How to create Jiffy DOS Adapter sockets for the Commodore 64 and 1541 disk drive.

If your lucky enough to own an EPROM burner you probably bought it to create ROMs for your projects and vintage computers. My first project was to install JiffyDOS into my Commodore 64 and 1541 Disk drive. The Kernel ROM in the 64 and the DOS ROM in the 1541 are, in most cases, 8K 2364 ROM chips. In order to switch between using JiffyDOS and the stock ROMs it is necessary to use a 27C128 EPROM because it holds 16K of data, twice that of the original ROM.

This article assumes you have an EPROM burner and eraser, ROM image files of JiffyDOS, and soldering skills. If your resourceful, the actual cost of building the project is very low once you have a burning setup.

To start with you need the JiffyDOS ROM images for the 64 and 1541, they should both be exactly 8k. The capacity of the EPROMs we intend to install is 16K, this leaves room for the stock ROM to reside on the EPROMs as well. Like the original JiffyDOS, you can select between JiffyDOS and the stock ROM by installing switches. If you do not already have the stock images, download the 1541 1541-e000.901229-05.bin image and the C64 kernal.901227-03.bin image from Zimmers.net.

Get all four images together in the same directory and create images to burn into the 27C128 EPROMs. Create new images where the  JiffyDOS images are appended to the end of the stock images. If you are running windows a HEX editor should work, something like notepad will not! Notice the new images are 16K instead of 8K.

If you are running Linux you can use cat:

cat 1541-e000.901229-05.bin C1541.ROM > J1541.bin
cat kernal.901227-03.bin Kernal.rom >JC64.bin

One you have completed the new ROM images, burn them and make adapters, 1 per 64 and 1541 in your setup.

Most of the pins of the 27C128 and the 2364 in your 64 and 1541 match, consider the following diagram of the 27C128 ROM installed over a 2364 socket. (credit: Ray Carlson)

 By bending out the proper pins on a 28 pin socket and installing it on top of 24 pin socket, we can realize the wiring shown.

To build one adapter you need:

One 27C128 EPROM
One 24 Pin DIP IC Socket
One 28 Pin DIP IC Socket
One 4.7K Resistor
One SPST or SPDT Switch
A length of Solid Core CAT5 Wire

Solid core CAT5 cable for wiring is easy to work with and is already twisted together for the switch wires. You do not need long leads on resistors and they can be about 4.7K. I cannibalized the EPROMs and resistors from old circuit boards.

The selection of sockets will affect the height and strength of the adapter. High precision sockets will be easier to work with but cost more, cheaper ones work well but are fragile. For a shorter adapter you can omit the 28pin socket and solder directly to the EPROM.

Technique improves with each adapter built, depending on your skill level and sockets used, the wiring can be very tidy. Tin all solder joints before you solder to them.

Start by bending pins 20, 23, & 26 outward on the 28 pin socket. Pay attention, the 24 and 28 pin sockets are numbered differently.

Wire up the 24 pin socket first, flip the socket over and solder wires to pins 12, 18, & 21 on the base of where the pins come out of the socket. Small solder joints are a must here and to much heat will weaken the structure of the socket. I actually attach the ground wire for the switch to pin 24 of the 28 pin socket after every thing else is done, as opposed to the diagram.

Figure out the best way to route the wires to the 28 pin socket. Some sokets allow you to snake wires through the center. Once figure the routing of the wires connect the sockets together and solder the wires to the 28 pin socket. Leave the ground for last, it will be attached at the same time as the switch.

Congratulations, the hard part is over.

Now connect the power wire from pins 1, 27, & 28 on the 28 pin socket to pin 24 on the 24 pin socket. I usually use a solid length of wire for power and then solder the resistor between the power wire and pin 26 on the 28 pin socket.

Now we install the switch. Use a 6-8" length of one of the twisted pairs from the CAT5 cable. Solder the striped wire to the center of the switch and the colored wire to the outside contact. The striped wire will be attached to pin 14 of the 28 pin socket at the same time as the wire coming from pin 12 of the 24 pin socket. Solder the colored with to the side of the resistor that is connected to pin 16 on the 28 pin socket.

You should now have a functioning replacement JiffyDOS ROM.

 Note: 27128 EPROMs work but consume more power than 27C128 EPROMs. A larger EPROMs can be used to switch between more than 2 ROM sets, the wiring is slightly different. You must own a legal copy of JiffyDOS for each device you install it into. Credit goes to Ray Carlson for his wiring diagram, http://symlink.dk/nostalgia/c64/rom/ for a helpful tutorial that is much like this one, and the owners of the archive sites the files are from.