Sega Master System / Mark III / Game Gear
This page explains how to create a 32K rewritable cartridge for the Sega Master System. It involves modifying an existing single-chip cartridge with 32-pin integrated ROM.
Here's a parts list - prices (UK Pounds) and part numbers given are Maplin's.
|Component||Qty.||Maplin Code||Maplin Price (ea.)||Total Price|
|Harris/ICL 7673 (battery backup watchdog chip)||1||*||1.51||1.51|
|16V 100nF Ceramic Capacitor||1||YR75S||0.12||0.12|
|2.2kohm micro-miniature resistor||1||U2K2||0.06||0.06|
|4075 triple 3-input CMOS OR gate IC||1||QW45Y||0.42||0.42|
|CR2032 3 volt lithium battery||1||ZB74R||1.69||1.69|
|Ultra-miniature panel mounting SPDT toggle switch||1||FH98G||0.93||0.93|
|62256 LL 28 pin 32KByte 8 bit SRAM chip||1||UH40T||2.54||2.54|
|Total (excluding VAT)||7.27|
|Total (including VAT)||8.54|
* This part is no longer stocked by Maplin. It's available from Farnell, order code 408657. Farnell also stock Varta CR2032 batteries with built-in solder tabs; the order code is 249877, price 1.77.
You'll also need:
Note: You can, if you prefer, use a single-chip cartridge containing a 28-pin ROM, but bear in mind that all of the photographs show a 32-pin board.
Below is a schematic of the circuit. Pin numbers are shown.
The 7673 serves the function of switching seamlessly between the main +5v and backup +3v (battery) supplies when required. The logic output, PBAR, is low (0v) when the main +5 volt supply is present, and high (+3v) when the backup supply is present.
The OR gate (4075) is used to enable the 62256 only when all of the following are true:
If any of those three lines are high, then the output of the OR gate will also be high, disabling the 62256 and switching it to "stand by" mode. (This reduces current consumption and prevents data corruption.)
The 62256 can only be written when WE (Write Enable) is low. A physical switch is used to implement a "write protect" mode which keeps WE high at all times, preventing accidental overwriting of data.
Finally, output from the 62256 is only permitted when the RD (Read) line of the cartridge is low.
Open up the cartridge, and desolder the original ROM as follows. Place the free end of the desolder braid on the solder joint, and press the hot bit of your soldering iron on top of the braid. Apply gentle pressure and the solder will be "soaked up" by the braid. Cut off the solder-impregnated end of the braid, and repeat for the remaining 31 (or 27) pins of the ROM.
When you've desoldered all of the pins, wiggle each pin gently and it should break free. (If it doesn't, repeat the desoldering process). You should then be able to lift the ROM clean out of the board - use a flat screwdriver to lever it up gently at each end, then pull it out with your fingers (or use IC tweezers).
The cartridge board with the ROM removed. Front view (above), reverse view (below).
Now, remove the 62256 from its protective package. Lift pins 1, 20, 22, 27 and 28 of the 62256 so they are "sticking out" at right angles to the other pins, as shown below. Trim the legs of the pins you've lifted, so only the wide part of the pin remains.
Solder the remaining chip legs into the holes on the board. If the original board was a 32-pin, then leave the top four holes empty, as shown here:
Remove the 7673 from its packaging. Lift all of its pins, then trim each one except pin 8, like this:
Now, here's a neat trick if you can pull it off! It helps if we can minimise the number of wire links we need to make, so see if you can solder pin 1 of the 7673 to pin 28 of the 62256; and pin 8 of the 7673 to the positive pin of capacitor C2 on the board, like so:
(On a 28 pin board, C2 is too close to the 62256 - you'll need to trim pin 8 of the 7673, and solder a wire link in place instead.)
Make sure the 7673 itself is as close to the board as possible (preferably touching it) - if components are too high off the board, then the board may not fit back into the cartridge shell! It's a good idea, after each step, to place the board back in the cartridge casing to make sure it will fit.
Now solder the 2.2kohm pull-up resistor between pins 6 and 1 of the 7673. The one shown is a micro-miniature type - a standard resistor is fine, but is a tighter squeeze!
Connect the 100nF decoupling capacitor between pins 1 and 4 of the 7673 (first picture, below). Take the capacitor lead going to pin 4, and bend it round to reach a suitable ground point on the reverse of the board, in this case the negative leg of capacitor C2 (lower picture, below).
Remove the 4075 from its packaging, then lift and trim all of its pins as previously described. The 4075 contains three triple-input OR gates; we only need one of these, so we wire all of the six remaining inputs to the Vcc line (pin 14) to reduce current consumption. Do this now: take a bare piece of wire, and use it to connect pins 14, 1, 2, 3, 4, 5 and 8 together. This picture shows what is required:
Now solder pin 7 of the 4075 to pin 14 of the 62256, on the reverse side of the board. Orient the 4075 so it does not foul the board mounting posts in the cartridge shell - see the picture below.
Connect a wire link between pin 1 of the 62256 and ROM hole 29 (or 27, if it's a 28 pin board) on the board itself. (This is the hole which pin 27 of the 62256 would have gone into, had you not lifted and cut it.) Front view, showing pin 1 connection:
Reverse view, showing hole 29 connection:
Connect a link between pin 4 of the cartridge connector, and pin 22 of the 62256. (Pin 4 is the second pin from the right on the front of the board.) This is a tricky manoeuvre - take care to solder only to the very top of the gold area, as shown below. You'll also need to cut a notch in the front part of the cartridge shell, to accommodate the wire.
If you're using a "gap-toothed" board which doesn't have a pin 4 on its cartridge connector, i.e. there's just bare board where pin 4 should be, you'll have to connect a link between pin 22 of the 62256 and Ground (pin 14 of the 62256) instead. You can do this anyway if you can't be bothered to cut the shell and solder to the cartridge connector pin.
Now five more wire links:
This is what the board should look like after you've made these links. Note the routing of wires to avoid the board mounting holes. Front view:
The penultimate step is to add the Write Protect switch. This is a single-pole, double-throw (SPDT) switch which toggles the Write Enable pin of the 62256 between the Z80 WR line (write enabled) and the Power line ("always off", i.e. write protected).
Connect the centre pin of this switch to pin 27 of the 62256. Connect one of the other pins to pin 1 (i.e. the power line) of the 7673, and the remaining pin to ROM hole 3 on the board. (Hole 1, on a 28 pin board.) This is the hole which pin 1 of the 62256 would have gone into, had you not cut it.
See the above picture for an illustration of the switch connections. Ensure that the wires you use are long enough to reach the side of the cartridge case (or wherever you plan to mount the switch).
WARNING - soldering wires directly to lithium batteries is very dangerous. The battery could explode if you heat it up too much. Unless you're incredibly stupid (like me), you may prefer to use a battery holder, or buy special CR2032s which have solder tabs already built into them. For now, though, I'll describe the procedure for soldering directly to the battery. Just remember - I warned you of the risks, so don't blame me if you hurt yourself!
CR2032 batteries look like small coins - the side with the manufacturer's name is positive (and should be connected to pin 2 of the 7673), while the other (blank) side is negative (and should be connected to pin 4 of the 7673).
You should gently roughen a small area of the battery surface (with dry wet-and-dry paper) before soldering - the solder seems to "take" better if you do. Just make sure you clean up the metal filings - they could cause a short circuit.
Solder the negative terminal first. Mount the battery in Blu-Tack and set the wire in position - do the soldering, then remove the iron very quickly. Let it cool before soldering the positive terminal. This picture shows the solder connection on the positive terminal:
Here's the completed board:
Test the board using your multimeter - in particular, check that pin 20 of the 62256 is at +3 volts (not 0 volts).
You should now drill a hole in the case to mount the switch. The top left of the front portion of the case is a suitable position - see below...
Now reassemble the cartridge and test it!