SCREEN 12
VIEW (1, 40)-(638, 348), 8, 4                  'Grey box, red border
WINDOW (0, -40)-(10, 40)                      'Scale 0 to 10, -40 to 40
LINE (0, 0)-(100, 0), 1                        'Axis in blue


PRINT "Simulation of a second order open-loop system with `live input'"
PRINT "Press number keys 0 to 9 to change the input; try to follow the blue line."

dt = .00001         'Make dt larger to speed up running
x = 10             'Initial values for x
v = 0              'and v
t = 0
u = -.2            'Initial value of u until a key is pressed
PSET (0, x)
DO
   a$ = INKEY$                            'These two lines change the value
   IF a$ > " " THEN u = 4 * (VAL(a$) - 5) 'of u whenever a key is pressed

   x = x + v * dt                         'This is the simulation
   v = v + u * dt                         '
   t = t + dt
   LINE -(t, x)                           'This displays the result
LOOP UNTIL t > 10