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Unidentified XT Clone m/b

Chr$

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Posting in case anyone recognises this board. It's a generic XT clone with 7 x 8bit ISA and 5 EPROM sockets. The BIOS contains the strings "(C) COMPUTER TURBO version 2.0 1985 International Business System" and "(C) PC/XT TURBO 8 MHz SYSTEM". On the solder side it has "PMC' and separately "OEM/PC REV B1".



I've googled a lot, as you do! This chap here has a 99% identical board:

Restoring the old PC-XT clone mainboard - MCbx (oldcomputer.info)

I can't find a way to contact him though and it would also be nice to have full manual info on the 15+ jumpers that relate to the EPROM configuration as I'm not as clever as the above website owner. The other 4 'proms on my board contain IBM Personal Computer Basic, v C1.10. (incidentally...that's obviously the cassette basic, so how on earth do you use that on an XT clone, which has no way of hearing or playing sound?).

I am fairly sure that the little block of 8 switches is the same configuration as this similar example:

TURBO - V4 (arvutimuuseum.ee)

The board condition is very nice, as you can see. It turned on twice (without a monitor connected) before the PSU exploded. I need to take care of that first as I don't have another to hand that isn't in something else. Speaker doesn't work, but apart from that I think it made all the right noises. All voltages were present. In the pic there is a missing 74x logic chip as I found it was faulty and am waiting on a replacement.

The generic serial ports/extra RAM!/RTC card is another story as it had a NiCd battery on it that made a right mess, as usual. Fortunately it seems to have been stored upside down and it only damaged the card it was on.
 

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Chuck(G)

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I can't find a way to contact him though and it would also be nice to have full manual info on the 15+ jumpers that relate to the EPROM configuration as I'm not as clever as the above website owner. The other 4 'proms on my board contain IBM Personal Computer Basic, v C1.10. (incidentally...that's obviously the cassette basic, so how on earth do you use that on an XT clone, which has no way of hearing or playing sound?).

The 5150 did? The 5150 BASIC in ROM works just fine; you don't get to use the cassette features. If you're going to run IBM PC-DOS BASICA, however, you need something in ROM.
 

Chr$

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The 5150 BASIC in ROM works just fine; you don't get to use the cassette features. If you're going to run IBM PC-DOS BASICA, however, you need something in ROM.

Ah, I see. BASICA requires Cassette BASIC in ROM but allows you to save load to/from disk. I didn't realise that. It's my first XT, the earliest was a 286 up to now. Hope it works.
 

Eudimorphodon

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I didn't realise that. It's my first XT, the earliest was a 286 up to now. Hope it works.

Sadly IBM machines actually kept the same Cassette Basic in ROM through at least the first generation of PS/2s machines. (Anyone happen to know off the top of their head when it finally disappeared?)
 

vwestlife

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Sadly IBM machines actually kept the same Cassette Basic in ROM through at least the first generation of PS/2s machines. (Anyone happen to know off the top of their head when it finally disappeared?)
The PS/1s and some of the Aptivas also had BASIC in ROM.

They even modified QBASIC in PC DOS 5.0 to require ROM BASIC in order to run:

http://www.os2museum.com/wp/ibm-dos-...n-ibm-systems/

That requirement was removed when IBM began selling PC DOS 5.0x on the retail market for clones as well as IBMs.
 
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Chr$

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Dimensions are 27 x 34.5cm. Attached is the BIOS bin.
 

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Chr$

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So that's about two inches wider and half an inch longer (front to back) than a standard XT/Baby AT motherboard.

Lucky it came in a suitable case then! It takes up floor space in this attractive top-hinged housing.
 

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SomeGuy

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Chr$

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That looks like a (DTC) Display Telecommunications Corporation "Megaboard".

I've got one, but just the motherboard.

https://www.vcfed.org/forum/forum/g...play-telecommunications-corporation-megaboard

There was another thread about another similar one here: https://www.vcfed.org/forum/forum/genres/pcs-and-clones/67645-identifying-xt-clone-mainboard

Very non-standard form factor.

I'd be interested to see photos of the inside of the case.

Excellent, great info, thanks. They certainly seem identical to all intents and purposes (mine doesn't have the ISA PCB connector on the edge or the reset button).

The 2nd topic you linked to by MCbx appears to be the same person that I chanced upon with the repair write-up and the eprom setup woes.

If 256k is the max, despite full RAM population, then that would explain why mine came with a multifunction (SixPac clone) card containing 384k of RAM. Hopefully I can sort out the battery damage on that. Case pics to follow. For now, here's part of the back showing the PMC OEM info. I just noticed the '3' with a crown-like thing on it in a circle.
 

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Chr$

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Here are some case pics. I did also power up the board today... more on that in the next post as I'm not sure how many pics I can attach.
 

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Chr$

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Testing the beast:

I couldn't wait on the PSU parts to get things fixed up, so re-purposed a PSU from a later AT PC.

The +12v was shorted. I wonder if that's what caused an already weak PSU to blow (and/or it has no shutdown protection?). The culprit was a tantalum on the CGA card. Removed that and it powered up. It counts up to 256k RAM despite the fully populated board. With no drive controller cards I was able to get to the IBM Cassette basic. All the keys on one of my own keyboards seemed to work fine.

Then I attached the floppy controller card that it came with and one of the 360k drives that it also came with. I'd previously tested both drives on another machine and the one I used did seem to be fine on the other machine (the other was not - perhaps more on that in another post).

It boots, but only after it reports 'Disk Boot failure' or 'Non-System disk or disk error' about 20 times after pressing any key. When it does eventually manage to boot to PC-DOS (3.2) all seems well. I can dir and load things from the boot disk as normal. I suspect the unreliable booting from floppy is possibly down to the disk - which was an NOS, never formatted 1.2mb disk that I virgin formatted in another 360k drive and then wrote the PC-DOS 3.2 image to. I think I read somewhere that non-formatted 1.2mb disks can be reliable when virgin formatted to 360k. Maybe a dreamt that!

Then I tried adding the Seagate S10 MFM controller card that it came with, but that simply displays 'ST10 v2.4' on the screen before anything else appears and then hangs. The Seagate ST251 drive that it came with is also quite loud and seems very much on it's way out. I also tried the S10 card in another machine and even with no drive or ribbon cables attached to it, it does the same thing, i.e. shows 'ST10 v2.4' and then hangs. I'd love to hear if anyone has any ideas about that.

Next, I installed a known working WD controller card and ST225 HDD pair with DOS 6.22 pre-installed and it booted to C: without issue (apart from the obvious incompatible himen etc stuff).

So, some teething troubles but overall a good result I think.
 

modem7

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Then I tried adding the Seagate S10 MFM controller card that it came with, but that simply displays 'ST10 v2.4' on the screen before anything else appears and then hangs. The Seagate ST251 drive that it came with is also quite loud and seems very much on it's way out. I also tried the S10 card in another machine and even with no drive or ribbon cables attached to it, it does the same thing, i.e. shows 'ST10 v2.4' and then hangs. I'd love to hear if anyone has any ideas about that.
Some of these XT-class HDD controllers have timeouts as long as 2 minutes. If you wait, say 3 minutes, do you see a 1701 error appear at some point ?
 

Chr$

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Thanks for the tip. But no, it stays hung well beyond 5 mins. It also does the same with no drive or cables attached, which is probably normal.

I'm getting on better with the floppy drives though. It seems my NOS main boot disk was flaky. The second drive is reliable with disks that it formatted itself, so suspect alignment. And the better drive seems to read factory retail disks pretty well. With the right disks in the right drive(s) it's happy and I can get it to boot every time.
 

modem7

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Thanks for the tip. But no, it stays hung well beyond 5 mins. It also does the same with no drive or cables attached, which is probably normal.
Normal for an XT-class HDD controller is that it issue a 1701 error after a timeout, i.e. allow the boot process to continue.
I have an ST10 card (with v2.4 BIOS) in storage. I will dig it out and see how it behaves with no cables/drives attached.
 

Chr$

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Normal for an XT-class HDD controller is that it issue a 1701 error after a timeout, i.e. allow the boot process to continue.
I have an ST10 card (with v2.4 BIOS) in storage. I will dig it out and see how it behaves with no cables/drives attached.

Thanks very much. I did try it with the ERSO v2.42 BIOS, just in case, but it made no difference except the ST10 v2.4 message comes after the initial few lines of text, and then it hangs again. I did also check the BIOS on the ST10 card and that matches exactly the v2.4 BIOS on your website.

Another issue is now getting the Fountain Technologies Inc MFT-384 multifunction card RAM to be seen. The m/b has 256k fitted and when I set the card to start at 256k (the same settings it came with) it is simply not seen but some of the card RAM chips do get a little warm. I have also read that the m/b may not be capable of having more than 256k. Or well, it's all good fun.
 

modem7

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Another issue is now getting the Fountain Technologies Inc MFT-384 multifunction card RAM to be seen. The m/b has 256k fitted and when I set the card to start at 256k (the same settings it came with) it is simply not seen but some of the card RAM chips do get a little warm. I have also read that the m/b may not be capable of having more than 256k. Or well, it's all good fun.
I have just now replied in that thread.
 

modem7

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Normal for an XT-class HDD controller is that it issue a 1701 error after a timeout, i.e. allow the boot process to continue.
I have an ST10 card (with v2.4 BIOS) in storage. I will dig it out and see how it behaves with no cables/drives attached.
Thanks very much. I did try it with the ERSO v2.42 BIOS, just in case, but it made no difference except the ST10 v2.4 message comes after the initial few lines of text, and then it hangs again. I did also check the BIOS on the ST10 card and that matches exactly the v2.4 BIOS on your website.
I located my ST10 card. I put it into my IBM 5160 (IBM XT). No cables connected to it. Shortly after the 5160's POST completed its memory count-up, the ST10 displayed "ST10 v2.4", then about 45 seconds later, the ST10 displayed "1701 (04)". I was then prompted by the 5160's POST to press the F1 key to continue.
 
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