Decoder

The decoder IC can be viewed with the analogy as shown above. The input code (ABC) is analogous to the delivery instruction written on the package - the name/address/date etc. and the outputs (Y0:Y7) are analogous to dispatching the delivery man. The enable input (G1, G2A, G2B) are analogous to the cash payment. Once received, the delivery service will be enabled. Note that, when enabled, for each code there will only be one output activated.

For example, when the input code (written on the package) is "110", and the enable inputs G1G2aG2b = "100", the decoder will be enabled and activates the output signal on output pin Y6, since binary "110" = decimal 6.

Active High/Active Low

Some students find it difficult to understand the concept of "active high" or "active low". They are just different ways of control, depending on the requirement of the next stage. As an analogy, you can either ask "those who agree put up your hand" or "those who disagree put up your hand", although the purpose of that is more psychological than technical.

Hence, for the decoder 74LS138, only the output that is activated goes low, the other outputs stay high (inactive). Thus, we can construct the truth table of this decoder:




[more: timing diagram]