Patent ID: 8766221

Claim:
A bidirectional communications system comprising: a bidirectional transmission medium coupled between a master device and a slave device and having a high logic state associated with no data transmissions between the master device and the slave device; a first data transmission path along the transmission medium from the master device to the slave device via an encoder circuit coupled to the master device and a first opto-isolator circuit, the first data transmission path associated with a first low logic state generated by the master device; a second data transmission path along the transmission medium from the slave device to the master device via a second opto-isolator circuit and a decoder circuit coupled to the master device, the second data transmission path associated with a second low logic state generated by the slave device; a diode coupled between the first opto-isolator circuit and the slave device and arranged to prevent activation of the second opto-isolator circuit in response to a first low logic state generated by the master device, wherein said encoder circuit is arranged to prevent activation of the first opto-isolator circuit in response to a second low logic state generated by the slave device; wherein the first low logic state is associated with a voltage on the transmission medium having a value less than a first predetermined threshold voltage; wherein the second low logic state is associated with a voltage on the transmission medium having a value less than or equal to a second predetermined threshold voltage, the second threshold voltage being greater than the first threshold voltage; the encoder circuit further comprising a comparator having a first input coupled to the master device via the transmission medium and a second input coupled to receive a reference voltage approximately equal to the first threshold voltage; an output of the comparator is coupled to the base of a switch, the comparator output arranged to turn on the switch when the comparator detects a voltage at the first comparator input of less than the first threshold voltage; and wherein turning on of the switch provides a current path arranged to activate the first opto-isolator circuit.