Home-Brew Tandy 200 RAM Expansion Modules (DIY)
Kent Nickerson
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.
Fred McKenzie
nick...@nimios.eng.mcmaster.ca (Kent Nickerson) wrote:
> 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!!).
Kent-
I purchased the two memory modules from Jameco Electronics, several years
ago. They turned out to be made by Purple Computing, a company that made
numerous upgrades for the Tandy 100/200 lines. Their price was probably
much less than Tandy's, but I don't remember what it was. The cost of your
mod almost certainly is less! I believe Purple Computing is still in
business. I'm not sure if Jameco still carries the modules.
Regardless which way you go, 24K per bank can be restrictive. Programs
written for the 32K Tandy 100, may not fit in a Tandy 200 memory bank.
Even though there may be a total of 72K, you can only access 24K at a time.
In your research, did you come across any hint, as to how one might
increase the bank capacity to 32K? I would guess that the limit was
related to the Model 200 having a larger ROM than in the Model 100, with a
maximum address space of 64K.
Fred McKenzie