Working principle: When any input in the T1 emitter is 0, the output of the Y terminal is 1; When T1 emitter input is all 1s, Y terminal output is 0. The function of and not gate is realized. When using TTL circuit, we should pay attention to the problem of input hanging. When all T1 emitters are suspended, the power supply UCC can still provide base current to T2 through the R1 and T1 collector junction, resulting in T2 and T3 conduction, T4 and D cutoff, and Y terminal output is 0.
When there are 0 inputs in the T1 emitter and the rest are suspended, the T2 and T3 cut-off, T4 and D conduction are still determined by the 0 input emitter, and the Y output is 1. It can be seen that the TTL input is suspended equivalent to 1.