device to select u or v as the carry out signal applied to its 34 y 34 output terminal in particular u is selected as the carry out signal if the dedicated carry in signal is logic 0 and v is selected as the carry out signal if the dedicated carry in signal is logic 1 programming fce 2008 to produce logic 1 allows the c input to select which of nand gates 2010 and 2014 will produce logic 0 and which will produce logic 1 if the c input is logic 1 then nand gate 2010 produces logic 0 and nand gate 2014 produces logic 1 due to the inversion of c by inverter 2012 on the other hand if the c input is logic 0 then nand gate 2010 produces logic 1 and nand gate 2014 produces logic 0 if nand gate 2010 produces logic 0 and nand gate 2014 accordingly produces logic 1 then plc 2016 selects v as its output and plc 2018 also selects v as its output on the other hand if nand gate 2010 produces logic 1 and nand gate 2014 accordingly produces logic 0 then both of plcs 2016 and 2018 select u as their outputs from the last two sentences it will be seen that the c input effectively selects u or v as the output of both of plcs 2016 and 2018 it then does not matter what the carry in input signal on lead 54 is because the 34 a 34 and 34 b 34 inputs to plc 2020 are both the same the u or v signal selected by both of plcs 2016 and 2018.