Genesis Reproduction Board Guide

Here’s a quick guide on how to populate my custom designed Genesis/Mega Drive reproduction PCBs. It supports most Genesis games.

Front Side

PXL_20211101_042910055

parts_list

U2 – EPROM

Needed for: Every game
Part Number: 27C322 or 27C160
Function: Holds the ROM file
How to Program: Use an EPROM programmer and load the ROM file; be sure it is byte-swapped before programming. ROM preparation is detailed below.

C1 – Electrolytic Capacitor

Needed for: Every game
Value: ~22 uF, at least 10 V rated
Function: Smooths out supply voltage for the board due to transients on the power supply, prevents quick changes in supply voltage when power is turned off.

C2 – Ceramic Capacitor

Needed for: Every game
Value: ~0.1 uF, at least 10 V rated
Function: 
Filters out high-frequency noise that can interrupt the EPROM.

C3, C4, C5 – Ceramic Capacitors

Needed for: Games that save
Value: ~0.1 uF, at least 10 V rated
Function: 
Filters out high-frequency noise that can interrupt the SRAM, Flip-Flop, and Decoder.

R1, RC, RB – Resistors

Needed for: Games that save
Value: ~1 kΩ for R1 and RC, ~10 kΩ for RB
Function: Part of the battery management and saving logic circuit.

D1, D2 – Diodes

Needed for: Games that save
Part Number: Almost any schottky diode will do, I use 1N914 or BAT85 diodes
Function: Combines battery and main power supply to power the SRAM, and prevents backfeeding power into the battery.

Q1 – NPN Transistor

Needed for: Games that save
Part Number: Most NPN transistors work, such as the 2N3904 or 2N2222 series
Function: Part of the battery management and saving logic circuit, also forces the SRAM into a low-power state when the game is turned off.

U3 – D-type Flip-Flop

Needed for: Games that save, for ROM files that are larger than 16 Mbit
Part Number: 74HC74 or equivalent
Function: Part of the saving logic circuit.

U4 – Decoder

Needed for: Games that save
Part Number: 74HC139 or equivalent
Function: Switches between activating the ROM or the RAM during gameplay.

U5 – SRAM

Needed for: Games that save
Part Number: 62256 series SRAM (be sure to get low standby current model)
Function: Holds save game data.

B1 – Battery

Needed for: Games that save
Part Number: C2032
Function: Keeps the SRAM on to retain data while power is off.

Solder Pads

If your game does not use a 74HC139: Bridge the solder pad on the right of the 74HC139 socket to bypass the circuit
If your game uses SRAM: Bridge the solder pad in the right hand corner – if you want to clear a save game, desolder this pad to discharge the SRAM power supply, and then reapply solder to re-enable it.

Back Side

PXL_20211101_042935712

Solder Pads

EPROM selection: Bridge the pads in the middle of the board based on which EPROM you are using – the 27C160 or 27C322
ROM size selection: Bridge the middle pad to the left for games 16 Mbit or less, to the right for games larger than 16 Mbit (this is only necessary for games that save). Solder based on the original ROM file size – if you expand a 16 Mbit game to 32 Mbits, for example, you should still use the “16 Mbit or less” solder pad.

Preparing the ROM

When programming Genesis games, you need to make sure you byte swap the ROM file before you program it. For this, I use the GQ-4×4 programming software – you don’t need to have a GQ-4×4 to use this software! You could just use it to byte swap your ROM and then save it to use a different programmer. To byte swap, just load up your ROM in the software, and click the “A<>B” button.

byteswap1

You can then save the ROM to use in another programmer, or go ahead and program with your GQ-4×4.

Important rework for older board revisions!

If you’ve got a board that’s revision v1.1, v1.2, or v1.3, then you might need to do a slight rework.

Needed for: Games that save, that have ROM files 16 Mbit or smaller
Rework: Solder pins 2 and 3 together on the 74HC139. Make sure the 74HC74 is not installed on the board.

IMG_20200922_222528

11 thoughts on “Genesis Reproduction Board Guide

    • No men, instead support this guy buying some of the boards on his store, he made all this guides for free and even answer your questions on facebook.

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  1. That’s cool and all, but why don’t you explain why you need to solder pins 2 and 3 on the 74HC139 on older board revisions? Why musn’t the 74HC74 be installed?

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    • Honestly I forgot I had that note on there, those boards are pretty old at this point so I doubt there are many out there who still haven’t used them. This was supposed to just be a quick guide for assembling the board, not a deep dive explainer, so I didn’t feel like it needed to be included.

      Basically, the RAM is located at memory address $200000-$3FFFFF. For games less than 16 Mbit, there’s no ROM overlap with RAM, as addresses past $200000 would be located after the first 16 Mbit of ROM data. So you only need a ‘139 to toggle between the ROM and RAM chips based on where you are in the memory.

      But if you have a game larger than 16 Mbit, which will by definition have memory space overlap with the dedicated RAM space, you need to have a ’74 flip-flop to bankswitch, between reading ROM data or RAM data when accessing memory location $200000-$3FFFFF. The flip-flop CLK input is wired to the /TIME pin on the Genesis cartridge, which will pulse when a specific register is set ($A130xx) by the ROM. This tells the cartridge to either allow ROM or RAM data to be used when accessing that particular memory location.

      On the v1.x revisions of my Genesis boards I had neglected to include support for games 16 Mbit and smaller that use RAM space, so the fix I described restores that functionality. All revisions after v1.3 have solder pads on the back to choose between which mode to use.

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  2. Hi! I want to program a 27c160 with a 1mb game, but I don´t really know how to do it because I use the tl86 with an adapter. Can you please help me a bit?

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  3. Could you please share the schematics of tgis board?
    I’m interested specially on the battery backup support.
    Thanks in advance.

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