ATX backplane prototype...

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Dave
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ATX backplane prototype...

Post by Dave »

Here's a snapshot of my ATX backplane prototype.
Isn't she pretty?
Isn't she pretty?
It can take a 20- or 24-pin ATX PSU, using the 5v line to supply power to cards. +12v and -12v are also provided. A flying lead is provided if you want to provide 12v or 5v to the microdrive end of a QL mainboard. Aurora systems won't need it. (It is not shown in this pic.)

All signal lines are in a middle layer between two huge ground planes. There are two LEDs indicating power good and power on, a connection for the case power switch and reset switch and two local buttons with the same function, if you prefer to use the system outside of a case.

I will 3D print a mounting bracket in black plastic that will have guides for the QL and expansion cards. It will allow mounting either from behind or from the bottom, so you'll have choices about orientation. The entire system with QL requires 40cm x 12cm deep by 11cm high - which will fit inside most full size ATX cases and a few mini-ITX cases too. The expansion cards sit 21mm apart, and the lowest expansion card sits 35mm above the QL mainboard.

Here's your chance to give me some feedback! I want your opinions!

What changes can I make to this backplane to make it better than an AT-style backplane? Besides a new case and a fresh new PSU, what would you want to get from re-casing your system?

When the design is finalized I will order 25 PCBs and once they're gone, they're gone.


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Dave
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Re: ATX backplane prototype...

Post by Dave »

I had a couple of interesting bits of feedback from PMs and a nice conversation with Nasta about this board.

I'm going to sit on it for a little while. I need to let the ideas percolate and see what comes out of it. I'm still open to ideas and suggestions, so if you have a requirement or opinion, please let me know so I can take it into consideration.


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genetika
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Re: ATX backplane prototype...

Post by genetika »

I'm interested!
:-)


Paul
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Re: ATX backplane prototype...

Post by Paul »

I am also interested in these. I guess this backplane is unbuffered. Wouldn't it be better to have a buffered version?
Also I think some extra holes for cardholders would be good.
I use these for example.
https://www.reichelt.de/Distanzhuelsen- ... OUPID=7758


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Dave
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Re: ATX backplane prototype...

Post by Dave »

It has a 3D printed frame with card guides already, so.... ;)

As for it being buffered, that's a complex subject. I welcome your thoughts.


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Dave
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Re: ATX backplane prototype...

Post by Dave »

I did a little more work on the ATX backplane to make some signal quality improvements, based on the prototype PCB that came in before the weekend. I had some measurable crosstalk on the first prototype, so I did some modeling based on feedback from Nasta and have got it down to almost immeasurable levels.

I altered it from supplying 8.7V to supplying 5V. You get 8.7V as the difference of 12V and 3V3, which is fine for some PSUs but others have difficulty. I figure most people with AT backplanes already bypass their regulators and/or have (S)GCs. Also, it's more fault tolerant to plug a 9V card into a 5V supply than the opposite.

The switch circuit works very nicely.

I added locations to mount LEDs to show STANDBY mode, POWER ON and POWER OK status. I also added an auxiliary power on button on the other side of the backplane, which is the side most likely to be accessible from inside an open ATX case.

I will now add a couple of positions to add screw terminals or allow soldering of +12/GND/GND/+5 cables for floppy/hard drives. I will also provide a pair of +3v3/GND and a -12V/GND/-5V/GND pair too. This allows a flying lead to supply 5V at the microdrive end of the QL also. I will also aaa a large electrolytic capacitor at the QL socket and another between each pair of expansion slots.

I have other options I can add: a ROM cartridge slot, and a reset button - assuming the reset pin on J1 functions.

Top view
Top view
Bottom view
Bottom view


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Dave
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Re: ATX backplane prototype...

Post by Dave »

Aaaaand the new prototype PCBs came in. I'm not happy with them AT ALL. The board is too wide to fit in standard packaging. I have instead made the board narrower and longer by moving the ATX connector and control circuitry to the end above the top expansion connector. Another 8-12 days pause, waiting for prototype PCBs to be made and shipped!

I had an interesting conversation about signal quality, and power for the QL motherboard.

Signal quality:

The mPlane is widely regarded as the cleanest backplane, though the miniplane beats it by a little simply by having shorter tracks. The mPlane used 2k2 pulldowns on many of the lines, and though I am told this was not beneficial, I can see a gentle improvement in signal quality if the pull-down resistors are fitted. For this reason, I will be using pulldown 2k2 resistors on active high lines and pull-up resistors on active low lines in the ATX backplane. I'll also be using a few large electrolytics on the backplane, as close to the connectors as possible, and a few ceramic caps for quicker response. The ATX backplane alternates tracks, and every track is adjacent to ground on both sides and the opposite layer. Testing shows it is by far the cleanest backplane in terms of signal quality... The mPlane has inducted noise at twice the level and the QPlane at over 10 times the level.

Power:

The QL motherboard was never designed to be back-fed power from the J1 connector. I have been given all kinds of warnings of the magic smoke leaking out of components, poor voltage levels, etc. As a naturally conservative fellow, I have taken a very cautious approach. For the 5V line, the board includes a flying lead to provide regulated 5V to the regulator connector. Over the next few days I will do testing to find the consequences of applying +12v and -12v, which are needed for functional serial ports. I welcome feedback from people on how they have solved this issue in practice. It may just require removing the 7812 and 7912. However, it may require more. Even then, for some soldering will be too much to ask. So, how have you resolved this issue in your re-cased QL? Do you have 5V only systems, or do you feed 9- or 12v in to the QL and let it do the regulation?

Expansion:

I have put FOUR expansion connectors on it. If there's a strong demand for FIVE, I'll do it. It could have up to eight, I suppose. but I don't know anyone who could plug that much non-conflicting hardware into their system. And knowing the other item or two we're working on, *grins* even four will be tough to fill. :)

I'm guessing the ATX backplane has about two weeks more of active development and waiting for prototype PCBs, and then it should be finished.

I will be able to supply fully assembled backplanes, or with just the SMD components applied, or bare PCBs for the adventurous.

More, and photos, in a few days.


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vanpeebles
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Re: ATX backplane prototype...

Post by vanpeebles »

Would this be suitable for a Pandora/Aurora?


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Dave
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Re: ATX backplane prototype...

Post by Dave »

Very suitable, yes.


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