SAMPLES = 20
TOLERANCE = 0.01
TIMEOUT = 10.0

intensity =  ChannelDouble("intensity", "SARFE10-PBPG050:PHOTON-ENERGY-PER-PULSE-US")
intensity.initialize()

gap = Channel(GAP + ":GAP_SP", alias = "gap")
readout = Channel(GAP + ":GAP")

av = create_averager(intensity, SAMPLES, interval = -1, name = None)

def before(position, scan):    
    caput(GAP + ":GO", 1)  
    start = time.time()  
    while abs(readout.read() - gap.read()) > TOLERANCE:        
        time.sleep(0.1)
        if time.time() - start > TIMEOUT:
            raise Exception ("Timeout waiting gap change")    

ret = lscan (gap, av, 17.5, 19.5, 40, latency=10.0, before_read=before)