NOTE: The following article also applies to expanding C1700 128K REU's to
512K with the following changes:
- You will need to remove the 64x1 DRAMS, and replace them with
16 256x1 DRAMS.
- You will need to cut the "J1", aka "CUT 512K" jumper.
EXPANDING YOUR 1764 RAM EXPANSION UNIT
FROM 256 TO 512 KILOBYTES OF MEMORY
by Scott A. Boydman (ScottB30 on QuantumLink)
This article details the steps required to increase the 1764 RAM
Expansion Unit (REU) to a total of 512K. Although the article has been
tailored primarily for the GEOS 64 user, it is also applicable to all 1764
REU users who wish a 512K expansion. I have had a great deal of correspondence
with people, both by mail and over Quantum-Link, and I would like to share
some of the knowledge gained from these rewarding interactions. You should
note that since the Japanese "chip embargo" the prices have been inflated
and availablity is scarce. One of the best sources of chips may be found in
the various advertisements in the "COMPUTER SHOPPER" magazine.
The 1764 REU consists of a plug-in cartridge, a deluxe power supply, and a
disk containing software for using the REU with both BASIC (side 1) and GEOS
(side 2.) The deluxe power supply is required with the C-64, since the
original was not designed to meet the power requirements of the many chips
present in the REU. The deluxe power supply is rated for 2.5 amps DC, which
is sufficient to meet the power requirements of the computer, a modem,
and an REU with up to 512K RAM. The C-128 power supply already has enough
amps to handle any REU. The REU itself is nonfunctional without appropriate
software. The front side of the disk contains Commodore's RAMDOS software
for use with BASIC. The back side of the disk contains a GEOS upgrade which
installs the GEOS Kernal V1.3 on your system boot disk, as well as other
upgraded applications and input/printer drivers.
The latest release of GEOS (V2.0) is a two-disk system and already supports
the REU. If you have Kernal V1.2 or lower, you will have to upgrade by one
of two methods. The first is to run the GEOS Upgrade program on the back of
the disk supplied with the REU. This will upgrade your GEOS System disk to
Version 1.3. However, this technique does not always work properly, so many
people have upgraded directly through Berkeley Softworks. The newest version
of GEOS (V2.0) supports the REU as well as Commodore's new 1581 3.5 inch disk
drive. Note that the GEOS and BASIC RAMDISKS are not compatible, therefore,
GEOS RAMDISK files are not accessible from BASIC. The original REU test
program on the BASIC side of the Test/Demo disk was designed to test the
256K of ramdom access memory that is supplied in the original 1764, but
Commodore has since released newer software (available for downloading
over Quantum-Link, or directly from Commodore) that also supports the
C-128, and any size REU from 128K to 512K.
The 1764 REU itself consists of a printed circuit (PC) board inside a
plastic and metal housing, which plugs into the expansion port of the C-64 or
64-C computers. The PC board is the same as that used in the 1700 (128K)
and 1750 (512K) REU's, intended for the C-128, but with the following changes:
Commodore's BASIC RAMDOS software (included with the 1764) is not
compatible with many commercial software packages. GEOS, however,
allows the expansion to operate as a fully functional disk drive in RAM.
The current versions of both GEOS 64 and GEOS 128 have provisions for both
1541 and 1571 RAMDISKS. Another available feature is the shadow mode.
GEOS is able to store data to the REU when reading files from the physical
1541 disk drive, so that further references of that file occur directly
from RAM. This results in a significant speed-up by eliminating
multiple disk reads of the same file. All writes go to both the physical
drive and the REU, which eliminates the RAMDISK's primary disadvantage of being
a nonpermanent storage device. In the shadow mode, your data is always
current and up-to-date on your real disk. Of course, when using the
RAMDISK, files must be copied to a physical disk, or be lost when the
computer is turned off.
- The 1764 has only one bank of eight 256K Dynamic RAM chips installed.
Each chip holds 256K bits of information; therefore eight chips are
required to obtain 256K bytes (since eight bits make up one byte).
- There is an empty row of eight chip positions labeled BANK II on the
PC board. The traces for additional memory chips are present,
and the holes for the pins on the chips are presoldered.
- There is a 390 Ohm resistor in the 1700 and 1750 REU's that is not
present in the 1764. The resistor's position is located in the
upper-left corner of the board, and labelled R4 in the 1764.
According to Fred Bowen of Commodore, this resistor "tweaks" the
signal that is supplied to the C-128 computer. There are C-64 users
that have installed unmodified 1750 REU's without reporting problems.
In these installations, larger power supplies rated at 2.5 amps DC are
used. Also, there are C-128 owners who have used 1764 REU's without
difficulty. Therefore, the presence or absence of this resistor seems
to be a non-critical factor for most C-64 or C-128 computers, at least
when using the REU with GEOS.
- A jumper, labelled with the notations J1 and CUT 512K is located on
the bottom side of the circuit board. The jumper is cut in the 1750
and 1764 REU's, but left intact in the 1700 REU. Apparently, the cut
jumper tells the Ram Expansion Controller (REC) that 256K RAM chips are
being used, rather than the 64K RAM chips used in the 1700.
The 256K REU does not contain the amount of memory needed to shadow two
physical 1541 drives, or shadow one drive while supporting the RAM 1541, or
support a RAM 1571. For this reason many people have desired their 1764's
to have 512K RAM. It is not very difficult for someone with basic
mechanical and soldering skills to install additional chips on the 1764 PC
board. I recommend soldering 16-pin DIP (Dual Inline Package) sockets to
the board, which prevents possible heat damage from direct soldering of the
memory chips. The chips are simply pushed carefully into the sockets.
Radio Shack stocks the 16-pin DIP sockets, and the chips may be
available from mail order houses.
NOTE: The steps required to install the chips should be undertaken by
someone with some skill in handling and soldering PC boards. Also, some
mechanical skill is needed to remove and install the PC board in its
housing. Opening the REU housing will void Commodore's warranty and is done
entirely at the owner's risk.
When you arrive at this point, CONGRATULATIONS! GEOS makes use of the
512K REU in the following ways; the first 64K holds a copy of the entire
C-64 RAM, which allows for fast memory access and rebooting GEOS from the REU.
If you select a RAM 1541, 165K plus directory space are reserved. This
amount is also allocated if you are shadowing a real 1541 drive. GEOS V2.0
also allows for a 1571 RAMDISK. The BASIC RAMDISK software included with
the REU also uses the expanded amount of REU memory, but the original version
of Commodore's RAMTEST program was only designed for 256K. If you have this
version, it can be modified to check the entire 512K in the expanded 1764 as
- The housing of the 1764 must be opened carefully. There are
four pins, one in each corner, holding the plastic case together
with friction. Starting at the expansion port opening, gently
but firmly pull the top and bottom halves apart. The plastic
case will begin to separate. Continue around the perimeter of
the case until the top can be removed from the bottom.
- Remove the metal shield from the plastic casing by pulling it
out at each corner. The PC board is inside the metal shield.
Open the shield, also starting at the connector.
You may need a small screwdriver to pry the metal slightly where
it is held together. When the shield opens, spread it only as
much as needed to remove the PC board. Then slide the cardboard
liner off from the expansion connector. This metal shield serves
as a barrier to radio frequency (RF) interference as required by
the FCC. When testing your expansion, the PC board can be safely
plugged into the computer prior to final reassembly of the REU.
- Examine the PC board. BANK I is the row of eight chips along the
top of the board. The RAM Expansion Controller (REC) is the square
chip in the lower portion of the board. The empty area labelled
BANK II can be seen just below BANK I.
- Solder eight DIP sockets into the locations in BANK II. This is
the most difficult step, because it is hard to position all 16 pins
at once. I recommend that a small, round toothpick be used to open
the 16 holes on the PC board for each socket position. Using a
soldering iron, heat the hole position until the toothpick can be
inserted. By rotating the toothpick during cooling, a hole will
be left upon its removal. An alternative method would be to use a
desoldering tool or wick (available from Radio Shack) to remove the
solder from the holes. Note however that removal of ALL the solder
can make it difficult to achieve good contact when soldering the
DIP sockets to the board. After doing this for all 16 holes, the
DIP socket can be inserted into the holes, and then each pin soldered
individually. Use care that each solder point makes a secure
connection on both sides of the PC board. When opening the holes,
be certain the metal traces printed on the circuit board are not
disrupted, otherwise your REU may not function at all when tested.
- Insert the chips carefully into the sockets, checking to maintain
the correct chip orientation. The notch or dot on the chip points
toward the top of the PC board. Be certain that all pins are inserted
into the socket, and that none bend or break.
- Before putting the REU together again, plug the PC board into
the computer (make sure the power is off), turn the computer on,
and boot GEOS V1.3 or higher. If necessary, run the UPGRADE conversion
program on the disk that came with the 1764 REU, or 1351 MOUSE to
convert your V1.2 to V1.3, but be sure to keep a V1.2 backup
copy should anything go wrong. If you have a two-disk GEOS system,
you already have a compatible version. From the deskTop, run the
CONFIGURE program. The RAM EXPANSION box that appears should indicate
512K if your work was successful. If this is the case, reassemble
the REU. If 256K or NONE is displayed, you should check the
board for improperly soldered or disrupted traces, a chip that is
not securely plugged in, or rarely a defective chip. Recheck all the
connections systematically. You may have to desolder and remove one
or more sockets to locate a bad connection point or points. Then
rerun CONFIGURE to see if the problem has been corrected. If you
are not a GEOS user, use Commodore's RAMTEST program to check out
your expanded REU (see below.)
You should be aware of some bugs in the original Commodore RAMDISK
- Copy the files 1764 RAMTEST.BAS and 1764 RAMTEST.BIN from the
demonstration disk onto another formatted disk. If the files have
different names than those noted above, then you may not have to alter the test
program (your disk may be an updated version of Commodore's RAMDOS). You
may use the FILE COPY utility included on side 1 of the disk to do this, or
even copy the files from the GEOS deskTop.
- From BASIC, type the following:
LOAD "1764 RAMTEST.BAS",8 (RETURN)
501 REM LINES 502 & 503 POKE THE CODES TO TEST 512K! (RETURN)
502 POKE 33093,7: POKE 33099,33 (RETURN)
503 POKE 36003,53: POKE 36004,49: POKE 36005,50 (RETURN)
- List lines 501-503 to confirm the changes have been entered. Then, list
line 100 and space over the beginning characters 100 REM (to erase them) and
press . This will resave the altered version of 1764 RAMTEST.BAS to
the disk, and delete the original version. Reload 1764 RAMTEST.BAS and
relist lines 500-503, rechecking for errors. Then, run 1764 RAMTEST.BAS,
and the program should test all 512K in the expansion. If this does not
happen, recheck the poke codes in lines 502 and 503 for errors.
The operation of the 1764 with GEOS seems flawless, although I have heard
of occasional incompatibilities with user-programmed software. This,
however, is a very rare case. It would be nice to be able to exit GEOS and
directly utilize the GEOS RAM 1541 files in the BASIC environment; but due
to the differences between the GEOS and Commodore RAMDOS formats, this isn't
- The DOS NEW command does not seem to work (a syntax error is returned).
However, the SCRATCH command using the wildcard symbol (S0:"*") will do the
same job. This bug has been corrected in subsequent revisions of the
Commodore RAMDOS. In fact, the RAMDOS has now been upgraded for use in C-128
mode. The latest revisions of RAMDOS are version 4.2 for C-64 mode, and
version 4.3 for C-128 mode.
- With the 256K REU, the BLOCKS FREE message in the original RAMDOS
directory reports an incorrect number (2014) of blocks, as if the expansion
contained 512K. This number remains about the same after the 512K is
installed. This bug has been corrected in the newest releases of
- In version 4.2 and 4.3 of RAMDOS (downloaded from Quantum-Link), the
unit (or device) number of the RAMDISK cannot be changed with the CHANGE UNIT
program. The only way to change unit numbers (i.e. from 9 to 8) is to
reinstall RAMDOS specifying the new unit number, but without reinitializing
the REU memory (in other words, files in the REU memory are retained).
In the area of troubleshooting, the majority of people who have upgraded
their 1764's have done so without problems. Of the people who have
experienced difficulty, the usual defect has been poor connections at the
soldering points, or disruption of the PC board traces occurring at the time
the presoldered holes were opened. In most cases, a careful inspection and
reheating of the solder points will reestablish the proper connection(s).
Use of a continuity tester (available from Radio Shack) may be helpful in
isolating any bad connections. In the case of an extremely hard-to-identify
problem, it may be best to desolder and remove the sockets and reinspect the PC
board, checking for disruptions on both sides of the board. A careful,
meticulous job of soldering is the best way to avoid trouble.
Regarding the compatability of the 1764 with the C-128, in most cases, no
alteration seems required. However, the addition of a 390 Ohm resistor to
the empty R4 position of the 1764 circuit board makes it virtually
identical to the 1750 REU. Conversely, removal of this resistor makes the 1750
equivalent to the 1764.
I hope this information will be helpful to all 1764 owners who desire
to have 512K RAM rather than the 256K supplied by the factory. If you need
assistance, feel free to contact:
Scott A. Boydman,
25447 Bryden Road,
Beachwood, Ohio 44122
or send E-Mail to ScottB30 on Quantum-Link.