For months I had been coveting the 32K RAM expansion banks available for the MEWS, but as the price of the basic machine plummetted it became clear that my hopes for a commensurate fall in the price of such add-ons were in vain. So armed with a handful chips, a soldering iron, and complete lack of common sense I pried the back off my little friend and began to operate. The first step was to decide where and how to place the components. I finally decided that the open area on the back of the CPU board, on the opposite side of the ROM and expansion sockets was best. It gave the easiest access to the signals I needed, and it was open enough for what I had in mind. After thoroughly studying the schematics for the 100 and looking through the technical references for the RAM chips I intended to use, I finally settled on a simple design which I would be entirely enclosed within the machine. Leaving the ROM and expansion slots. I also decided, for the sake of simplicity, to use a simple slide type switch to change banks. The work went quickly and I was ready for my first try after about three hours. To my horror the machine was totally dead, but after my initial panick I realized the memory power switch was off, flipping it on I hit the power and was, to my great relef, greeted by the normal screen. Then came the acid test, I shut off the power and toggled the bank selector. It worked perfectly, and has continued to do so ever since. If you are good with a soldering iron, and don't want to spend more than the cost of another machine for addidional RAM in your MEWS, read on. I am sure you will find this project both enlightning and rewarding!! NOTE!! I can not take resposibility for any damage that may result from this modification, and I do not warrant that this document is 100% correct. It is correct as far as I know, but errors may still exist. Opening the case ----------------- Place your MEWS face down on a clean, static free work surface. Be sure that you are wearing static free clothing (no sweaters etc.). A humidifier will also help reduce the potential for static damage. After turning off the memory power, and taking out the batteries remove the four screws that hold the bottom of the M100 to the keyboard-display section. Pressing inwards on the sides of the black bottom portion of the case lift it off of the white upper portion. Flip it over onto the work surface, being careful not to strain the cables that connect the two halves. Using a small screwdriver to help pry, remove the grey ribbon cable and the power wire that go to the display. They are simply pressed on to sockets on the CPU board. Similiarly remove the black and the white keyboard connectors and set the upper portion of the case aside. Preparing the memory chips -------------------------- Take two of your 8k*8 low power memory chips (toshiba 2364 , 2365 , Hitachi 6264 , or equivelant) and bend pin 20 of one them away from the case at a 45% angle. Place the chip with the bent pin on top of the other, making sure the pins are alligned, and that both pin ones are on top of each other. Using a low wattage soldering iron gingerly solder all of the pins on the top chip to their counterparts on the bottom. Using your wire wraping tool attatch a four inch piece of wire wrap wire to pin 20 of the top chip, and another to the bottom. Repeat these steps with the other two chips. You now have two 16k modules ready for positioning in the machine. Preparing the CPU board ----------------------- In order to fit the memory into the system you must remove the capacitor C9 and relocate it to the other side of the board. Using a piece of stiff wire, or the tip of a pen, as a form make a hook at one end of a piece of 3 inch, pre stripped, wire wrap wire and cut off the excess. Tin the hook with your soldering iron and tack solder it to pin 1 on the back of the rom socket ( conveniently silk screened with pin numbers) with the wire pointing downward. Repeat this process for pins 2-20 and 24-26. Installing the RAM chips ------------------------- Using a dollop of silicone RTV glue, attatch the two memory modules to the CPU card with the pins upward and the pin ones towards the top of the board. Be sure that the bent out pin 20 does not touch the adjacent module or any traces on the CPU board. After the glue has dried thoroughly take 27 pieces of one inch pre stripped wire wrap wire and, using your wire wrap tool, attatch all of the pins on module 1 with their counterparts on module 2 with the exception of pins 20. Finally attatch pin 14 to pin 22 on either module. Using the connection chart (figure 5) attatch the wires from the back of the ROM socket to the proper pins on the RAM chips. Remember, with the chips in this confuguration pin 1 is on the upper right. Routing is your major concern. Since the two modules are interconnected 1 for 1 run your wires to the pin number indicated on whichever module allows the neatest path. Installing the address decoder ------------------------------ Now you are almost done. Just the address decoder must be installed. Using your silicone glue attatch a 40139 or 74hc139 cmos dual 1 of 4 decoder to the cpu board next to IC M5. Using connection chart (figure 6) wire in the decoder. Attatch pin 28 of the memory module to the output (end with the band) of diode D22. Adding the bank select switch ----------------------------- Using a non-shorting SPDT slide switch, attatch three pieces of wire wrap wire long enough to reach from the decoder IC to a convenient place on the case. I chose a location just next to the modem control switches but you may choose your own location. Attatch the common lead of the switch to pin 13 or 14 of the decoder (these two pins should be tied together and to pin 6 of M4). Then attatch one of the leads to Vdd (I used pin 16 of the decoder) and the other to gnd (I used decoder pin 8). When the switch is selected to ground you will be running out of your new bank, when it is set to Vdd you will be using your original ram. The moment of truth ------------------- Making sure that the leads of your bank select switch aren't touching anything, temporarily re-attatch the keyboard and the display. Don't try to put the halves of the system back together yet though. Just re-install the batteries and turn the memory power back on. With the bank select switch set to Vdd turn on the system power. Now peek at the screen. You should see the usual screen. If you see nothing turn off the power immediatly and check your work. now turn the system off and switch banks. Turn the system back on and look again. If you see the normal screen you are all set. If not check your work again. Closing the case ---------------- In order to close the case you will have to fold the pins on the memory modules over onto the bottom of the chips. Then cut a small notch on the side of the black portion of the case and attatch the memory select switch. Close the case and you are ready to go. I found this project most enjoyable, but be advised this is definitley not a project to be attempted by the novice. You should have experience in handling CMOS circuits, soldering, and wire wrapping. A more than passing understanding of the circuits involved is also helpful. One final warning, it goes without saying that you will void your warranty by doing this. But if none of this scares you, you should find this an enjoyable, and very worthwhile project. (Figure 5) Memory module(s) Attatch to ---------------- ------------------ Pin 1 no connection Pin 2 - Pin 19 Rom socket pin 2 - pin 19 Pin 20 Four 4" wires to be used for chip selects. Pin 21 Rom socket pin 20 Pin 22 Ground to module pin 14 Pin 23 Rom socket pin 24 Pin 24 Rom socket pin 25 Pin 25 Rom socket pin 26 Pin 26 IC M5 pin 13 Pin 27 IC M9 pin 25 Pin 28 Diode D22 positive. (Figure 6) 40193 decoder Attatch to ---------------- --------------------- Pin 1 Decoder pin 12 Pin 2 Rom socket pin 22 Pin 3 Rom socket pin 28 Pin 4 - Pin 7 Attatch to the 4 wires leading to pins 20 on the RAM chips. Order does not matter. Pin 8 Module pin 14 (ground) and one side of the bank select switch. Pin 9 - Pin 11 No connection Pin 13 Decoder pin 14 and common of bank select switch. Pin 14 IC M4 pin 6 Pin 15 IC M5 pin 15 Pin 16 Rom socket 1 and other side of the bank select slide switch. Parts Required ---------------------------------- 4 Low power 8k*8 CMOS RAMs xx64 1 40139 or 74hc139 CMOS decoder 1 SPDT slide switch 1 Pkg 1" wire wrap wire 1 Pkg 2" wire wrap wire 1 Pkg 3" wire wrap wire 1 Tube silicone glue Tools Required ---------------------------------- Low wattage soldering iron Wire wrapping tool Rosin core solder Screwdrivers Small wire clippers For more complete instructions with diagrams etc. send a business sized, self addressed and stamped envelope to. Randy P.O.Box 27855 St. Louis, MO. 63146