R help - Dec 2012 - suggestions for nls error: false convergence

Hi, 
  I'm trying to fit some data using a logistic function defined as 

y ~ a * (1+m*exp(-x/tau)) / (1+n*exp(-x/tau) 

My data is below: 

x <- 1:100 

y <- c(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 
0,0,0,0,0,1,1,1,2,2,2,2,2,3,4,4,4,5, 
5,5,5,6,6,6,6,6,8,8,9,9,10,13,14,16,19,21, 
24,28,33,40,42,44,50,54,69,70,93,96,110,127,127,141,157,169, 
178,187,206,216,227,236,238,244,246,250,255,255,257,260,261,262,266,268, 
268,270,272,272,272,273,275,275,275,276) 

My first attempt was to use nls as below: 

    d <- data.frame(x=x, y=y) 
    model <- nls(y ~ a * (1+m*exp(-x/tau)) / (1+n*exp(-x/tau)), data=d, 
    start=list(a=277,m=100,n=101,tau=10), 
    algorithm='port', trace=TRUE, 
    control=nls.control(maxiter=5000, minFactor=1/2048)) 




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