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PCXT640K.ZIP
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PCXT.TXT
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1980-01-01
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196 lines
IBM PC XT Memory Boost
by Barry Humphfus
There are lots of relatively expensive ways to increase
the memory of the older model PC XTs that involve the use of
add-on memory cards, multi-function cards, and the like. The
least expensive by far takes about thirty minutes and costs
only the price of one IC and 18 256K RAM chips.
The idea has been around so long that even IBM has
caught on and is using this same technique in all XTs built
since the summer of 1986. It only took them 3 years.
The idea is very simple. You will make a small (and to
allay any fears of warranty violations, reversible) change to
an IC on your system board, add one IC to an empty DIP
socket, and 18 256K RAM chips. The result -- a 64-256K
system board is transformed into a 256-640K system board.
For those of you in need of expansion slots, this means
that you can now remove that old memory card and free up a
slot.
Those of you who's hands start to shake while holding a
dripping soldering iron over your PC, can usually find a
service technician to do this for you at a reasonable price.
You can make it cheap buy removing the system board yourself
and bring in only the IC. Tell him what you are doing. Many
of them will have already done this for other people.
What You Need
1 screw driver (to remove the case)
1 pair of needle nose pliers and a rubber band or a hemostat
1 25-35 watt soldering iron
1 IC (74 LS 158 N) from JDR Electronics, Radio Shack, and
most electronic parts houses
1 three inch strand of wire. This should be a strand from
inside a piece of "zip-cord" also called lamp cord. Don't
be confused by this. What I mean is not the whole group
of strands - just one - it should be a little larger than
the thickness of a hair. An alternative would be what is
referred to as wrap-wire.
18 256K RAM chips (I recommend 150 ns)
What to Do
When IBM built the 64-256K system board, they were
thoughtful enough to number all of the IC position sockets on
the board. We will be dealing with only two of them: numbers
U44 and U84.
1. Park the fixed disk (if you have one) with either the
IBM Diagnostics diskette or the one that came with your fixed
disk if you added it later. Turn off the power to the PC and
disconnect the power cord. Remove the case.
2. Remove all of the expansion cards (display card, floppy
controller, etc.).
3. Unplug the speaker cable (usually a yellow and black
pair of wires going from the speaker to the system board).
4. Depending on which model of system board you have (PC XT
or PC Portable), find and remove the screws holding the board
to the chassis. One of these screws is located at the right
rear and the other is located at the front of the board.
If you have a PC XT, you will need to remove the right-side
disk drive. If you remove the data cables from the drive,
note which side the red stripe is on -- you will need to plug
them in the same way later. Unplug the power cables coming
from the power supply to the system board. (I know we are
unplugging a lot of stuff, but they only can be plugged back
in one way -so don't worry).
If you have a PC Portable, you will have to remove the
lower disk drive. This is a bit tedious, but is the only way
you can get to the front screw holding down the system board.
Just note how you do it. Replacement is the reverse.
5. The system board should now slide out of the chassis.
Note that the system board has little plastic feet. If one
of these falls out of the system board, pick it up off the
floor before the cat finds it, and pop it back into the
system board. Be sure you have a flat work area on which to
lay the system board.
6. Now is the time to plug in your soldering iron.
7. READ THIS CAREFULLY. Locate socket number U44. To find
it, position the system board with the expansion slots away
from you. The chip we are after is on the forth row counting
from front to back, second chip from the right. In it you
will find an IC number N82S129 (or one very close to that
number). With a small screw driver or chip puller, carefully
remove the IC. You will note that it has a small U-shaped
indentation on one end. With the indentation facing away
from you, pin number one is on your left, closest to the
indentation. On the right side of the chip, the third pin
from the indentation back toward you, is pin number eleven.
It is these two that will be connected by the piece of wire.
8. Take the piece of wire and carefully wrap two turns
around pin number one. Try to wrap it around the large upper
part of the pin. Make certain that the wire, once soldered,
does not come in contact with another pin.
9. This is where your hands start to shake. Very carefully
clamp the pliers (with a rubber band around the handle to
keep them closed) or a hemostat over the tip of the pin.
This is being done because I'm cautious. The pliers or
hemostat acts as a heat sink - it draws excess heat from the
soldering iron away from the chip while you are soldering.
Allow a small amount of solder to melt onto your iron. Check
your work. Momentarily touch the tip of the iron to the wire
where it is wrapped around the pin. After about three
seconds or less, the solder will have flowed onto the wire
and soldered it to the pin.
10. Take the other end of the wire (under or over the chip
-- it doesn't matter) and wrap it around pin eleven as above.
Check your work twice - count the pins again if you are not
sure.
11. Solder this end as you did before. Remember to use the
pliers or hemostat as a heat sink. Check your work. Snip
off any excess wire and check to make certain that only pins
one and eleven are connected.
12. Take a small break, get your breath. The rest is easy.
13. Reinsert the chip. If you didn't notice which way it
was oriented in the rush to remove it, look around on the
system board. You will find that all the chips in the area
of U44 have the little indentation oriented in the same
direction. Insert the chip, making sure that all the pins
are in correctly.
14. Find position U84. This will be one of those mysterious
empty sockets IBM left unfilled. It is located along the
first row of chips (as the system board faces you) just right
of the center line. Note the orientation of the chips around
it and insert the LS 158 chip.
15. Remove the 64K RAM chips from banks 0 and 1. Do this by
carefully prying up each one with a small screw driver --
better yet, use a chip puller. Leave the 64K chips in banks
2 and 3 in place. Insert the 256K RAM chips into the sockets
on banks 0 and 1. Take care that you insert the chips in the
correct orientation. If you are uncertain, look at the
remaining 64K RAM chips on the board. You now have a 256-
640K system board.
Putting It Together
16. You may have noticed that the system board has little
plastic feet. When you slide the system board back into the
chassis, these should slide into little holes in the chassis.
17. Find the screws that hold down the system board and
screw them in to place. Reconnect the speaker plug. It only
goes in one way.
18. Reconnect the power cables. They only plug in one way.
If you had to remove a disk drive (PC Portable or two-drive
XT), reinstall it.
19. Reinsert the expansion cards. If you have a multi-
function card with memory, you may discard it or install it
as you wish. Most of them have a battery powered clock and
extra ports that you may want to use. The memory chips may
be removed as you wish. If you do this, you may have to set
the board's switches to tell it that it has no memory chips.
20. Before buttoning everything up, plug in the power (don't
forget the keyboard) and turn on the PC. If everything went
right, you should see the POST crawling up to 640K. If it
doesn't. check to see that the PCs system board switches are
set to 256K of memory and try again.
21. Once you've seen the miracle of 640K displayed, you can
put the case back on.
That's it. Have fun!
Barry M. Humphfus
GENIE Mail: BHLB1114