Wednesday, July 10, 2013

Using a Dell Optiplex 745 Motherboard in a Dell Dimension 5100 Case.

Give a guy an empty case and he’ll think it’s a PC
My previous workstation was a Dell Dimension 5100 I bought in 2006. When I upgraded to my current machine (Dell Studio XPS 8100) in 2010, the 5100 was consigned to a shelf. Note to self: this means you can justify upgrading your desktop PC next year. It sports a Pentium 4 single core processor which really isn’t up to much these days. I hated to let the machine languish so I looked into upgrades and found that some people had installed Pentium D dual core processors. I tried it and it worked but had an annoying message about the processor being unsupported and I had to hit F1 every time I booted up. The Pentium D is the worst dual-core processor (unless used as a space heater), so the performance gain wasn’t that great.
Then a month or so ago I reanimated a used Dell Optiplex 745. It supports Core2Duo processors with a 1066MHz FSB, which is still somewhat limited but far zippier than a P4. It also has Gigabit LAN, etc. Long story short I bought a spare 745 motherboard for $18.00 on Ebay. I had a Pentium Dual core e2200 laying around surplus to my needs (it’s about twice as capable as a Pentium D in spite of the low amount of cache and clock speed). I proceeded to cram the 745 motherboard into the 5100 case.
Did I mention that the whole hassle arose because Dell chose to use the BTX form factor during this period? Yeah, that’s right, an ATX motherboard will not fit. If it had been ATX/micro ATX I would have had a wide array of motherboards to use.
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I did have to nibble out the I/O plate (which is not removable) to fit the different I/O configuration. It’s ugly but what the heck, the alternative was not doing it at all. Did I mention that this whole project is a monumental waste of time?
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The front panel connection is identical however.
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When I booted up I was more than irritated to see this message. Once I hit F1 to boot the fan ran at 100% speed (think small turbojet engine), making the machine way too noisy for general use. It ran at this rate even with the operating system running. I did boot into Linux, perhaps Windows controls this better, although being a Dell I doubt it. They often have weird hardware overrides in their various BIOS-es. Like the irritating one that detects that you’re using a non-Dell charger on your laptop and throttles CPU speed.
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Optiplex 745 front panel board top.
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Bottom.
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If you look closely at the top you can see what I believe is a nickel thin film resistance temperature detector.
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Dimension 5100 board, top. There are some interesting unpopulated pads on the board. But I’m not going to add an air temperature sensor.
Why? Because the whole notion is incredibly stupid. All the sensor is measuring is the temperature of the room air entering the case. If the PC overheats because of ambient air temperature then something has gone seriously awry in the case design. It might make sense if the sensor was inside the middle of the case, or maybe up top by the power supply.
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Bottom.
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Off to the bench for some poking and prodding. The microscope was of help.
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Pins identified.
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The scrawl of a madman.
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Wires identified, cut and stripped. I then added female connectors to them.
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I hooked a 5k potentiometer up to the wires and tried different resistances. Of interest is that adjusting the potentiometer once the machine had booted did not seem to cause any sort of fan speed adjustment, so I had to boot repeatedly. It was also quite sensitive with a small range of resistances being acceptable. I wanted a relatively quiet fan speed. In theory the processor temperature should dictate fan speed once the PC is running.
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I settled on a 3.3K Ohm resistor.
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Made a little pluggable unit just in case I need to swap it out.
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Installed.
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Anyway now it boots up fine with no sensor warning and the fan doesn’t run full bore. As a bonus the existing XP install booted up fine on the Optiplex board. A couple of missing drivers but that’s easily taken care of. At least I kept it out of the landfill…

12 comments:

triquarter said...

oh man thank you so much for this info. I ran into a similar situation. I'm installing a optiplex 145 board in an optiplex 155 case. Same deal, everything is the same but front panel. The blank pin is at number 8 on the 155 board, so i cut it out on the cable, but the only thing that worked was the usb ports, and the lan lcd. I also get fan error on boot. I just cut the cable and cross wired it as to your specs. Just like installing a car stereo. (much smaller wires) Thanks again, your pin-out saved me 20 dollars, for a 145 F/P board.

Felice Luftschein said...

Glad to hear it was helpful!

Phil Croft said...

Hey , good stuff , im doing the very same thing , i just wondered if the resistor is across the motherboard side of the frnt panel side ? and is it pins 10 and 11 ? i cant quite see which end is which in the photos ? cheers

Felice Luftschein said...

Added a pic, 2nd from bottom (the mis-sized one). Should clear it up.
Nick

Phil Croft said...

Thanks Nick..i shall give it a go..
i have to ask one more obvious question: Both front panels appear to have the same dimensions and component positions etc , would the 745 one not go straight in the 5100 ?

Felice Luftschein said...

I think so.

George Bradbury said...

Hey I recently got a dell optiplex 775 from a friend and decided to use it for a cheap home pc. I bought a new btx case and atx power supply and transfered the parts to that case so I could use my old 9800gt for some light gaming. I only just realised the front panel connectors are different (40 pin) compared to standard one which was on the case. From your digram I see one power pin but my understanding is you to short two to turn a pc on, do you know which other one I have to short to power my pc up?

Nick Carter said...

You'll want to somehow reuse the Dell case's power button and other connected elements if possible. Otherwise you need to search the internet for a pinout of the connector or more laboriously trace each wire to understand what it does.

George Bradbury said...

I don't think that's an option as I don't have the tools to modify the case to fit the physical power button on. I can't find anything online about it I have been searching for a while. I just need a quick fix so that the pc can be powered on from the front. The front panel connector will fit over two pins I just need to identify them. What did the pin you labeled power do? Thanks for the reply.

Nick Carter said...

Power would be one lead to connect to the power button in your case, try connecting your new case's power button to "power" and one of the grounds. That should work, if not I'm out of ideas barring some soldering.

Anonymous said...

Wow nice work! I have the same PC, the 5100 but I also recently got a E520 which is 99% identical except Core 2 support and a slightly different IO plate layout (unfortunately). However, there is another, bigger catch, the e520 mobo has bulging and one leaking capacitor! And also a broken cpu pin! Miraculously, it still works (also with the same e2200 as you're using) and seems stable. But I do want to replace the capacitors before I really start using it. Now I read your article and it got me thinking what is the best (optiplex) mbtx motherboard out there that will fit? From my research so far the optiplex 980MT was the last btx optiplex and it has nehalem (core i7 1st gen) support. Motherboard isn't easy to get and expensive on eBay. I wonder if anyone knows of a great mbtx motherboard that could fit into this dell with cpu support beyond core 2?
The best cheapest core 2 mobo I could find is the optiplex 755 one, it supports 1333 fsb instead of 1066 the 745 supports. Also 45nm is supported.
The best solution for me is to fix the capacitors but I don't know how difficult and reliable that will be.
Thanks again for the post and regards

Kimi said...

Help me a lot.Thank you very much.from malaysia.