Here’s a quick guide on how to populate my custom designed Genesis/Mega Drive reproduction PCBs. It supports most Genesis games.
Front Side
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.
CB – Electrolytic Capacitor
Needed for: Games that save
Value: ~22 uF, at least 10 V rated
Function: Prevents transient voltage drops on the SRAM during power-down.
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, R2, R3, R4, R5 – Resistors
Needed for: Games that save
Value: 1 kΩ for R1 and R4, 10 kΩ for R3 and R5, 100 kΩ for R2
Function: Part of the battery management and saving logic circuit.
D1, D2, D3 – Diodes
Needed for: Games that save
Part Number: Low reverse leakage diodes (BAT85 is suggested)
Function: D1 and D2 combine the battery and Genesis voltage rails to power the SRAM and keep it working after the power is turned off. To lengthen save data retention, it’s better to get diodes that have a low reverse leakage. D3 keeps /VRES from staying high while the supply voltage drops.
Q1 – NPN Transistor
Needed for: Games that save
Part Number: BC547 is recommended, 2N3904 works as well
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. Please note which pins are emitter, collector, and base.
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. Note: buying name brand batteries from reputable sellers will guarantee you rated battery life – unbranded ones might not last as long.
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
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 pads on the left for games 16 Mbit or less, on 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.
You can then save the ROM to use in another programmer, or go ahead and program with your GQ-4×4.
Mouse Bite Labs 
Dear friend, can you give Gerber files of this Board?
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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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I want to buy this board. When they will be available?
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I just ordered a restock – should be here in a few weeks!
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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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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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Check out this article: https://thepoorstudenthobbyist.com/2019/09/02/27c322-160-programming-adapter/
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Thanks for answering, but if I want to program a 1mb game in a 27c160 with the adapter, do I only need to use the first two banks?
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Yeah if your game is smaller than the EPROM you only need to do the necessary parts. But doing all four wouldn’t hurt anything
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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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How can I buy boards from you with delivery to Ukraine?
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Check my Etsy page, I believe I can ship to Ukraine from there
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Having a bit of an issue I’m getting a blank screen on the system I’m using 27C160 I have C1 and C2 in place along with <16Mb and 27c160 soldered on the back at first I thought maybe a bad write on the eprom, but if I take your cart and use my MD Flashkit programmer to rip the rom and upload it back to their cart it plays fine. I'm a bit stumped at this.
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Got it had bad solder joint
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Bought some boards back in March and only just got around to using them. The boards I received are v2.3 boards and don’t have a spot, “CB”, for the second electrolytic capacitor. Do I still need to use one for games that save? And if yes where do I solder it to the board?
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No, you shouldn’t need to worry about it. The added components like CB are protections against rare save glitch edge cases. But I’ve never encountered an issue with the older boards (or gotten any reports about the older boards).
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Thanks for the quick response Bucket. Much appreciated 👍
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Used this PCB to program a copy of James Lopez’s GenTest diagnostic cartridge (https://gitlab.com/Jlopez450/genesis-diagnostic), worked perfectly!
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Can this PCB be used to flash Sega Master System games for play on Genesis as well? I wanted to try to make an original Phantasy Star that would play on my Genesis 😎
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