››››››››          THE ATARI 130XE MEMORY UPGRADE››               The Atari 130XE lends itself to memory increases quite›          easily.  For those who may care the Freddie chip (Part #›          CO61991-29) has been around for a while.  Remember the 1400›          and 1450XL computers?  These machines used Freddie strictly›          for memory management of 64k.  The pin on Freddie dhat we›          will be concerned with is pin 36.  This pin is labeled›          16KCAS.  Pin 36 is used to enable the extra 64K bank(s) and›          is active low.›››          Technical Overview››               Now for the good part.  If we redirect the output of›          pin 36 to another bank of 64K ram chips we can via software›          select any one of 8 banks of 64K.  In this configuration›          one would have a maximum of 589,815 bytes of memory.  The›          only disadvantage of this is the fact that under these›          constraints the hardware becomes more complex and the›          software to drive it does too.  I recommend that for›          general purposes the 130XE be upgraded to a maximum of›          320K.  It gives the user sufficient "horsepower" and yet at›          the same time the software does not become too intense.›          Now for the bad news.  To implement the 320K mod one has to›          remove the internal basic ROM which really isn't that bad›          because most users are using Basic XL/XE anyway.  The›          reason for this is quite good.  That bit is required for›          the addressing of the extra bank(s) of memory.  Actually›          one could use the Self-test bit but that requires›          additional hardware or a reburn of the O.S. ROM.  More on›          that later.›››          The Mod››               What we will do is to wire up a 74LS138 to the 6520›          PIA and to pin 36 of Freddie.  This chip can be best›          described as a routing switch.  The data that we want to›          re-direct is presented at pin 4 of the 138 and the data›          output is pins 15, 14, 13 and 12.  The pins that tell the›          chip to what bank to directs it's output to are pins 1, 2,›          and 3.  Now for the installation.››               1)  Take 32 64K x 1 RAM chips and bend out pin 15 on›          all of them.  Pin 15 is the CAS line.››               2)  Solder these chips onto the existing RAM chips in›          the 130.  Do not solder anything to pin 15 yet!  For ease›          of servicing and soldering I staggered these new RAM chips›          onto the existing ones.  I recommend this highly.  When you›          has completed soldering in each new bank take a piece of››››››››››››››          insulated wire and solder this wire to pin 15 of each new›          RAM chip.  Wire wrap wire works nicely for this.  Continue›          with this process until all 3 additional banks are wired.››               3)  Just to the right of Freddie is an area to solder›          in a 14 pin IC chip.  We will use this area to supply +5›          and ground to our 74138.  Take a 74LS138 and bend out all›          of the pins except pins 8 and 16.  Solder pins 8 and 16›          into the holes of the unused chip area next to Freddie.››               4)  Next to the RAM chips is a chip with the part # of›          CO25953.  Behind the chip are two 33 ohm resistors.›          (orange, orange, black, gold)  Unsolder the right-most lead›          of the rear resistor.  (R111)  Solder a wire from the free›          end of the resistor to PIN 15 of the 74138.  Solder another›          wire from the land where the resistor used to go to pin 4›          of the 74138.››               5)  Solder two wires from pins 11 and 16 of the 6520›          PIA chip (Part # CO14795-12) to pins 1 and 2 of the 74138.›          Also ground pins 3 and 5 of the 74138.››               6)  Solder the CAS line from each new bank of 64K to›          pins 14, 13, and 12 of the 74138.››               7)  Unsolder the Basic ROM chip from the board.  This›          is the 24 pin chip that is located closest to the front of›          the machine just to the left of the 555 timer IC.››               Thats it.›››          Conclusion and Tech notes:››               To test each bank boot with DOS 2.5 with your basic›          cartridge, POKE 5439,49, set the appropriate PIA port bit,›          go to DOS and reformat D8.  Continue through all the banks›          and check your directory.  If you write a file to one bank,›          switch banks, and write another file, you will not lose›          what you wrote to the first.  The real advantage is that›          you can have your ramdisk and Basic XE too.  Around the›          bbs's here in Chicago there is a file floating around›          called RAMDISK2.  The source code is available on›          compuserve.  By re-writing the routine you can have your›          ramdisk invisible to Basic XE and/or double density.››               I mentioned previously about using the Self-test bit›          in the PIA port.  To use this bit you have to disconnct the›          line on the PIA that runs to the PAL MMU and connect it to›          a 556 timer that will enable the self-test input to the PAL›          for about 3-5 seconds.  Half of the timer is used for›          timing and the other half is used as an inverter.  The›          reason for using a timer is that on boot-up the OS uses››››››››››››››          some of the routines in the self test to check for valid›          RAM and to determine RAM size.  I have done this and then›          pulled the circuit out.  It was just too "messy" to suit›          me.››               One potential problem is the 555 timer used in the›          130.  This chip is used for system reset timing.  If you›          press your system reset key down and hold it there the›          system should not reset.  If it does you will notice that›          if you release it the system will reset again.  The reason›          for this is because the 555 timer is putting out a spike›          that the system sees as a valid reset.  To solve the›          problem replace the chip.  It seems about 1 in 10 are bad.››               That's it!  Hope you enjoy the mod.››          Rich Andrews›          Box 229-1 RR#7›          Lockport, Il 60441››ČÉÔ ÁÎŮ ËĹŮJJJJJJJJJJJJJJJJJJJJ