Instructions to modify XT-IDE to support a 32K EEPROM.  (28C256)

Rev 1 - 02 AUG 2010
Bill Lewis - bill@wrljet.com

This document is found at:  http://www.wrljet.com/XT-IDE

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This relates to the XT-IDE project of the Vintage Computer forum.

http://wiki.vintage-computer.com/index.php/XTIDE_project

http://www.vintage-computer.com/vcforum/forum.php

http://code.google.com/p/xtideuniversalbios


The modification adds jumper selection of A13 and A14 to the
decoder and the EEPROM to permit the use of a 32K (28C256) memory.

To avoid bus contention, do *not* change the jumper settings
while the computer is powered on.

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Physically mount a 3-pin header and a 5-pin header to the
board.  I drilled holes and milled away the ground plane from
around the holes, so the pins could be through the board.
I used epoxy to glue the headers to the board.

You can also glue the headers flat against the solder side of the
board to avoid drilling holes.

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Nomenclature:

The new 5-pin header is W1.
The new 3-pin header is W2.

When I refer to U9-15 that means IC U9, pin 15.
W1-3 would mean the new W1 header, pin 3.

Cut two traces, A13 from the edge connector to U9-15,
and A14 from the edge connector to U9-13.  (see photo)

Add the following wires:

- W1-1 to GND (any ground that's close by and easy to solder to)
- W2-1 to GND
- W2-5 to GND
- A13 from edge connector (see pic) to U10-26 and W2-1
- A14 from edge connector (see pic) to W1-3
- U9-15 to W2-2
- U9-13 to W1-2
- U10-1 to W1-4

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Jumper Assignments:

                W1    W2
              -----------
Original ckt | 2-3 | 1-2 | EEPROM A14 is low (low half on 28C256, n/c on 28C64)
 8K          | 4-5 |     | EEPROM A13 is connected (n/c on 28C64)
28C64B       |     |     | A13 and A14 go to decoder
             |     |     | To use a 28C256 in 8K mode at any address,
             |     |     | the contents must be duplicated at 0000h and 2000h.
-------------|-----------|
 16K         | 2-3 | 2-3 | EEPROM A14 is low (low half selected)
28C256       | 4-5 |     | EEPROM A13 is connected
             |     |     | A14 goes to the decoder
             |     |     | A13 on decoder is low (don't care)
-------------|-----------|
 32K         | 1-2 | 2-3 | EEPROM A14 and A13 are connected
28C256       | 3-4 |     | A13 on decoder is low (don't care)
             |     |     | A14 on decoder is low (don't care)
             |     |     | Only high 3 bits of switch are used
-------------------------  

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Notes:

- To avoid bus contention, do *not* change the jumper settings
  while the computer is powered on.

- There's no reason to perform this mod if you're only going to use 8K.
  But if you do, and use a 28C256 chip, you'll need to duplicated the
  8K contents in the EEPROM at addr 0000h and 2000h.

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