resistor circuits 110 fig3 illustrates an embodiment of an impedance termination circuit 300 of the present invention circuit 300 is an example of programmable resistor circuits 110 in fig1 circuit 300 includes field effect transistors 301 302 303 304 305 306 and resistors 307 u2013 308 n channel transistor 302 and p channel transistor 301 form a first pass gate that is controlled by signal bit 1 and its compliment bit 1 _b n channel transistor 304 and p channel transistor 303 form a second pass gate that is controlled by signal bit 0 and its compliment bit 0 _b n channel transistor 306 and p channel transistor 305 form a third pass gate that is controlled by enable signal en and its compliment enb signals bit 0 bit 1 bit 0 _b bit 1 _b en and enb can be generated by memory or logic elements on a field programmable gate array pass gate 305 306 is turned on to enable impedance termination circuit 300 and turned off to disable circuit 300 transistors 301 u2013 304 are coupled across both terminals of resistor 308 as shown in fig3 therefore transistors 301 u2013 304 cannot bypass the current paths through transistors 305 u2013 306 thus when transistors 305 306 are both off current flow between pin in and common mode driver 111 is blocked when transistors 305 306 are on current can flow between pin in and common mode driver circuit 111 transistors 301.