Hello,
sorry for the english, i'm feeling very lazy ...
i'm mod'ing one of the 486DX2 boards.
model is PDX 1019-09 / 0675.
fortunately, i had downloaded the docs
that were scanned, though it would have
been better if the resolution was 1.5x or 2x higher,
particularly for interpreting the schematics
and the layout. but until now it's almost OK.
First mod :
a 1500uF condo between the unpopulated
J18 and S1. I have removed the solder
and the distance between the ground pad
and the Vcc pad was within correct range.
I am really a decoupling freak, particularly
for audio systems (and this board will be used
as a diital synthesizer board).
I also removed solder out of other unpopulated
connectors. Currently for the backup battery
and the power connector.
I still haven't completed the power system
but if i can avoid polarity inversion risks, it's
better. So i consider soldering a keyed connector
or at least wires to the power supply.
More complex is the CMOS battery system :
i dislike NiCd for many reasons, and i have
a good supply (tens) of 0.22F condos.
I could also install a Lithium battery but i'm not
sure that it is electrically ok : applying 5V to it
would damage the cell, i think.
Or i could put one
of my Lithium-Ion cells that i got from broken cellular
phones ( a silicon diode would drop the 5V to near
the 4.2V charging tension and would prevent backwards
current at the same time) but the size and the complexity
are not worth it. The 0.22F will keep the CMOS
config during less time, but there is no worry at all
about the health of the battery cells.
I also solder the keyboard connector, this
will prevent other potential plug misplacement problems.
HDD and diskette are no worry, except that the diskette
needs 12V, while i use a laptop HDD so it takes
less current and only needs 5V.
Note : i have found Y2K ISA boards in a shop in Paris.
The nice thing is that it removes some of the worries
with the RTC thing (according to the doc, the clock is
managed by the BIOS through an IRQ, "the old way",
so the clock is not updated when the board is unpowered).
The 8-bit ISA board contains a Dallas chip with integrated
Lithium battery, which is cool, but needs a "driver"
because it's not replacing the "integrated" RTC, it's located
in another address. I don't need this for my current application.
But there's more : the PCB contains an altera MAX7064 with
a nice JTAG header, so the CPLD could be reprogrammed for
other purposes :-)))
I have grand plans for this board.
When SW developement is finished,
i'll remove the VGA sandwich for reducing the power
consumption, and eventually will replace the DX2
with an "overdrive"-style CPU.
But more importantly i need a high quality sound output
so i'll make a custom board for this with a comfy FIFO.
Big question : will i be able to add a custom memory chip
that will take over the BIOS before it polls the Diskette
and the HDD ? Is there a support for the "ROM BASIC" or
(as used later) "Network BIOS" ? The documentation doesn't
seem to indicate anything like this. I simply want to run
a smallish SW stored in FLASH or backuped SRAM, and forget
about the rest.
Any hint ?
YG