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Unknown XT board with 1 MB of RAM

Ruud

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Nov 30, 2009
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Heerlen, NL
I have this XT board that can handle 1 MB of RAM. By accident I found a picture on Modem7's page. The photo is part of an entry on http://minuszerodegrees.net/xt_clone_bios/xt_clone_bios.htm, look for "Motherboard: Unbranded XT clone (CPU: V20)". As you can see it doesn't mention the capability of handling 1 MB of RAM, maybe for the simple reason that fact was unknown.

How do I know it can handle 1 MB? Because it in my case the board has eight 414256 DRAMs on the board plus four 41256s for parity AND the BIOS counts up to 1 MB.

I looked on the internet for more info but none found so far. In fact I looked for software that can use the extra RAM but no luck here as well. I started to disassemble the BIOS and soon had the idea that a register at IO address 0E0h was used to select the extra RAM. But then I also ran into the registers 70h and 90h and so far I have no idea what they are used for. I hope time (or on of you) will tell.

I attached a photo of my board plus a ZIP containing the JUKO V2.30 BIOS.
 

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Tronix

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Russia, Moscow
AFAIK you can use add ram on UDL chipset as RAM-drive, PRINTER-buffer or HDD cache. No UMB or EMS.
 

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modem7

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Ruud

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Some findings:
- The manual doesn't mention the possibility to place up to 1MB of RAM except the dip switch settings. Remark: the board on Modem7's site has a NEC chip, my board has a VDL one. It could be that the NEC one isn't capable of handling 1 MB.
- It explains the use of port 70h :)
- It also mentions the fact that it runs at 12 or 15 MHZ. Mine runs at 8 MHz. Hmmm, could it be possible to upgrade it to at least 10 MHz? Not much work involved, just replacing a crystal.
 

modem7

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- The manual doesn't mention the possibility to place up to 1MB of RAM except the dip switch settings.
In the manual that dkedrowitsch pointed to, the front page is titled 'UX Turbo (44256 VERSION)'. 44256 is the main type of RAM chip on my motherboard. Because your board uses 414256 RAM instead, perhaps there was a 'UX Turbo (414256 VERSION)' version of the manual produced.

It also mentions the fact that it runs at 12 or 15 MHZ. Mine runs at 8 MHz. Hmmm, could it be possible to upgrade it to at least 10 MHz?
On the clock pin of the V20 on my board, I measure 4.77 MHz (non-turbo) and 12 MHz (turbo). That matches nicely with the fitted crystals: 14.31818 MHz and 24 MHz.

On my board, the fitted V20 is rated at 8 MHz. I presume that a test was done in turbo mode to verify that the fitted V20 was stable at 12 MHz.

On my board, the ratings of the fitted RAM chips are a mixture of 100, 120, and 150 ns.
 

Ruud

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Because your board uses 414256 RAM instead, perhaps there was a 'UX Turbo (414256 VERSION)' version of the manual produced.
Different code, same type of RAM.

On the clock pin of the V20 on my board, I measure 4.77 MHz (non-turbo) and 12 MHz (turbo). That matches nicely with the fitted crystals: 14.31818 MHz and 24 MHz.
Are you sure about measuring 12 MHz? I just checked the data sheets and the 8284 divides the incoming clock by three, whether it is coming from the internal clock generator or the external clock. The only thing I can think of is that you measured the wrong signal. The pin CLK on the ISA bus is the chosen frequency divided by TWO. That would explain your 12 MHz.

On the clock pin of the V20 on my board
So it your own board? Would it be possible to solder some sockets, fill them with 4256 DRAms and using the BIOD I provided to see if your board is capable of handling 1 MB as well? If not, it means there are different versions of that custom IC around.
 

modem7

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Are you sure about measuring 12 MHz?
Yes, as measured by oscilloscope.

I just checked the data sheets and the 8284 divides the incoming clock by three, whether it is coming from the internal clock generator or the external clock ...
There is no 8284 on my board.

The pin CLK on the ISA bus is the chosen frequency divided by TWO. That would explain your 12 MHz.
On my board, the CLOCK pin (pin B20) of the ISA connectors is connected to the CLK pin (pin 19) on the V20, which in turn, is connected to one of the pins on the NEC flat pack chip.

So it your own board? Would it be possible to solder some sockets, fill them with 4256 DRAms and using the BIOD I provided to see if your board is capable of handling 1 MB as well? If not, it means there are different versions of that custom IC around.
I will have to see whether or not I have the required RAM type. And then the activity would be added to my to-do-list.
 
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