GRiD Custom HD Geometry.txt

Posted by Michael R. on June 25, 19101 at 00:17:18:

Hello everybody,

I found a way to connect a Conner CFS210A Harddisk to my GRiDCASE 1530. (Yes, I have a 200MB HDD AND a 3,5" FDD in my 
GRiDCASE!) I think it should work with each Conner Harddisk which fits mechanically and is below the 520MB border.
The trick is, you have to patch the BIOS.

One (or a few more ;-)) word before you start patching your EPROM:

DON'T USE THE ORIGINAL BIOS EPROMS OR MAKE A BACKUP OF YOUR BIOS EPROMS!
YOU MAKE ALL CHANGES ON YOUR OWN RISK!
Note the Checksum of each original EPROM, you need it later on.

How the BIOS works:
My GRiDCASE has the Version "80386 ROM BIOS Version 3.06 10/25/89".
First the BIOS uses the Harddisk-command ECh to identify it; then it looks into the identification for the Harddisk-ty
pe and compares this with a BIOS-internal table of allowed Harddisk. In my BIOS the table starts at F000:F63C and incl
udes the following types CP3022, CP3024, CP3044, CP3042, CP344 and CP3104. You can check this with DOS debug and using
 "d f000:f63c" it corresponds with the EPROM-address 763Ch in byte- and 3B1Eh in word-mode.
Then there must be a second table with six pointers, one for each HDD-type, I didn't search for this table till now, b
ecause I found a workaround by chance.
This pointer points to an entry in a third table, the table of HDD-parameters. It starts at BIOS-address F000:E400, EP
ROM-address 6400h in byte- and 3200h in word-mode. Each entry is 16 (10h) bytes long. Then the BIOS uses this paramete
rs.

Now to the solution. You have to change three points into the BIOS:
1. the HDD-type; 2. the parameter entry; 3. the BIOS-checksum.

Step 1:
I changed the first and the second entry in the type-table (3B1Eh):
The original content was (in ASCII):
CP3022CP3024CP3044CP3042CP344CP3104
to the following:
CFS210ACP344CP3044CP3042CP344CP3104
The first entry is the type of the HDD you will use (for me CFS210A), I changed the second entry to another valid entr
y (CP344). So I didn't change the table length and all enntries are valid. You may change more then this entrys, but u
se only valid entries and don't change the table length.

Step 2:
By chance I found out, that entry two in the parameter-table belongs to entry one in the type-table. Entry two starts 
at BIOS-address F000:E410, EPROM-address 6410h in byte- and 3208h in word-mode.

The CFS210A HDD has the following parameters:
Cyl 685 2ADh
Hds 16 10h
WPComp 0 0h
LZone 2477 9ADh
Sect 38 26h

meaning ? Cyl Hds ? ? WPComp ? ? ? ? ? LZone Sect
original entry 00 | 67 02 | 04 | 00 00 | 2C 01 | 00 00 00 00 00 | 67 02 | 11
patched entry 00 | AD 02 | 10 | 00 00 | 00 00 | 00 00 00 00 00 | AD 09 | 26

Remember, you have to exchange high and low byte (Cyl, WPComp and LZone).

Step 3:
Change the checksum of each EPROM to the original value. By making the above patches, you changed the checksum of the 
EPROMs. I used a trick to restore the original values. At the beginning of each EPROM is a string with a copyright not
e "Copyright 1985,1986 Phoenix Technologies Ltd.". You may change this strings in a form, that brings you back to the 
original checksum. This string is only (and not more) a copyright note. The checksum of my even-EPROM is F600h and of 
the odd-EPROM is 8600h.

May be you can help me, I am looking for a battery slide in module (is it the correct formulation?) for my 1530 becaus
e it came without one and for an ethernet adapter. 
If you have documentation about the connectors, specially the internal and the external peripheral or if you find out 
more about connecting HDD's, please mail it to me.

Please, don't use this information commercially (without giving me a little bonus ;-}), I worked hard for two days to 
find out this informations!

I wish you good luck and success, many greetings from germany

Michael