halt one stepper until other has revolved once, then continue simultaneously

hi everyone,
i 2 motors following:

1. stepper1 , stepper2 turn simultanously 6400 steps
2. stepper1 stops until stepper2 has completed 3200 steps
3. stepper1 , stepper2 turn again simultaneously 6400 steps
4. stepper1 stops until stepper2 has completed 3200 steps
3. stepper1 , stepper2 turn again simultaneously 6400 steps.

all of should happen when send "1" serial communication. using accelstepper library (http://www.airspayce.com/mikem/arduino/accelstepper/).
my code far, including serial communication , other important chunks have stay in  is below. how , have modify code? have insert if-loop in  if (letter=='1')  loop? have tried around different loops , commands have not been able figure out.


any advice appreciated!
thanks,
isa

code: [select]




#include <accelstepper.h>




// define 2 steppers , pins use
accelstepper stepper1(1, 9, 8);
accelstepper stepper2(1, 7, 6);


// define variable record when instruction arrived.
unsigned long instructiontime = 0;


void setup() {
  
   serial.begin(9600);
   stepper1.setmaxspeed(8000);
   stepper2.setmaxspeed(8000);
   stepper1.setacceleration(100000000);
   stepper2.setacceleration(100000000);
}




void loop()
{
    if (serial.available() > 0)
    {
      char letter =  serial.read();
      if (letter=='1')
      {
 
         stepper1.move(-6400);
          stepper1.setspeed(3350);


         stepper2.move(6400);
         stepper2.setspeed(3350);


         instructiontime = millis();
      }
      else if (letter=='2')
      {
      
         stepper1.move(-6400);
          stepper1.setspeed(3350);


         stepper2.move(6400);
         stepper2.setspeed(3350);


         instructiontime = millis()+500;
      }


  // run if either stepper1 or stepper2 has distance left go.
    if (stepper1.distancetogo() != 0 || stepper2.distancetogo() != 0)
    {
      // run stepper1 regardless of time
      stepper1.runspeedtoposition();


      // run stepper2 if time more 500ms when the
      // instruction received
      if (millis() >= instructiontime)
      {
        stepper2.runspeedtoposition();
       }
  
if (stepper1.distancetogo() == 0 && stepper2.distancetogo() == 0)      
 
    {
       serial.print('x');
     }  
   }
 }

}

as far understand 2 lines

code: [select]

stepper1.runspeedtoposition();

and

code: [select]

stepper2.runspeedtoposition();

should not are. should both @ bottom of loop() , not within if statements called @ every iteration of loop() regardless of else goes on.

try first.

...r

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