mod

Use mod( ) to achieve periodic pattern:

surface zinTest()
{
float pulseFreq = 5;
float pulseUU = (step(0.3,mod(u*pulseFreq,1)) - step(0.6,mod(u*pulseFreq,1)));

Oi = Os;
Ci = mix(color(1,0,0),color(0,1,0),pulseUU);
}