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UC1843B-SP: Rt/Ct not oscillating

Part Number: UC1843B-SP

I am making a boost converter in Orcad, but the Rt/Ct is not Oscillating (only a flat line at 5V). I have 25v VCC, Gnd connected and RT/CT connected exactly like below with R= 1.73k and C= 1200pF. I am measuring at the RT/CT pin. Is it the model or am I doing something incorrectly?