SWITCHLESS JIFFYDOS... how to install it yourself
                 latest updates and/or corrections 9-13-2020

     JiffyDOS is well known as a computer and disk drive enhancement chipset.
One drawback of a standard installation is the need for a bypass switch to
enable the original Kernal code in the computer. There are some programs that 
conflict with JiffyDOS, so a way to bypass it without drilling holes in the 
computer case is preferred by many users.
     With the addition of a single IC and some soldering, an external switch 
can be eliminated and the JD/normal selection can be done via the RESTORE 
keyboard button at bootup. The boot defaults to JiffyDOS but if the RESTORE 
key is held down at boot, a normal C64 Kernal is selected. 
     There is a plug-and-play version of switchless JiffyDOS available via
Ebay and Facebook. The builder is known by several names: Coprolite Computer
By-Products, AAASCRAPDOGSUNLIMITED and BOULDERDASH. The module he makes has 
JiffyDOS in it so it just plugs into the socket where the C64 Kernal was 
installed. If soldered in, that IC must be removed and a socket installed.
The module has two wires with color coded clips to attach to two points in 
64 or 128 computer, namely the reset and restore lines. Instructions indicate 
where the wires are to be connected. Switchless JD for the Plus/4 has only 
one clip wire to attach.

NOW FOR THOSE WHO LIKE TO "ROLL THEIR OWN"...

     A professional JiffyDOS module with switch is available from Jim Brain 
(Retro Innovations). He also sells just the code which I buy and use to 
program my own EPROMs when I install switched or switchless JiffyDOS for
users during repairs or upgrades. Notice the generic EPROMs in all my photos.
     The additional IC used is a common TTL chip 74LS74 available at any parts 
house. One supplier I use frequently is Jameco Electronics. Only one of the 
two gates in this IC is needed for the installation. Other than power and 
ground, three connections to the IC are used: reset, restore and a line to 
the JD module. Placement of the added IC is not critical and any convenient 
spot close to all the connections needed is preferred. Rather than a circuit 
board, for something this simple I used to attach the IC upside down on the 
motherboard with Superglue so all the pins are sticking up which allows easy 
access for soldering. I now use a "piggyback" arrangement: mounting the new 
IC on top of the EPROM. It's important that the install doesn't interfere with 
any possible future work needed on the computer. The first installation I ever 
did was in my own C128 and since in all versions of the 64.
     The diagrams show how the IC is wired. Note that each computer is a bit 
different in where the connections are found for power, ground, reset and 
restore. Photos show the different PC motherboards and where the connections 
are located. Before mounting the IC, mark pin 1 so there is no confusion after 
it's glued down. See photos.

SWITCHLESS JD FOR THE 128 USING THE CAPS LOCK KEY contributed by Jason McCarver

     One possibility with the C128 is to use the Caps Lock key to disable
JiffyDOS instead of the Restore key. Using the Restore key has the disadvantage
on the C128 of being quite awkward to hold down while also having to hold the
C= key when booting into C64 mode. Using the Caps Lock key avoids this and
makes it easy to diable JiffyDOS. I mounted the 74LS74 near the bottom right
of the board near the keyboard connector with hot glue. Power, Ground, Reset,
and Caps Lock are all easy to pick up close to this area: see photo capslock.jpg

     +5V       - lower side of R38
     GND       - vias right below the keyboard connector
     Caps Lock - right most pad of J7
     Reset     - Left of CIA 6526 at U1 between pins 7 & 8

SWITCHLESS JD FOR THE PLUS/4, A BIT TRICKY TO IMPLEMENT...

     Because the Plus/4 keyboard matrix is different than a C64 or 128 keyboard,
a few changes to the standard switchless circuit must be made. First, there are 
no keys similar to RESTORE as on the other computers, so a source must be found 
that is suitable but that will not upset the existing keyboard circuits. There 
are pulses on all the keyboard lines which must be filtered out so only a logic 
level change is detected. One NPN transistor, two resistors and a cap form a
filter, buffer and inverter to feed the 74LS74 D input. The reset line is easy, 
an IC nearby has that pulse on it. If you'd rather not solder to an IC pin, run 
the reset line over to the left side of R21 or to the center pin of the serial 
port, pin 6. 
     Operation of switchless JiffyDOS in the Plus/4 is a bit different than in 
other computers for this arrangement of added components. When the computer is 
powered up or reset, JiffyDOS is selected. Subsequently, if either CONTROL key 
is held down at power up or RESET, the OEM Plus/4 Kernal is selected. Note that 
holding down any key will work to select the Plus/4 Kernal but the control key 
doesn't affect a normal boot screen such as holding the spacebar would do. 
     There is an alternate way to control JiffyDOS in the Plus/4 by setting the 
added IC to "toggle" between the normal Kernal and JD, so the transistor circuit
is not needed. When powered up, the computer may come up in either mode and a
reset switches between normal and JD. Of course this method is easier to 
implement since it only requires the reset line and no other components. The 
keyboard is not used to control it. 
     Since space under the reattached keyboard is minimal, the added components
must be low enough to clear the underside of the keyboard (which is metal) with
components and wires on it. Keep all parts as low to the board as possible. 
Shine a light down into the case as the keyboard is being put back to see if
there is adequate clearance and no shorts are possible. Two physical arrangements
are shown for the added 74LS74 switch IC. One is mounted on the motherboard and 
the other is Superglued right side up onto the top of a home-made JiffyDOS EPROM 
after the pins are cut short. 

SWITCHLESS JIFFYDOS FOR THE SX-64...

     This one would be easy except working on the SX is difficult as everything 
is packed into a small space and boards must be removed to do just about anything.
The switchless JD IC is mounted on the I/O board and lines run from it to the 
Kernal and drive JD modules. The photo tells all. 

Ray