Patent ID: 7764479

Claim:
A communication circuit for a load control device, the load control device operable to be coupled to a communication link having two conductors, the load control device operable to transmit a digital message by alternating the communication link between an idle state in which a link voltage is developed across the conductors on the communication link and an active state in which the conductors of the communication link are substantially the same electrical potential, the communication circuit comprising: a receiving circuit coupled between the conductors of the communication link, the receiving circuit operable to conduct an idle current when the communication link is in the idle state; and a transmitting circuit coupled between the conductors of the communication link, the transmitting circuit comprising: an optocoupler having an input and an output, the output comprising a phototransistor and operable to become conductive when the input is driven with an input current, the output operable to conduct the idle current immediately after becoming conductive; a voltage clamp coupled across the output of the optocoupler, the voltage clamp operable to clamp the voltage across the output of the optocoupler in the idle state, wherein the voltage clamp limits the voltage across the output of the optocoupler to approximately two diode drops; a controllably conductive device responsive to the output of the optocoupler to electrically couple the conductors of the communication link together when the output of the optocoupler is conductive and to stop electrically connecting the conductors of the communication link when the output is non-conductive; and a current source coupled to the phototransistor of the optocoupler, the current source operable to generate a source current when the controllably conductive device is electrically coupling the conductors of the communication link together; wherein the output of the optocoupler is operable to conduct a first portion of the source current and the voltage clamp is operable to conduct a second portion of the source current in the active state, such that the phototransistor of the optocoupler is maintained in an active region of operation.