Combination Cartridges for the Plus/4 By James Hehl This article is the result of just doing something that was convenient, for me. Athough this article is mostly Plus/4 specific, much of the construction and methods of switching applies to the other Commodores. I talked with Jack Vanderwhite, a short time ago, and mentioned that I had decided NOT to waste a couple of cartridges for the Script Plus and Calc Plus programs. Told him that I'd stuck them both in a single cartridge, just switching in whichever one that I wanted to use. I'd hardly gotten the description out of my mouth, before he says, "Write it up!". So folks, here we go again... The typical Plus/4 cartridge will contain either a single 128k ROM, or a pair of them. In the cases of Script Plus and Calc Plus, there are two pairs of 128k ROMs. Although Plus/4 cartridges are not super-scarce, they CAN be hard to find. I really hated to tie up a couple of perfectly good cartridges (with as much experimenting as I do) for the two productivity programs that I use the most. So I decided to modify one of the cartridges, with a switch, to be able to "bank in" whichever program that I want to use. This is as easy as sliding a switch, on the side of the cartridge, and pressing the reset button. There are two graphic files with this article. They are "EPROM.LAYOUT" and "PIGGYBACK.ROM". The first shows how to modify a Plus/4 cartridge, to switch between two 128k banks, in a pair of 27256 EPROMs. The second shows how to "piggyback" the original ROMs (from Script Plus and Calc Plus cartridges) and fit them into just ONE cartridge. First, let me remind you that Script Plus, for the Plus/4, is the companion program to the C64's Easy Script. The Calc Plus cartridge, for the Plus/4, is the companion for Easy Calc on the C64. I'm not sure if Jack will be including the two EPROM data files for the Combo cartridge. They are each 130 disk blocks long. If he doesn't, they will be available, by your request, from either Jack or myself. BEFORE YOU START!! Before opening the cartridges or working with them, be sure to use a grounded work surface. This will prevent any possibility of static electricity damaging your ROMs. GETTING STARTED First, let's address the way to piggyback the two original ROMs. Take a look at the graphic file: "PIGGYBACK.ROM" (it would be a good idea to print it out). You'll see a top view of the circuit board on the right side of the graphic. Note ,carefully, the locations of the notches on the ROMs. They will ALWAYS be facing the pins on the edge of the cartridge board. There are two small capacitors that aren't shown on the graphic. We won't do anything to these, so just ignore them. Mark the ROMs, with a bit of tape and pen, with their locations (ie ROM1, ROM2). Then remove them with either an IC puller or by (carefully) inserting a blade under the ends. Gentle, alternate prying will lift the ICs from their sockets. If your ROMs are soldered in place, then just leave them alone. The following steps will apply to both socketed and soldered ROMs, in your cartridge. Pin number 28 on ALL the ROMs (which is the +5 volt line) must be bent straight out. If your ROMs are soldered in place, you must either unsolder it or clip it close to the board. Bend the pins, carefully, with a small pair of needle-nosed pliers or a pair of tweezers. Take a look at FIG.1 in the drawing. It shows how each of the second set of ROMs, is positioned to "piggyback" over the first set. Make sure the notches on the IC cases are at the same end. Align the pins of the top IC, right over the corresponding pins of the bottom IC. With a bit of care, the top IC can be pressed down on the lower one. Make sure the pins are still aligned with each other, before going further or soldering. At this point, all of the pins of the top IC will be aligned with and slipped over the outside of the pins on the bottom IC. Pin 28, on both, will be sticking straight out to the side. These pins (pin 28) will be connected to the switch, a bit later. With everything aligned (as shown in FIG.2) use a low wattage soldering iron and lightly "tack" solder all the pins (pins 1 through 27). If you aren't handy with a soldering iron, please get someone who is... TO DO IT. Failing in that, you can use the following alternative. ALTERNATE ROM CONNECTION Instead of tack soldering the pins, you can use a conductive paint to make the connections. This is available at most Electronic stores. If you can't locate some, then you'll find a substitute in your local automotive store. They carry a conductive paint, that is used to repair rear window defroster strips. This stuff works quite well and is very handy for repairing pathways, on most circuit boards. If you decide to use this conductive paint method you should put a dab of adhesive (shoe goop is perfect) between the bodies of the ICs, before you align the pins (as in the above paragraphs). This will keep the ICs in place and prevent the conductive paint from breaking free. If your ROMs are socketed, you need only remove them to bend pin 28 straight out. Then replace the first set, into their proper sockets. Position the second set over the first (putting the dab of adhesive between the plastic bodies), making sure the pins of the upper IC are overlapping and aligned with the bottom set... then gently press it down. The pins of the top ROM will partially enter the socket that holds the bottom ROMs. The bodies of the ROMs need to be as close as possible (for the board to fit back into the case properly, so you may have to trim a bit off the length of the pins (of the top IC). Do this with a pair of small wire snippers or scissors. When both ICs are in their proper positions, you are ready to use the conductive paint to make the electrical connections. Be sure to shake the bottle of conductive paint WELL, as most paints of this type will settle quickly. Using a toothpick, dip into the paint and transfer enough to each set of pins (pins 1 to 27) to wet them both. Take care not to use too much, which can flow over and short unwanted pins. If you do, stop and clean up the excess VERY WELL. The conductive paint dries very quickly and as long as it is not disturbed (the reason for the adhesive between the ICs) it will give you a permanent job. MOUNTING AND WIRING THE SWITCH Looking at the front of the top case cover (with the opening for the circuit board toward you), the switch is mounted on the right side. A cutout is made, starting 3/4 of an inch from the top, to accomodate a small slide switch. The type to use is a SPDT switch. SPDT means Single Pole Double Throw. When mounted, solder about four inches of insulated wire to each of the three contact lugs. Set the top half of the case aside, for now. FINISHING THE CARTRIDGE WIRING Look at the right side of the graphic. It shows that a wire is soldered between pin1 of ROM1 and pin1 of ROM2. Refering to FIG.3, you'll see that it is done with both the top and bottom ROMs. That is: Pin 1 of ROM 1 (top) is soldered to pin 1 of ROM 2 (top). Pin 1 of ROM 1 (bottom) is soldered to pin 1 of ROM 2 (bottom). Lay the two case halves side by side. Place the cartridge board in the bottom half, in its proper position. Now solder the wire from either outside lug, on the switch, to one of the pin1 to pin1 loops. Solder the wire from the OTHER outside lug, on the switch, to the OTHER pin1 to pin1 loop. Locate the heavy pathway that starts at pins 2 & 3 of the connector edge of the cartridge board. Scrape an area of the varnish off of it. This is the +5 volt pathway. This is where the center lug of the switch will be soldered. The right side diagram, on the graphic, will help you locate it. Once you are done, CHECK ALL WIRING and replace the top half of the case. This finishes the wiring of the cartridge, using the original ROMs. If all is correct, you should have either SCRIPT PLUS or PLUS CALC indicated as active when the cartridge is plugged in and turned on. In order to access the other program, it is a good idea to turn off the machine, slide the switch and then power up. The other program will now be active. The ROM switching is accomplished by simply connecting the +5 volt line to the ROM that you want active. The other ROM will not function. ------------------ NOTE TO C128 USERS ------------------ I use the above method to install the 64k video RAM chips in all my C128 flat machines. I just "piggyback" the new 64k chips over the existing 16k chips. First clipping the +5 volt line to the 16k chips. Then I tack solder the new chips, as above. In some of my machines, I've installed the SPDT switch (also as above) to make my machines 16k/64k switchable. DOING IT WITH EPROMS Now we will cover the method of creating a 512k EPROM cartridge, with both Script Plus and Calc Plus in it. Many cartridges and replacement ROMs (like JiffyDos) take advantage of the fact, that you can switch between the "upper" and "lower" halves of a 27256 EPROM. Each half or bank will be 128k in size. This done by either grounding or applying +5 volts to pin 27, of the EPROM. That is what we will do in this part of the project. This will allow us to put Script Plus in one bank and Calc Plus in the other. The EPROM data files must be programmed into a pair of 27256 EPROMS, with an EPROM programmer. I use the Promenade C1. A SPST (Single Pole Single Throw) slide switch, which is a simple on-off type, is installed in the top of the case. This is described in the first part of this article. Look at and print out the graphic "EPROM.LAYOUT". This will help guide you throught the steps that follow. The use of a grounded work surface is necessary in this project, too! Remove any ROMs from the cartridge and wrap them in foil or stick stick them into conductive foam, for storage. Turn the cartridge over (to the bottom side) and compare it to the bottom view, as shown in the graphic. You will need to solder a short length, as shown, between pins 1 and 28 on both ROM solder pads. Be careful NOT to short any of the other pins with solder. Turn the cartridge over to the top side. Compare it to the TOP VIEW as shown in the graphic. Before installing the EPROMS in their proper sockets, you will need to bend pin 27 (on each) outward. Do this carefully with a pair of needle-nosed pliers or with tweezers. This is the pin, that will be switched between ground and +5 volts, to bank in the program that you want. Fist, as in the drawing, solder a loop of insulated wire between pin 27 of EPROM 1 and pin 27 of EPROM 2. This will be our grounding wire for switching banks. Next, locate the wide pathway that starts at pins 2 & 3 of the circuit board edge. The graphic shows this as a wide black line. Scrape an area of the varnish off (to bare copper) near pins 27 of both EPROMs. This is the +5 volt line, for the cartridge. Solder a couple of 10k (10,000) ohm resistors as shown. They can be as small as 1/8th watt units. They are solder between pin 27 and the +5 volt line on both EPROMs. When you are finished, there should be both one lead of the resistors and the switch loop wire attached to pin 27... on both EPROMs. Scrape away an area of varnish in the top right corner of the board, to the bare copper. The area around the outer edges of the cartridge board is grounded. This will be our ground connection for the switch. All that remains is to solder one of the two leads, from the switch, to the newly cleaned ground. The other switch wire is soldered to the loop (from pin 27 to pin 27) as shown in the graphic. RECHECK your wiring, then assemble the cartridge. Plug in the cartridge and turn on your machine. Depending on which position the switch is in, either Script Plus or Calc Plus will be active. To change programs, with this set up, just slide the switch and then press the reset button. The second program will now be active. For those of you that are interested, the resistors act as "pull-up" resistors. They hold pins 27 at +5 volt until the switch is closed. When this happens, the pins are pulled to ground potential. The resistors limit the amount of current that flows, so that you won't be shorting out your power supply (when you ground pin 27). Well, I guess that covers it for now, folks. Just use your imagination and you'll see the potential for using this arrangement for other applications. Any additional questions, concerning this article, can be addressed to either Jack Vanderwhite or myself. Have fun EPROMming!