any of the bread-board inverters I had seen, I felt like I could do this, and, sure enough, I could.

The 74HCT04 is a 14-pin IC. Rather than solder directly to the pins of the IC, it is much better to use an IC socket. (Both IC and socket together are less than $1 at your local parts store.) The pins on the socket and IC are numbered down one side and back up the other. Be sure you know which pin is which on both the socket and the IC.

IC socket

The purple (violet) 5V trickle (stand-by) line is attached to socket pin 14. This line is the same color on both the iMac and the ATX cables. On the iMac cable end it is cable pin #24. On the ATX cable end it is cable pin #14. I added a 'tap' to this splice.

A black ground (common) line is attached to socket pin #7. Ground (common) lines are the same color on both cables. There are several on each cable. I picked pin #1 on the iMac connector end and pin #3 on the ATX connector end. I added a 'tap' to this splice.

The green Power-on (PS-on) line from the ATX cable end (pin #8) goes directly to socket pin #2.

The brown PFW (Power Failure Warning) line from the iMac cable end (pin #24) goes to socket pin 1. I added an 'extension' to this wire.

I put heat-shrink on the active pins, and then put a skirt on the socket with electricians tape to cover the unused pins. I inserted the IC, and put another piece of electricians tape across the IC and socket to cover the exposed parts of the IC pins. The Hex Inverter 'package' sticks out from the cable like a tree limb from a tree trunk.

inverter

It fits nicely in the original plastic handle for the iMac motherboard chassis.

The only quirk about Soft Power On/Off with this installation is that, if you perform 'Shut Down' immediately after boot, without opening any programs, the unit powers down, but rebounds as soon as power is off, and boots up again. If, however, you open a program, and do something with it, 'Shut Down' runs normally, and the computer stays off at the end. Guess how long it took me to figure that one out.


Links to iMac re-casing articles which have contributed to this project:


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