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The first gate has the input inverted then passed through the buffer while for the second the input is buffered then output is inverted. Both of them are VALID NOT gate symbols.
The first one protection is done at the input while for the second protection is done at the output. Inversion of signals can be used to protect against signals with higher values than the expected (usually +5V).
A CD4069 is a package of six Cmos Logic Inverters. They are not called Not Gates.
They are an N-channel Mosfet and a P-channel Mosfet connected as a push-pull inverter with input protection diodes to each supply.
The datasheet shows the normal way of drawing a Logic Inverter.
mabauti,
Functionally there is no difference. The two different symbols are used for clarity. For example, if the input is an active low signal, it is clearer to use the symbol on the left to indicate that when the input is active, a logic "1" will appear on the output. The same reasoning is used for alternate symbols for other gates. For example, the standard symbol for a NAND gate is an AND gate with a bubble on the output. An equivalent symbol is an OR gate with bubbles on the inputs. If the output is active high, then it makes sense to use the alternate symbol to indicate that the output is active when a "0" is applied to either input. On the other hand, if the ouptut is active low, then it makes sense to use the standard sysmbol to indicate that the output is active when a 1 is applied to all the inputs (input a AND input b AND input c ...). There are times when these rules do not apply, in which case, the standard symbol is usually used.
Regards,
Kral
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