The Flash/EPROM Adaptor

This adaptor allows you to install Flash or EPROM chips with custom firmware into the ROM sockets of many Commodore computers. With the help of some configuration jumpers it can be plugged into the sockets intended for 2364, 23128 or 23256 ROMs. Many different EPROM and Flash types can be used, from 2764 up to 29C010. If the replacement chip contains more memory than the original chip, a switch can be connected to switch between several different firmware versions.

Building the hardware

The PCBs are double sided and require some skills to be built. The layout files were created with Cadsoft's Freeware version of Eagle 4.09 and can be downloaded here. The file contains layouts and schematics for other projects, too. You can also buy the PCBs from me, just visit the order page.

The following components are needed:

IC1 Flash-ROM or EPROM 27C64-27C512, 29C256-29C010 and pin compatible devices (so 27xxx, 28Cxxx and 29Fxxx should work fine)
2364 PCB

CN1 precision socket adaptor 24 pin (for 2364) or
28 pin (for 23128/23256)

R1,2,4,5 SMD resistors 4.7kOhm, 0805 package (optional)

S1,2,4,5
switch
(optional)

IC socket
32pin or 28pin IC socket
(depends on EPROM type used)

For details on assembling the device, please refer to the construction page. Before you start soldering parts, you have to configure the board's jumpers!

Options

The adaptor can be used in many different ways. I will show the most common configuration options in a table, with a picture showing the changes that have to be done.

Use of 28 pin EPROMs or Flash-ROMs in a machine with very limited space: In this case you can cut off the extra board space with a small hacksaw, but be careful not to damage the part of the board you still need. The edge to cut is shown with the yellow line. This picture also shows the place where a 24 pin socket adaptor has to be installed, if you want to replace 2364 ROMs. Installation in a 24pin 2364 socket does not need any special changes to the board, except the adjustments needed for the EPROM bank selection (see below), because the board comes preconfigured for 2364 mode.
If you want to install the adaptor in a 23128 socket (usually found in the C64E board and the C128 and (plastic case) C128D machines), perform the following changes: cut the traces that connect the pads of SJ1 to SJ4, then close the other pads of SJ1 to SJ3 with a blob of solder and a short piece of wire, if needed. Also cut the two traces on the bottom side of the board, as shown in the picture. These traces are located between J4 and J5, and between pins 27 and 28 of the DIP 28 adaptor socket. Do not install a resistor to R5! If you want to use an EPROM, and/or you want to make use of the extra memory banks, you have to cut the traces between J1, J2, J3 and J4 as well, and install the three resistors that correspond to J1, J2 and J4. Refer to the picture below for details.
If you want to install the adaptor in a 23256 socket (usually found in the (metal case) C128DCR machine only), perform the following changes: cut the traces that connect the pads of SJ1 to SJ4, then close the other pads of SJ1 to SJ3 with a blob of solder and a short piece of wire, if needed. Also cut the two traces on the bottom side of the board, as shown in the picture. These traces are located between J3 and J4, and J4 and J5. Do not install resistors to R4 and R5! If you want to use an EPROM, and/or you want to make use of the extra memory banks, you have to cut the traces between J1, J2 and J3 as well, and install the two resistors that correspond to J1 and J2. Refer to the pictures below for details.
Flash/EPROM size and banking adjustments: Since a 29C010 can hold 16 times the contents of a 2364 ROM, many different ROM images can be stored in it. To select one of these images, the extra address lines have to be set to the desired levels. In the standard configuration of the adaptor all lines are grounded with the tiny traces between the solder joints J1 to J5, so bank 0 of the Flash chip is selected. If you need the other banks, these traces have to be cut open (yellow lines), pull-up resistors must be installed on the bottom side of the board in the space shown by the yellow rectangles. See the construction page and the next chapter for details.

Bank selection switches

Depending on the type of FlashROM or EPROM you use, and also depending on the type of socket you want to install the board into (2364, 23128 or 23256) you have to cut some jumpers, as it is described in the table above.

The following table will only discuss the availablility of address lines, their use for banking is optional! You can also use only some of them, then the others will be connected to GND with the copper trace on the PCB. Be careful when you do this, because the GND connection of J1 is routed through J2, and the one of J5 through J4. It's easiest if you simply install all the resistors and cut all the GND lines if you want to make use of banking (except the resistors that are marked as "do not install" in the configuration table, of course).

But there is an exception: If you use some older (and small) EPROMs, the higher address lines are used for other signals and must be brought to high level with the pull up resistors.

address lines available on Flashs/EPROMs

address line	chip size	chip type
A16	1Mbit	29C010
A15	512kbit	27C512/29C512
A14	256kbit	27C256/29C256
A13	128kbit	27C128

2364 mode banking

banks	address line	jumper	resistor	cut trace	notes
16	A16	J2	R2	J2-J3	requires A15, too!
8	A15	J1	R1	J1-J2	required on 2764-27256
4	A14	J4	R4	J3-J4	requires A13, too! required on 2764-27128
2	A13	J5	R5	J4-J5

23128 mode banking

banks	address line	jumper	resistor	cut trace	notes
8	A16	J2	R2	J2-J3	requires A15, too!
4	A15	J1	R1	J1-J2	required on 27128-27256
2	A14	J4	R4	J3-J4	requires A13, too! required on 27128

23256 mode banking

banks	address line	jumper	resistor	cut trace	notes
4	A16	J2	R2	J2-J3	requires A15, too!
2	A15	J1	R1	J1-J2	required on 27256

J3 is the common ground connection that must go to the common contact of all switches or to the center contact of a rotary coding switch. Here is an example picture of how to wire four switches to select from 16 banks:

Special note for the C128

Normally the C128 and plastic C128D use four 23128 ROMs, unlike the metal C128DCR that uses only two 23256 ROMs. If you want to replace the C64 and C128 mode KERNALs, you need only one Flash adaptor board in the DCR as opposed to two on the older machines. With a small modification the older machines can also use the DCR style ROMs, so you need only one adaptor board to replace all KERNALs, but you have to install a new C128 BASIC chip, e.g. a 27C256 EPROM or another Flash adaptor board. I wrote an article describing the procedure long ago, it is archived in the Commodore Knowledge Base.

Contact

If you have questions about this project, you can mail me: please visit my contact page.

IC1	Flash-ROM or EPROM	27C64-27C512, 29C256-29C010 and pin compatible devices (so 27xxx, 28Cxxx and 29Fxxx should work fine)
CN1	precision socket adaptor	24 pin (for 2364) or 28 pin (for 23128/23256)
R1,2,4,5	SMD resistors	4.7kOhm, 0805 package (optional)
S1,2,4,5	switch	(optional)
	IC socket	32pin or 28pin IC socket (depends on EPROM type used)