Sega Master System Development Cartridge
Introduction

 Here are the instructions to modify a SMS cartridge to accept a Am29F040
 flash memory chip.

 I used a 4-megabit cartridge that had a 315-5235 paging chip, in this
 case Phantasy Star. This cartridge also has battery backed SRAM, which
 is an additional bonus. The 315-5235 supports paging in all three banks,
 making it compatible with virtually every SMS game.

 For more information on the 315-5235 and games which use it, take a look
 at this document at SMS Power:

Instructions

 Instructions:

 1. Remove the 32-pin ROM in the upper right corner of the board. It has the
    text "IC2" printed beneath it. I didn't want to keep the original ROM,
    so I clipped the legs off to remove the ROM, and then desoldered each
    pin by hand on the back of the board. Not hard to do, just tedious.
    For me, this process removed most all of the existing solder.

 2. Solder a 32-pin socket where the ROM chip went. It's easiest to do this
    by having the board oriented vertically, as this ensures there are no
    pins next to the current one you're working on.

 3. The Am29F040 has a slightly different pin configuration than the
    original ROM:

                Am29F040        ROM
    Pin 1       A18             N.C.
    Pin 31      WE#             A18

    You can leave WE# unconnected. As for handling A18, I didn't want to
    modify the PCB in case it would be used for something else later on,
    so I added a second socket over the first in order to route A18
    correctly. Here's the details:

 A. Using another socket, remove pin 31 (shown as an empty spot in the
    picture) and bend pin 1 outwards so it's at a right angle in respect
    to the other pins.

 B. Solder a small bit of wire from pin 1 of this socket to pin 31 of the
    *original* socket.

 C. Use a piece of electrical tape to cover the underside of where pin 31
    was on the new socket, and put a piece over the hole for pin 1 of the
    original socket. This ensures there are no short circuits.

 D. Finally, put the new socket into the old one.

 That's all there is to it. I added a ZIF socket plugged into the whole
 mess to make swapping flash memory simpler and to remove any wear and tear
 on the existing sockets.

 If you want to modify the board and omit a second socket, you could cut the
 traces to pin 1 and pin 31, then tap A18 elsewhere and wire that to pin 31.
 I don't know anything about doing that, but it should work.

 I only had two 40-pin sockets handy, which is why they extend over the
 right edge of the board in the screenshots. But I refer to the pin numbers
 as if it was a 32-pin socket, so don't worry about that.

 Some versions of this board have the battery mounted on the opposite side
 of the board as well, your cartridge may look a bit different.

 Many thanks to Mike G. for information on the 512k ROM pinout.

Screenshots

Component detailAdded ZIF socket, case, flash memory

Links

SMS Powerwww.smspower.org
SMS Readerwww.smspower.org/smsreader/

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