Copyright Graham Knott 1999
| The "pull down" resistors
hold the cathodes of the diodes low, making the diodes forward biased.
The output at the anodes is held low, Tr1 base is low and its collector high. The base of Tr2 is high and its collector low. A and B and C must all go high together, to reverse bias the diodes, and make the base of Tr1 high. The collector of Tr1 goes low, together with the base of TR2. The collector of Tr2 goes high. The output has gone high because A and B and C have gone high. |
Copyright Graham Knott 1999