Sega Master system controller to 7800 mod

[J.Ivy]

Okay, ive finally scored a sega master controller on ebay. Im getting ready to attempt my first controller mod.

 

Ive seen a few people link this http://www.instructables.com/id/Retro-Atari-7800-Mod-Sega-Master-System-controlle/ and it looks fairly straight forward.

 

Before I begin, any advice on this?

 

Edited June 5, 2014 by J.Ivy

[Lynxpro]

I know the SMS pad has its fans but I fail to see how it's better than the 7800 Gamepad.

[+Gemintronic]

I know the SMS pad has its fans but I fail to see how it's better than the 7800 Gamepad.

 

The US 7800 joystick gives me hand cramps within 15 minutes. It's not an age related thing either. This happened since I first bought the thing as a kid. The euro gamepad is kinda hard to find.

[gorfcadet]

Euro gamepad buttons are too far apart. Makes games where pressing both buttons simultaneously is needed a chore. I also find the Euro pad's buttons are prone to sticking. Nes and Genesis pads have them both beat, but the Master System is better.

I love the 7800. I just don't get the folks who stick to their guns that the ProLine or Euro pad is somehow better than a nes pad or anything else. If the T16 pads were cheaper, I would say those are the way to go. Even when the 7800 was new, I would use a Atari 2600 stick when I could.

[Prosystemsearch]

Won't the Master system pad work fine on its own? If not , just why??

Edited June 5, 2014 by Prosystemsearch

[Lynxpro]

 

The US 7800 joystick gives me hand cramps within 15 minutes. It's not an age related thing either. This happened since I first bought the thing as a kid. The euro gamepad is kinda hard to find.

But I wasn't talking about the 7800 ProLine joystick...

[Jinks]

Won't the Master system pad work fine on its own? If not , just why??

Both buttons wont work.

Edited June 6, 2014 by Jinks

[Lynxpro]

In principle, I liked the NES Max. I really should pick one up to test if the 360 Degree capable joypad works with the port of 720 Degrees. That title really should be ported to the 5200...

[NIKON]

7800 Proline controllers, in my opinion, are a joke... a slap in the face to all gamers...WHAT were they THINKING??? WHAT MOOK thought that this was a good controller???

 

When I first bought the 7800 system back in 1986 I gave them a chance and tried to work with them...but hand cramps and frustration prevailed. So I just used a 2600 controller until I had a Genesis controller...did not know about modding the controllers back then so I just did not play games that required two buttons...but then in the 90s I discovered emulators and been with them ever since because I can use any controller I want and set it up anyway I want.

 

 

I wish you luck on your modding adventure...I know that you will be pleased with the results.

Edited June 6, 2014 by NIKON

[gorfcadet]

In principle, I liked the NES Max. I really should pick one up to test if the 360 Degree capable joypad works with the port of 720 Degrees. That title really should be ported to the 5200...

The Max is still a 4 directional pad. I have one; its nice, but not for the slider. 720 is still fun though.

[Lynxpro]

The Max is still a 4 directional pad. I have one; its nice, but not for the slider. 720 is still fun though.

Well, that's disappointing.

[Prosystemsearch]

Why won't the sega master system buttons work on a 7800 under normal circumstances??

[cobracon]

I believe they are wired differently.

[+-^CrossBow^-]

I believe they are wired differently.

 

^^^^ Basically this...

 

However, what will really bake your noodle is that you can use a standard stock 7800 controller with an SMS and both buttons still seem to work?! Or at least a stock 7800 works on a Power Base converter on a Genesis with both buttons acting as 1 and 2. I found that out with my copy of SMS Montezuma's Revenge. A stock Genesis controller wouldn't work correctly, but a 7800 controller did.

 

So I wonder if modified SMS controller made to work on a 7800 would also still work as an SMS controller?

[The Usotsuki]

I thought the difference was that the polarity of one of the lines was swapped, such that it's "0=down" on one platform, and "1=down" on the other.

[Andromeda Stardust]

Okay, here's the rundown on the controller pinnings:

 

Up, Down, Left, Right, and Ground are the same on all 2600, 7800, SMS, and Genesis controllers.

 

VCC on Atari systems is not connected on SMS and to the select line on Genesis.

 

VCC on Genesis and SMS is mapped to 2600 paddle 1 and the 7800 I input.

 

Genesis button B maps to SMS I and 2600 Fire. Button C maps to paddle 2,

SMS II and 7800 II.

 

The 7800 controller is a weird beast because it uses a resistor lattice connected to buttons I and II. Pressing a button on the 7800 controller will pull down two pins instead of one. For one-button games, both buttons pull the fire input low. For two button games, each button pulls low one of the 2600 paddle pins. These pins are normally low on the 2600 unless a paddle or Genny controller is inserted, so pulling the paddle input low by hitting either 7800 button has no effect. In 7800 mode, the paddle inputs are both normally high and pressing the I button will pull the first paddle input low; pressing II would pull the second paddle input low. So one button 7800 games read the fire input, and two button games presumably read the paddle inputs.

 

SMS gamepads on a 7800, in theory would pull the Fire input low by pressing SMS I, and SMS II would in theory pull the 7800 II input low but not Fire. That is how it would work if SMS gamepads were direct wired, but some have a chip in them. This chip will not function without a VCC 5V power supply from some source, and the Atari VCC feeds into the Select line which is disconnected on SMS.

 

The reason the Genesis controller works on Atari is because the Genesis controller has an internal pull up resistor conected between VCC and Select. This pull up resistor allows Genesis controllers to be backwards compatible with SMS by holding the select input high so that the 74ls157 will properly send joypad and button inputs to the SMS. B and C are equivalect to SMS 1 and II. When the select line is low, Start and A signals are sent to these pins instead. Anyway, there is enough current leakage through the pull up resistor to power the chip allowing the Genesis controller to work on Atari sytems. Because the Genesis VCC is connected to the 1st paddle line, a flag is set telling the system that paddles are connected. C button pulls the second paddle high as well, allowing homebrew games to detect input to allow two button operation.

 

It is highly likely that neither 7800 or SMS controllers will operate properly on the other system due to incompatible pinouts. Fortunately due to the resistor network inside the 7800 controller (buttons are not direct wired), pressing the I button on the 7800 controller will not short or damage the SMS. If you can recall, Sega VCC is connected to Atari paddle 1 or 7800 I.

 

I have an idea for an NES controller mod (not tested) that might allow it to dual function on SMS and 7800. It would need a 1st party NES controller and a 9-pin third party extension cable. When the 4021 IC is desoldered from the PCB, there is a pin in the DIP layout for each button on the controller as well as ground and VCC. There is also a network of pullup resistors which keep the business end of the buttons high when not pressed. Normally this resistor bank is connected to VCC, so it would be a problem because the VCC pins are different on Sega and Atari controllers. The pull-ups will be either discrete resistors or embedded black pads within the PCB traces. These would all need to be cut or removed. The idea is to build a 7800 compatible resistor lattice, Then break sections of the lattice using a DPDT switch when in SMS mode. In SMS mode, button I will connect only to Atari Fire/SMS I and button II will connect to 7800 II/SMS II. In 7800 mode, the full lattice is restored. Now button I connects to 7800 I/SMS VCC and Atari Fire/SMS I. Button II connects to Atari Fire/SMS I and 7800II/SMS II. Remember, no short or ground fault will occur if button I is connected to VCC on SMS, due to the resistor lattice acting as a shunt. The resistor would only cause a button press to draw slightly more current when actuated. The action buttons just won't operate properly on either system without the switch in the appropriate position which is expected. I'll need to study the schematics of both controllers more carefully before I start drawing/building test schematics. Hopefully one DPDT switch installed in the side of the controller will be enough; otherwise multiple switches would need to be ganged together which would hurt the cosmetics.

[jmetal88]

I made simple adapters to use my SMS controllers on my 7800 a while back. I was thinking I got them to work with two buttons, but now that I think of it, I'm not sure I actually have any real two-button games, so I don't know if I got real two-button support out of it or if I just got both buttons pulling down the same line.

[Andromeda Stardust]

^^Forget my earlier post. There's more going on in a 7800 joystick than meets the eye. Apparently, either button pulls the fire pin low in 2600 mode, but pulls it's respective paddle pin high in 7800 mode. How the dickens that is possible I have no idea, but a 2600 controller will operate both systems assuming you only need one button to play. I have a feeling a DPDT switched controller is still possible, but 2600 games would now need to use the SMS setting in order to work.

[alex_79]

There's a circuit inside the 7800 that pulls high pin 6 of the controller port when 2-button mode is enabled. It only works in 7800 mode, so a 2600 game cannot take advantage of that even on a 7800 console.

[7800]

I know the SMS pad has its fans but I fail to see how it's better than the 7800 Gamepad.

 

The D pad on that controller is not very comfortable OR accurate, where as the D pad on the SMS or NES controllers seem to work MUCH better and aren't very rough on the thumbs.

[ColecoGemini]

Yeah, the 7800 europads are worse than the ProLines. Too light and flimsy, fire buttons too far apart and the dpad sticks and is very inaccurate. Also the dpads and buttons are too tall. You have to push them too far to register, so reaction time is slower than Karateka.

[jmetal88]

I made simple adapters to use my SMS controllers on my 7800 a while back. I was thinking I got them to work with two buttons, but now that I think of it, I'm not sure I actually have any real two-button games, so I don't know if I got real two-button support out of it or if I just got both buttons pulling down the same line.

 

 

I was wrong regarding this post. My adapters were NOT for SMS controllers, but for 3-button Genesis controllers. They don't work at all with my SMS controllers. I'm still not sure whether the second button is doing anything as the only two-button game I have is Pole Position II, and I'm not sure whether I'm actually causing the car to brake by pressing the second button or if I'm just imagining it slowing down faster because I'm holding a button down.

[Andromeda Stardust]

There's a circuit inside the 7800 that pulls high pin 6 of the controller port when 2-button mode is enabled. It only works in 7800 mode, so a 2600 game cannot take advantage of that even on a 7800 console.

7800_Joystick_circuit.jpeg

Thank you so much for the insight. So when in 2-button/7800 mode, Q6 is pumping a very low impedance high logic signal into pin 6 (the FIRE input). This low-impedance current source overrides the resistors inside the 7800 joystick which are grounding the paddle lines, throwing the paddle inputs high during a button press. Otherwise in 2600 or 1-button mode, the internal 10K pullup resistor is holding pin 6 high. This high impedance source is drained by the 520 ohm resistors inside the joystick, creating a logic low input on pin 6 during a button press.

 

However, there is one potential issue I see with this schematic: If you plug in a 2600 joystick while playing a 2-button game, wouldn't the 2600 Fire button short Q8 and Q6 outputs to ground? Depending on the gain of the transistors, the 2600 Fire button would drain excessively high amounts of current potentially damaging Q8 and Q6, unless a protective shunt resistor is placed inline to prevent over-current. If Q6 or Q8 were to blow, then "2-button" 7800 games would cease to function properly on the controller port. One button games would still continue to work with either controller.

 

EDIT: I looked up the data sheet for the 2N3906. It has a maximum power dissipation of .35W which would lkely be exceeded by grounding it. A half-watt 50 ohm shunt resistor between Q6 and joystick pin 6 would limit the Q8/Q6/Fire button current to a safe but still high 100mA when attempting to use 2600 joysticks on 2-button 7800 games. This 50 ohm value is also low enough to keep the joystick inputs above a 4V high logic level when using 7800 controllers even if both buttons are pressed. IMO, not including a shunt resistor is an extremely poor design choice since the transistors would get really hot being grounded by a 2600 controller, relying on their internal resistance to limit current instead of driving a proper load. The shunt resistor would dissipate any heat generated, not Q8 and Q6. Without a shunt reistor in place, holding down the 2600 fire button for an extended period of time might even burn them up...

Edited June 9, 2014 by stardust4ever

[RevEng]

[...]However, there is one potential issue I see with this schematic: If you plug in a 2600 joystick while playing a 2-button game, wouldn't the 2600 Fire button short Q8 and Q6 outputs to ground? Depending on the gain of the transistors, the 2600 Fire button would drain excessively high amounts of current potentially damaging Q8 and Q6, unless a protective shunt resistor is placed inline to prevent over-current. If Q6 or Q8 were to blow, then "2-button" 7800 games would cease to function properly on the controller port. One button games would still continue to work with either controller.

Good eye.

 

This issue was raised in this thread. In a nutshell, two button games should check for single-button presses and disable two button mode immediately if detected.

[Andromeda Stardust]

Good eye.

 

This issue was raised in this thread. In a nutshell, two button games should check for single-button presses and disable two button mode immediately if detected.

Thank you. I majored in ELET (Electrical Engineering Technology) and I can spot problems in electrical schematics. Studying the schematic which included the 7800 side gave me an understanding of how the controller works. 100mA with a 50 or 47 ohm shunt is a hella lot of current to drain through a joystick button, but the transistors would be safe because the majority of the heat dissipated would be across the shunt resistor. Bipolar transistors are high current, low impedance devices and should never be grounded that way. I have smoked transistors in the past by accidentally shorting them.

 

A logic fault (for example bus conflicts or tying an output to ground or VCC) in a low voltage circuit generally will not cause permanent damage because the logic circuits are by nature high impedance devices and the current generated by an improper connection is generally not high enough to damage them. I have even accidentally plugged cart PCBs in backwards on multiple occasions without incident.