brown@nicmad.UUCP (05/15/85)
I am the SYSOP for the Nicolet Bulletin Board system and one of my users
uploaded instructions for adding 640K to the mother board of an XT. Well,
I did it tonight and it works great.
Instead of buying AST memory expansion cards, you only need to buy the I/O
card to get COM1 and Date/Time.
The modification is very simple. Just follow the instructions as pointed
out below. I added comments to the end about my experience.
My understanding is that all late model shipments of the regular PC contain
the XT motherboard, making it real easy to add 256K memory chips to it as
well. It wouldn't hurt to look. The easiest way to tell is that there is
only one switch bank on the mother board, not two, like there is on the
older PC mopther boards.
Happy mods.
--------------------------------------------------------------------------
This document describes how to install 640k of RAM on the system board
in the IBM XT and the IBM portable (which uses the same mother board).
Parts list:
Quantity Description
---------------------------------------------------------------
18 256k by 1 Dynamic RAM chips (any of the following)
Manufacturer Part number
----------------------------------
Fujitsu MB81256-15
Hitachi HM50257-15
Mitsubishi M5M4256-15
NEC uPD41256-15
OKI MSM41256-15
TI TMS4256-15
Toshiba TMM41256-15
1 74LS158 Decoder/multiplexer Integrated circuit
IBM XT Instructions:
1. Turn off the system unit, and disconnect the power cables, monitor and
keyboard cables and any cables that may be connected to expansion boards.
Remove the monitor and the keyboard. Place the system unit in a convenient
work area.
2. Take off the cover from the system unit by removing the 5 screws on the
back (4 corners and top center), slide the cover forward and tip up to
remove completely.
3. Take out any boards installed in the expansion slots by removing the hold
down screw at the rear of the chassis, and pulling the card straight up.
4. The floppy disk drives will have to be removed to gain access to portions
of the mother board. This is done by removing the screws on the left side
of the drive(s). Gently slide the drive(s) out of the unit far enough
to get at the cables plugged into the back of them. Making note of where
each cable goes, remove the data and power cables by gently pulling them
away from the drive. When the cable are off, remove the drive(s) from the
chassis and set them aside.
5. Refer to figure 1 and locate the jumper block labeled E2 on the mother
board. It is located near the edge of the board near the power supply.
6. A jumper has to be installed between pads 1 and 2 on E2. This can be
done without removing the mother board using a short piece of wire. Hold
the wire with a pair of needle nose pliers and heat up one of the pads
with a soldering iron. When the solder melts, push the wire into the pad
and remove the soldering iron. Do the same thing with the other end of
the wire and pad.
7. Refer to figure 1 and locate the IC socket labeled U84 on the mother
board. This will be an empty socket near the front of the board,
underneath where the floppies were mounted. Install the 74LS158 chip in
this socket making sure pin 1 (marked with a dot or notch) is pointing
away from the front panel.
8. Remove the 64k RAM chips in the rows labeled BANK 0 and BANK 1 (9 in each
bank) on the mother board using an IC puller. If you currently only have
128k of memory on the mother board (BANK's 2 and 3 empty) you can move
these chips to those banks. Be careful not to damage the pins when
removing them (you can sell them to a friend whose machine has amnesia).
9. Install the 256k RAM chips in the now empty sockets of BANK 0 and BANK 1
making sure they are installed correctly with pin 1 pointing away from
the front panel. You should now have 2 banks (0 and 1) of 256k RAM chips,
and 2 banks (2 and 3) of 64k RAM chips, giving you a total of 640k.
10. Refer to figure 1 and locate the switch block, SW1 on the mother board.
Make sure that switch positions 3 and 4 are in the OFF position.
11. Re-install the floppy drive(s) by sliding them into the front panel about
half way and reconnect the data and power cables in the same locations
they came off of. Push the drive(s) the rest of the way in and anchor
them with the screws removed earlier.
12. Re-install the your expansion boards (minus any memory boards that used
to be in the system) in the reverse order of when you took them out.
13. Put the cover back on, re-connect the cables and install your monitor and
keyboard.
------------------------------------------------------------------------
| | |
| EXPANSION | |
| SLOTS | |
| | |
| | |
|--------------------------------------------- |
| |
| ------------------------------------ |
| | BANK 0 | ---- |
| ------------------------------------ E |12| |
| ------------------------------------ 2 |34| |
| | BANK 1 | --- ---- |
| ------------------------------------ |S| |
| ------------------------------------ |W| |
| | BANK 2 | |1| |
| ------------------------------------ --- |
| ------------------------------------ |
| | BANK 3 | --- |
| ------------------------------------ |U| |
| |8| |
| |4| |
| --- |
------------------------------------------------------------------------
Figure 1
(XT/Portable motherboard)
Note from Mike Brown (SYSOP of NICBUL):
I have modified my IBM-XT and it works great. But step number 4 need not be
done. The IC that needs to be installed in step 7 is only part way under the
metal lip that is used to help hold the floppy drive. So, removing the floppy
drive from the left side will not give you any more access to that socket than
there already is.
Removing the floppy drive will only help a litte bit in getting at the E2
jumper. The jumper is half way between the end of the floppy and where the
power wires go into the power supply. By moving the cables and wires out of
the way, you can use a long skinny soldering iron (I used a Weller WTCPN) to
attach the jumper. Use a long nose needle nose pliers tho.
I had everything done in approximately 1/2 hour.
Great mod. Another example of IBM thinking a little bit ahead, but not
telling anyone about it. This mod will also work in later versions on the
regular PC, as IBM is only shipping the XT mother board in all units.
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Mr. Video | | | o| |--------------|
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(!ihnp4!uwvax!astroatc!nicmad!brown)jordan@noscvax.UUCP (Martin C. Jordan) (05/21/85)
In article <166@nicmad.UUCP> brown@nicmad.UUCP writes: >I am the SYSOP for the Nicolet Bulletin Board system and one of my users >uploaded instructions for adding 640K to the mother board of an XT. Well, >I did it tonight and it works great. > >Instead of buying AST memory expansion cards, you only need to buy the I/O >card to get COM1 and Date/Time. Does anone out there in Netland know if similar mods can be performed on a COMPAQ ? I have a COMPAQ+ and would really be interested in using up one less slot if it is possible. Also, does anyone know the source/cost of a COMPAQ technical reference manual? With my limited hardware experience, I can't afford to assume that the COMPAQ and IBM mother boards are virtually identical. Any information on the above topics or any other interesting tidbits on the care and feeding of COMPAQ's would be greatly appreciated. Thanks in Advance, Martin Jordan ARPA: jordan@nosc