M200 RAM
by Ken Nickerson
"build them yourself"
A while back when I got my Tandy 200 (which is still a great little machine that nobody seems to have bothered to improve
on), I noted that it had two empty RAM sockets for memory expansion. Thus a T200 has one to three (3) memory "banks" (some
hacking will make it access more than three, but that's another story...) Tandy told me that they would kindly sell me RAM
modules for these sockets at only about $270 Canadian each (this in 1991!!).
This inspired me to find a way to make a RAM module for the price of a 32K static RAM and some small parts - about $20
each! (I've since found that others have preceded me in this discovery and have posted in Compuserve.)
Description of the Memory Banks
Each T200 memory bank is a 32K by 8 (256K bit) static RAM, of which about 20K is available for user files. The Tandy
modules were made up of four 8K by 8 chips on a carrier, presumably because single chips were not available or too
expensive at the time of the T200's introduction. Fortuitously, the RAM expansion sockets are almost pin for pin
compatible with present 32Kx8 (256K bit) static RAM chips. The RAM used is low power (naturally) with a "standby" or
"sleep" mode which allows the chip's contents to be preserved when not in use by a trickle of current (about 50 microamps
per chip).
Parts Required
A home-brew module requires the following: - a 32K by 8 static RAM chip in a 28 pin DIP package, with speed 150 ns or
faster and of low power type. Any of the major manufacturers make such a chip. I used PG2256-10L by OKI. Pinouts for these
chips are quite standard: Address pins are 1 thru 10, 21 and 23 thru 26. Data pins are 11, 12, 13, 15 16, 17, 18, 19 (note
that order doesn't matter for address or data pins in this application.) Chip Select, Output Enable and Write Enable are
respectively 20, 22 and 27.
- two 47K resistors, 1/8 or 1/4 watt.
- an NPN switching transistor, such as a 2N2222. I used a 2N5772 in a small plastic, but NOT METAL, case (e.g. TO-92)
Preparing the Circuit Board
The RAM expansion sockets are accessed by popping off the module bay cover, located on the case bottom beside the battery
compartment, with a dime or screwdriver. I removed the RAM expansion sockets to simplify my assembly. This will require
opening the T200 unit by removing the four corner screws on the case's bottom plate and gently prying it apart from the
top plate. The display and keyboard are attached to the circuit board by some ribbon connectors. The display ribbon can be
simply pulled out, while the keyboard can simply be flipped back. Remove the two RAM expansion sockets.
I did this by ripping the plastic part off, leaving the pins to be heated and removed one by one. Clear the socket holes
(with solder 'sucker' or wick).
You may be able to assemble the module by soldering to the socket pins (saving you the trouble of opening the case), but
if you try this, the module may sit too high to allow putting the module bay cover on again.
Assembly
Note: Treat the RAM carefully when installing, as it is static sensitive.
If only installing one module, install BANK #2 (RAM module #1). It makes little sense, however, to do the modification and
not do both banks!
On the chip, snip off pins 1, 20, 22 and 27 at the "shoulder", so that there is a stub left to solder wires to. Stretch
the rows of (remaining) pins out from the chip body a little so that they line up with the expansion module solder holes.
(If you think you can get by without removing the original sockets, you will have to devise the best way of doing this).
Insert a 1" length of thin insulated wire (wire wrap or up to 24 gauge hookup) into socket solder holes 1 and 27 (from now
on, called "holes") and solder in. Leave other ends pulled away from the socket holes. Insert one end of a 47K resistor in
hole 22 as close to the resistor body as possible, while having the resistor body parallel to the circuit board and its
unused end pointing towards hole 15. Solder hole 22.
Insert the emitter of the transistor into hole 20, while positioning the base lead to attach to the unused end of the 47K
resistor. The resistor and transistor body should be arranged to minimize lead lengths and to keep both components between
the module sockets (so that you can plug the chips in!). The collector lead should also be accessible. Solder the emitter
lead and solder (trimmed) base lead to the unused end of the 47K resistor (close in to the resistor body).
Insert the chip into the socket holes and solder in.
Bend the transistor collector lead to the stub of pin 20, trim, and solder together. Connect the stub of pin 22 to the
shoulder of pin 14 with a short piece of hookup wire.
Connect the wire from hole 1 to the stub of pin 23. Connect hole 23 and pin 1 similarly. Attach one end of the second 47K
resistor to the shoulder of pin 28 and the other to the stub of pin 20 and transistor collector.
The assembly is finished at this point, but some pieces of wire insulation and component rearrangement may be necessary to
avoid shorts and crowding the adjacent module socket.
Note: A dry run may help to see how the parts fit together for a more intelligent layout. One of the banks may demand
removal of the T200 bottom case section to allow room to work.
Reassemble the T200. The display ribbon will require some deft long fingers to reconnect (try half opening the display to
maximally extend the ribbon out of the case top.) You should now be able to switch between memory banks by pressing "F1"
or "TAB".
Enjoy,
Ken Nickerson
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also by Ken Nickerson: M200 to PC FileTransfer