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SR latch (bistable multivibrator from two transistors)

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Open this experiment in the circuit simulator

Objective

Build a memory cell from two cross-coupled NPN transistors; short set and reset pulses flip it and it remembers the state.

Procedure

Run and watch Q1 and Q2 on the oscilloscope: the SET pulse at 10 ms and the RESET pulse at 25 ms flip the latch.
Replace the pulse sources by switches (Controls tab) to set and reset by hand.

Theory

Each collector drives the other transistor's base through a resistor, so only two stable states exist: Q1 on / Q2 off or the opposite. A pulse on one base forces the change; positive feedback regenerates it and the circuit keeps it after the pulse ends.

Questions to answer

Why is S = R = 1 not used?

Components used

Battery / DC source, 2 × Ceramic Capacitor 0.1uF (104), 2 × Function generator (square), Oscilloscope (2-channel), 6 × Resistor 220 ohm 1/4W, 2 × Transistor BC547 (NPN)

About the model

Ideal wires; component models as listed in the catalogue (no parasitics, no temperature drift). Built without a breadboard so the schematic stays readable.