Mindboards

Very easy to connect !

Inside the pad is a 74HC157 ( Quad two input multiplexer ).
The chip inside the control pad is a 74HC157. This is a high-speed CMOS quad 2-line to 1-line multiplexer. Basically, how this works is there are two inputs ( A and B ) for every output ( Y ). There are four groups like this. There is one select signal for the whole chip. When the select signal is low, the output ( Y ) is the same as input A. When the select signal is high, the output Y is the same as input B.

Pad connector :
The line numbers are determined as follows, looking straight at the plug on the front of the Genesis the numbers are:

1 2 3 4 5
6 7 8 9

explanation :

Line 1 Up output.
Line 2 Down output. These are the only two direct connections.
Line 3 Pin 4 of the chip. Output 1Y.
Line 4 Pin 7 of the chip. Output 2Y.
Line 5 This line carries in +5V. It is connected to the +5V bus line.
Line 6 (TL) Pin 9 of the chip. Output 3Y.
Line 7 (TH) Pin 1 of the chip. This carries in a select signal from the Genesis. This is a signal which varies rapidly and controls which input goes through the output
Line 8 Ground. This is connected to the Ground bus line.
Line 9 (TR) Pin 12 of the chip. Output 4Y.

PCF8574 schematic :



List of matérial :

- PCF8574
- 2*82k ohm resistance
- Mega Drive PAD !!!!

line numbers/PCF8574 matching :
Line 1 : Pin 5
Line 2 : Pin 6
Line 3 : Pin 7
Line 4 : Pin 9
Line 5 : Green wire of NXT
Line 6 : Pin 10
Line 7 : Pin 4
Line 8 : Black and Red wire of NXT
Line 9 : Pin 11

NXC Example Code :

#define I2Cport S1 // NXT sensor port 1
#define I2CAddr8574 0x40 // 0x40 8574 or 0x70 for 8574A

byte WriteBuf[] = {I2CAddr8574, 0xFF}; // Sent to 8574
byte ReadBuf[]; // Receive from PCF8574
int RdCnt=1; // Number of bytes to read

void Changer_Sortie(byte Actuel,int sortie,bool valeur)
{
byte masque=0x01;
byte val;
masque=0x01 << sortie;
NumOut(0,LCD_LINE1,masque);
if (valeur==false)
{
val=Actuel | masque;
}
if (valeur==true)
{
masque=255-masque;
//val=0xFF;
val=Actuel & masque;
}
WriteBuf[1]=val;
}

task main (){
SetSensorTouch(IN_2);
SetSensorTouch(IN_3);
SetSensorLowspeed (I2Cport); // Configure NXT port
I2CBytes(I2Cport, WriteBuf, RdCnt, ReadBuf);
while (true)
{
Changer_Sortie(WriteBuf[1],0,false); // Put Line 7 to high
I2CBytes(I2Cport, WriteBuf, RdCnt, ReadBuf); // read the values of 4 first button
NumOut(0,LCD_LINE1,ReadBuf[0]);
Changer_Sortie(WriteBuf[1],0,true); // Put Line 7 to low
I2CBytes(I2Cport, WriteBuf, RdCnt, ReadBuf); // reas the last 4 values
NumOut(0,LCD_LINE2,ReadBuf[0]);
}
}

Source : http://segaretro.org/Control_Pad_%28Mega_Drive%29

Cool, thank you.

I've got one of these sitting in an old box in the cupboard under the stairs. WIll have to dig it out and play with it some time soon. (If only I had more spare time.)

I build a final version :

Mounting step :

1) open the pad, remove old wires replaced by new short ones :



2) desoldering and soldering the board with PCF8574



3) soldering the old wires to the PCF8574



4) the only place to put the board with PCF8574 is under the main board so I add insulation



5) close the pad, cut the old DB9 connector replaced with NXT connector



Total cost : 5$