UPGRADE: 800 -> 256K Memory

From: Atari SIG (xx004@cleveland.Freenet.Edu)
Date: 05/03/93-05:48:03 PM Z

From: xx004@cleveland.Freenet.Edu (Atari SIG)
Subject: UPGRADE: 800 -> 256K Memory
Date: Mon May  3 17:48:03 1993

Reprinted from A.C.E.C. BBS (614)-471-8559




for the



by David G. Byrd
1513 Commanche Drive
Las Vegas, NV 89109

Provided free for the personal use of ATARI enthusiasts everywhere, but not 
for sale.

Copyright (c) 1985, D. G. Byrd

Atari 800 is a trademark of Atari, Inc.

This article may be reprinted if printed in whole and proper credit is given to
the original authors and this publication.  For their repetitive failure to 
provide credit to the original author, this permission is expressly DENIED to 
Jay Torres and The Windhover Project.

First published in the July 1985 S.N.A.C.C Newsletter.
(Southern Nevada Atari Computer Club)

Revision A - Oct 1985

Revision A


288k Total Memory for the 800

by David G. Byrd, Las Vegas, Nv.

I am one of those incorrigible hardware hackers who is never satisfied with 
simply operating an appliance computer with purchased software.  I am 
fascinated by taking a commercial product and trying to extend its capabilities 
beyond those the original supplier provided.

After reading the "48/64K UPGRADE FOR THE ATARI 400" by Claus Buchholz, I was 
inspired to design a similar upgrade for the ATARI 800. Full credit for the 
installation of 5v only chips in the 16k board goes to Claus and the "MACE 
Users Group".

This article will describe how to modify, test and use a 256k memory board in 
the middle slot of the 800. This will cost you under $50, a few hours of your 
time and will give you a total of 288k of RAM.

All logic additions and track cutting is limited to the memory boards, so you 
may wish to pick up a "spare" to hack on. This will allow you to return to a 
stock 800 if you should desire. One jumper is installed in the Personality 
Module (ROM), and three backplane jumpers are necessary to provide the 
additional controls.


Remove the middle memory board (or use the spare) and Look at the memory board.

The eight chips along the top are the RAM chips.  The other four chips are the 
addressing circuitry.  The edge pin connectors at the bottom are labeled as in 
Figure 2.

The first step is to eliminate the 12V and -5V sources on the board and to move 
the 5V source to where the 12V used to be.  As shown in Figure 3, cut the trace 
going from pin "X" of the boards's edge connector to the capacitor C521.  Also 
cut the trace going from edge pin "Y" to C523.  Cut the traces cleanly and 
completely.  Be careful not to slip and damage adjacent traces.

Now remove the capacitors C521 and C523.  The trace coming from pin "W" carries 
5V.  Using a short piece of wire, make a solder bridge between this trace and 
the old 12V and -5V traces, at the point where C523 and C521 used to be (see 
Fig ure 3).  Next, remove the eight capacitors C503, C505, C507, C509, C511, 
C513, C515, and C517, which are usually in a row along the top of the board.

We now have 5V going to pins 1, 8 and 9 of the RAM sockets.  Remove the eight 
16k RAM chips and insert the new 256k RAMS in their place, properly orienting 
their notched ends.  With an ohmmeter, make sure there is NO connection between 
edge pin "Y" and pin 8 of the chips, nor should there be any connection between 
any two of the edge pins "W", "X", and "Y".

If all has gone well the board should function like a 16K memory board, since 
the addressing circuitry has not been altered.

Put the modified memory board in the middle memory slot of your 800, and check 
to see if it functions normally.  The blue screen should appear quickly. If 
not, recheck all modifications made so far.

Now take the 5V supply off pins 1 and 9 of the RAM chips.  To do this, cut the 
two rightmost wide traces on the chip side of the board (see Figure 4).

Pick up a new 74LS158 chip, which is the same as the chips Z503 and Z504 on the 
memory board.  With needlenose pliers, carefully bend up all pins except 1, 8, 
15, and 16 (see Figure 5).  Remove chip Z503, place the new chip on top so that 
the four pins listed above touch the same four pins on the lower chip.  
Carefully, solder each of the four pairs together, being careful not to get too 
much solder on the end of each pin. Now insert the pair in socket Z503.

The second new 74LS158 chip is to be mounted in the same fashion on top of Z504 
except that pin 1 is also elevated and not soldered.  Now insert this pair in 
socket Z504.

Next we will prepare the additional chips for mounting.  These chips will be 
mounted "Dead Bug" style, so it is a good idea to put a stick-on label on the 
underside of each chip indicating its type and the location of pin 1.

Install the four new chips in the vacant area below the memory chips. I used 
"Super Glue" to attach them.  Make sure you get them in the right place the 
first time.

Now wire the new chips per the diagram in Figure 6.  Connect 5V and ground pins 
first to each dead-bug chip. Convenient connection points for 5V and ground are 
the left and right ends of each capacitor located directly below each RAM chip. 
Connect each chip individually with short wires.

Connect a temporary jumper from pin V to W.


Return the memory board to the middle slot for testing as a 16k board.

If the blue screen doesn't come up quickly, turn it off immediately and check 
your work.

If it checks OK, remove it and remove the temporary jumper connecting pins V 
and W, and cut the track connecting pins S and T.

The 256k board is complete so put it aside for now.


Remove the two remaining 16k memory boards from the computer, and remove the 
case.  On the reverse side of the board, find the tracks connecting S to T. Cut 
this track on both 16k boards.

If you wish, you may reinstall these boards in their case.


Remove the ROM board, and take it out of the case.  Find Z401 and connect a 
jumper between Z401 pin 7 and pin 20 (the only unused pin) of the card edge. Do 
not allow solder to flow down the card edge pin.

Reinstall the ROM board in its case.


Now it will be necessary to get to the 800 backplane.  This requires removal of 
the case. First remove all cables from the computer.

Turn the 800 over, with the game-ports facing you, and remove the five recessed 
phillips screws holding the bottom cover.  It may now be removed by lifting at 
the rear and then sliding it forward to clear the game-port connectors. This 
will expose the bottom RF shield.

Remove the nine phillips screws holding the bottom RF shield and mother board 
in the RF cage.  Remove the phillips screws on the sides of the RF cage. Gently 
lift the front edge of the mother board and power supply boards and disconnect 
the keyboard ribbon cable, the power supply cable (note its orientation), and 
speaker cable.  Separate the power supply board and mother board.  The mother 
board and lower RF shield may now be lifted up and out of the RF cage.  Now 
remove the CPU card.

The bottom shield must be removed.  It is attached with four plastic expansion 
pins.  Use a screwdriver to push the center pin out until it can be removed 
from the bottom. Now remove the outer pins.

The bottom shield will lift off and expose the entire backplane area under the 
card cage.

Using Figure 7 as a guide, install the three jumpers using a fine, solid 
conductor, insulated wire.

Recheck your work with particular attention to correct pins, solder bridges and 

Reinstall the bottom shield and replace the plastic pins.


Reinstall the CPU board, place the mother board in the RF cage, and re-insert 
the power supply connector (remember proper orientation). Reconnect the 
keyboard cable and speaker cable.  Replace the screws in the RF cage and then 
the nine screws holding the bottom shield.  Reinstall the bottom cover and 
replace the five phillips screws.  The 800-PLUS 256k modification is now 


Turn the 800 upright and reinstall all cards making sure the 256k memory board 
is in the middle slot. Replace the top cover and close the cartridge lid.

Again, power up the computer and watch for the blue screen to appear.  As 
before, if it does not appear, turn the machine off and check your work.

If all is normal, congratulations, you now have an ATARI 800 with 48k of normal 
user memory and 240k of extended memory.


The most effective use of extended memory is as a disk emulator (virtual disk).
I used GENEMDE, (by H. V. Stacey), to extend Atari DOS II, version 2.0S.  This 
produces EMDE/OS which was written specifically for the 800 PLUS mod.  It 
effectively provides another "very fast" disk drive.  DUP.SYS and MEM.SAV may 
be made resident on the virtual disk and switching from application programs to 
DOS and back again will occur almost instantaneously. With 288K total memory 
installed the virtual disk may be configur

ed as a full 720 sector single or double density ASIC (slow) Extended Memory 
Diagnostic Programs.


The possibilities for use of this extended memory are many and varied. Very 
fast disk reads and writes can speed up Data Base searches.

Use of the "virtual disk" when running AMODEM will significantly reduce the 
upload and download times (and your long distance phone bill).

Disk duping becomes a breeze when you can use J. (Duplicate disk) or C. (Copy 
files *.*) to move data to and from virtual disk.

Programs that leave your drive spinning for extended periods can often be 
modified to perform much faster and with no wear on the drive.

I would be very interested in the uses you find. That is part of the fun and 
challenge.  Just send me a copy. 

                       800 PLUS 256k-PARTS LIST
                 Desig   Qty  Type       Description
                  ZX1     1  74LS02    Quad NOR Gate   
                  ZX2     1  74LS175   Quad Latch      
                  ZX4-5   2  74LS158   Quad Multiplexer
                  ZX6     1  74LS112   Dual Flip Flop  
                  ZX7-14  8  41256     DRAM (150ns)    
                  ZX3     0  Deleted in Rev. A

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