[WINGEN] - Data generation with IRT

Hello everybody,
I have a simulation study and I try to generate irt data with a program called WINGEN. I want to generate dichotomously scored, normal distributed and unidimensional data.
And I my number of examinees are 10000, number of items 40 and my irt model is 2PLM. WINGEN asks me about the parameters a and b. The distribution, mean and std of the parameters. As seen I enter normal distribution option for a and b and for a parameter mean=1 std=0.05, for b parameter mean=0 std=0.05. But these way, my data did not unidimensional. I want to ask you, for a unidimensional data, what is the a and b paramaters? Please help me, I'm very new in IRT
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