could otherwise occur in an example embodiment the direction detection flip flop 121 is provided by a 74hc74 integrated circuit the direction detection inverter 122 the first transmit enable inverter 141 the second transmit enable inverter 142 the third transmit enable inverter 143 and the fourth transmit enable inverter 144 are provided by a 74hc540 integrated circuit the transmit enable buffer 149 is provided by a 74hc126 integrated circuit the light emitting diode 161 is an if e91a from industrial fiber optics the pnp transistor 162 is a 2n2907 the first transmit resistor 163 is 220 u03c9 the second transmit resistor 164 is 3 9 k u03c9 and the third transmit resistor 165 is 4 7 k u03c9 the phototransistor 171 is an if d92 from industrial fiber optics the npn transistor 172 is a 2n2222a the first receive resistor 173 is 220 u03c9 and the second receive resistor 174 is 2 2 k u03c9 it should be recognized that many variations are also possible those of skill will appreciate that the various illustrative blocks and modules described in connection with the embodiments disclosed herein can be implemented in various forms some blocks and modules have been described above generally in terms of their functionality how such functionality is implemented depends upon the design constraints.