If I want to code a signed number into 2's complement I should use a XOR gate like this:
(Consider that A is an 8-bit input)
But if I want to code it in 1's complement how should I implement it in a logic circuit?
Electrical – 1’s complement of a number using logic gates
circuit analysiscircuit-designlogic-gatesmicrocontroller
Related Solutions
It sounds like you want something more along the lines of a monostable multivibrator. For example, the 74HC123. This device produces a pulse of a set length when it receives an input.
The internal schematic for 1/2 the device (it's a dual) is shown here:
You can see that there are internal feedback paths (the cross-coupled NAND gates) that allow it to retain states while the external RC circuit operates.
You can treat this as a black box with this behavior:
For CMOS logic parts like the 74HC family, the inputs are very high impedance, and don't assume any particular level when left unconnected. They must always be connected either directly, or through a resistor, to Ground or Vcc, in order for the chip to reliably see them as Low or High. ALL unused inputs MUST be connected to Vcc or Ground, even on the unused gates in the chip.
For bipolar TTL (plain 74xx, 74LS, 74ALS) the inputs source current, so will normally appear as high when not connected, and must be connected to Ground to be seen as a low.
As the comments mention, a bypass capacitor is desireable, but the lack of one wouldn't cause a problem with the simple test circuit you are using.
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Best Answer
Just invert all the bits. That's all 1's complement is.
In fact the circuit you show, assuming you replicated it for each bit, gives you the 1's complement. An XOR gate with an input tied high is basically just an inverter.