Electronic – Simulating step function in PSpice

There might be a better way to do it (look at the VPWL and VPWL_ENH sources), but to keep this simple, try putting two sources in series, each producing half the waveform.

I don't know which version of PSpice are you using. But after adding TR and TF a small value, it works. Usually a 1 picosecond rise/fall time is extremely small with regard to the simulation time, so it closely approximates the ideal step function. Set them to 0 may cause convergence problems in some transient analysis simulations.