Danny's Lego Mindstorms® RCX I/O Multiplexer

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You can call it RCX Output Expander.

You can call it RCX Output Extender.

You can call it RCX Output Doubler.

Because it can be used also for inputs, I called it

"RCX I/O MUX": RCX input/output multiplexer.

 

It's incredibly useful and easy to use in programs.

 

 

 

 

 

 

 

Pardon me, it's really ugly. I didn't have enough LEGO electric plates to sacrifice.

So I must use those horrible screws; I must admit that working on them ( fitting in the 4x6 plate, cutting) was terrifying but, at the end,  they allow a good electric contact.

The MUX must be connected to all RCX output ports to obtain 6 independent outputs.

Port A controls motors 1, 2 and 3.

Port C controls motors 4, 5 and 6.

Port B make the channel selection:

No power: motors 1 and 4 move as you want;

Negative power: relay2 is on, motors 2 and 5 move as you want;

Positive power: relay1 is on, motors 3 and 6 move as you want.

In any state, when a pair of motors work, the other are disconnected (no brake function is permitted).

 

 

 

 

 

 

 

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After many years, here's the schematic:

 

 

With simple programming, you can get a good switching rate, so it will seem all motors are on.

Here's an example, that makes all 6 motors turn both directions in cycle:

#define p Wait(10)
#define m 23
#define Fwd1(x) { OnFwd (OUT_A); Wait(x); Off(OUT_A);}
#define Rev1(x) { OnRev (OUT_A); Wait(x); Off(OUT_A);}
#define Fwd2(x) { OnRev (OUT_B); OnFwd (OUT_A); Wait(x); Off(OUT_A);Wait(5);Off(OUT_B);}
#define Rev2(x) { OnRev (OUT_B); OnRev (OUT_A); Wait(x); Off(OUT_A);Wait(5);Off(OUT_B);}
#define Fwd3(x) { OnFwd (OUT_B); OnFwd (OUT_A); Wait(x); Off(OUT_A);Wait(5);Off(OUT_B);}
#define Rev3(x) { OnFwd (OUT_B); OnRev (OUT_A); Wait(x); Off(OUT_A);Wait(5);Off(OUT_B);}
#define Fwd4(x) { OnFwd (OUT_C); Wait(x); Off(OUT_C);}
#define Rev4(x) { OnRev (OUT_C); Wait(x); Off(OUT_C);}
#define Fwd5(x) { OnRev (OUT_B); OnFwd (OUT_C); Wait(x); Off(OUT_C);Wait(5);Off(OUT_B);}
#define Rev5(x) { OnRev (OUT_B); OnRev (OUT_C); Wait(x); Off(OUT_C);Wait(5);Off(OUT_B);}
#define Fwd6(x) { OnFwd (OUT_B); OnFwd (OUT_C); Wait(x); Off(OUT_C);Wait(5);Off(OUT_B);}
#define Rev6(x) { OnFwd (OUT_B); OnRev (OUT_C); Wait(x); Off(OUT_C);Wait(5);Off(OUT_B);}

task main () //general motor test1
{
  while (true)
  {
  Fwd1(m);
  Rev1(m);
  p;
  Fwd2(m);
  Rev2(m);
  p;
  Fwd3(m);
  Rev3(m);
  p;
  Fwd4(m);
  Rev4(m);
  p;
  Fwd5(m);
  Rev5(m);
  p;
  Fwd6(m);
  Rev6(m);
  p;
  p; 
  }
}
 

As you could notice, the #define header is very easy to export, and so programming my MUX is really simple (pay attention to connect the MUX properly).

 

When I will get some electric plates, I'll do a new MUX, at least... more nice to look!

Also, using solid state relays, the MUX would save much power. If someone wants to develop this idea, please let me know.

 

     

Spring 2004, update April 2007