8550 Tinkering
Gereon Wenzel
   My 8550 has the old style 30M ESDI with the logic on the MCA card
and the disk plugging staigth to the MCA card. now I got hold of a card looking similar but without logic with connector for the bigger (up to 160MB) ESDI with onboard logic. Thinking its allways a good idea expanding disk space, just swapped to my 8550 and connected a working 80MB ESDI. Hard disk change notice( never got on the 8570 when swapping ESDI), autoconfig, Disk spins down and up, system frozen.
tried several times allways same. switched back to thr original 30MB.

    50Z only I believe.  Plain 50 didn't have support on planar.  50Z actually implements most of the controller onboard.  Need to scare up a 50Z planar to use that one.

The 50-021 / -031 lacks the ESDI part of the BIOS.

If you pick the 50Z BIOS chips (2 x 512) and split-burn them down into 4 x 256 (as the 50-0xx uses them) you can convert a 50-0xx into an "almost 50Z" without needing the "50-to-50Z upgrade controller", which is a rare item. It only adds the ESDI code to the BIOS and needs a different @DF9F.ADF, which allowes to set an adress for the BIOS ROM on the controller.

Did that a decade ago and it seemed to work fine with a 50-021 and a 120MB HD from a Mod. 70.

Ed Avis
   Couldn't you also replace bits of the BIOS in software?  Get the BIOS
copied into RAM as is sometimes done for performance, then overwrite
bits of it.  I dunno if anyone has actually done this, but it sounds

   The 286s don't copy the ROM into RAM after POST like the 386s do - and even if: you should do that *before* POST to have the ESDI controller available for the boot process .... but I don't think that was what you *meant*.
   [Rem: The "upgrade controller" in fact *does* something similar: it copies some routines into an appropriate area in the "adapter ROM area" (C000 - DFFF) and sets the bootloader entry point to the start of that routine. Usual "check
adapter segments for bootable controller ROMs" stuff.]
   Using "Split Copy" on an Eprommer is much better. You pick the "even" of the two 27C512 (64K x 8 bit) Eproms, read and store 0000-7FFF in the first and 8000-FFFF in the second process.  
   Then you pick the "odd" Eprom and repeat the step. The four files nicely burn into 4 x 27C256 (32K x 8 bit) Eproms and you have Odd-Low, Odd-High, Even-Low, Even-High.
It's *a little* complicated - but not much. (Even I managed ...)  ;-) I think I still have the codes in my data cemetry somewhere.  In the 50-021-format.

Tim Clarke 
   Better to mod. the planar BIOS to a minimal "IML" Stage-1 subset and add support for a "System Partition" with the "loadable" Stage-2 BIOS on it, then you can mod. the Stage-2 code to your heart's content. Not for the
faint-hearted, however.

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