Why are TTL and CMOS incompatible?how to interface them in circuits?
TTL and CMOS have a difference in the input voltage specified for high and low logic levels. TTL uses bipolar transistors which are basically current driven. Driving its input to logic low requires the driving circuit to sink a current to pull its voltage below the low level specified. CMOS is a high impedence input and therefore purely voltage controlled. On the output side, TTL circuits usually have a much greater capacity to sink current than to source current mainly because they normally drive other TTLs which require their input to be driven by a current sink. CMOS putputs on the other hand swing virtually rail to rail because they normally drive high impedence inputs of other CMOS logic. In practice, in most circumstances, the logic levels are such that it is possible to drive CMOS from TTL and vice versa. Only in rare circumstances this may not be possible.
