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View Full Version : XT clone motherboard upgrades.



Casey
October 30th, 2017, 05:24 PM
Just recently caught wind of the XT super bios. My biggest problem is that I don't have a burner. My lesser problem is that I'm not sure it would be that much help except to give me a less buggy BIOS.

I picked this box up a while ago, and haven't done much except pull the cards. Came with an Adaptec 2070a RLL hard drive controller, very nice. The cpu is a Siemens SAB8088-2-P, which is rated at 8Mz. It also has a Siemens oscillator, SAB8284AP, also rated to 8Mz. Thing is the timing crystal looks to be a bog-standard 14.318Mz crystal. Go figure.
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I've tried Ctl-Alt-+ and Ctl-Alt-\, and have seen only 4.77Mz speed from this thing. About the only "pure" non-turbo clone I've ever seen. ;)

So I'm wondering how hard it is to replace the crystal for a faster one, or is that necessary? Or do I need a new bios as well? This motherboard has 5 sockets and only one occupied.
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As you can see there's lots of info on the motherboard itself, but I never mucked around with that kind of stuff before. The closest I got was plugging in 1 MB of ram DIPS into a 286-12 motherboard I bought, then replacing the timing crystal as I accidentally pulled it out fitting the board into an XT box. Put it in backwards. When I took it back and the dealer showed me what I did, my face was red.

Anyway. Am I correct in my reading of the 8284 chip and the Siemens CPU? Are these in fact 8Mz capable? Is it worth the time & hassle to change the crystal (right now I have no soldering equipment) and do I need to change the ROM? Or would a ROM swap enable turbo mode as is? I doubt the latter.

For that matter I would need to ask someone else to burn the ROMs for me. Obviously I would pay them for the chips, shipping, and their time.

...Or would it be easier to just buy a replacement turbo xt motherboard? If nothing else I would get a kick out of booting into BASIC on this board, given a ROM swap.

Plan X involves dropping in a 486 motherboard, but that's a separate thread.

SomeGuy
October 30th, 2017, 05:59 PM
Not all clone XT boards and BIOSes supported switching speeds from the keyboard. More often, there would be a jumper or two pin header that went to a button. Might just want to double check and see if this has one, although that would be odd not to hook it up or place the jumper. Of course, they never labeled anything.

On the other hand, there were 4.77mhz-only generic motherboards like these.

Could you post a picture of the entire board?

What kind of ROM does it have right now?

KenEG
October 30th, 2017, 06:03 PM
Try other keyboard combinations, like Ctrl-Alt-D. Not all turbos used the +-.

SpidersWeb
October 30th, 2017, 06:25 PM
If it does turn out to be 4.77 only, I'd check out Dave's video regarding his Tandy 1000 from the back in the day:

https://youtu.be/av5NQ_3mW9A?t=6m00s

Chuck(G)
October 30th, 2017, 07:06 PM
None of the photos show an 8 MHz crystal.

If it is a turbo model, the very, very common way to switch is to use a pin on the 8255, reserved for the cassette See this discussion for details (http://www.vcfed.org/forum/archive/index.php/t-57609.html).

modem7
October 30th, 2017, 08:37 PM
The parts of the motherboard that are shown are a close match to the X'GOLDEN BOARD motherboard that member jh1523 has.
Photo at [here (http://minuszerodegrees.net/xt_clone_bios/X'Golden%20Board%20of%20revision%20G.jpg)].
A manual that may match is at [here (http://minuszerodegrees.net/manuals/unknown/Unknown%20maker%20-%2016-Bit%20Motherboard%20-%20User's%20Manual.pdf)].

modem7
October 31st, 2017, 12:17 AM
The cpu is a Siemens SAB8088-2-P, which is rated at 8Mz. It also has a Siemens oscillator, SAB8284AP, also rated to 8Mz.

Anyway. Am I correct in my reading of the 8284 chip and the Siemens CPU? Are these in fact 8Mz capable?
From what I see ([example (http://datasheet.datasheetarchive.com/originals/distributors/Datasheets-X/DSA842000-349.pdf)]), I say yes.
But what is the rating of other clock-fed chips, e.g. 8237 and 8255 ?


... and do I need to change the ROM?
Maybe. The present BIOS ROM may be designed/tested for motherboard operation at approx. 4.77 MHz only.


For that matter I would need to ask someone else to burn the ROMs for me. Obviously I would pay them for the chips, shipping, and their time.
See [here (http://minuszerodegrees.net/rom/misc/eprom_programming_services.htm)].