Patent ID: 7212740

Claim:
A transmitter-receiver device comprising a receiver unit arranged to via a first optical conduction path receive light and optical signals and comprising a first output which indicates whether the receiver unit receives light, a transmitter unit arranged to on a second optical conduction path transmit light and optical signals and comprising a first input which controls whether the transmitter unit shall transmit light, a supervising unit with a second input connected to said first output and a second output connected to said first input and arranged to via said second output prevent the transmitter unit from continuously transmitting light when the supervising unit via the second input detects that the receiver unit does not receive light, wherein the supervising unit is arranged to, when it detects that the receiver unit does not receive light, change to a test mode where the supervising unit controls the transmitter unit to intermittently transmit short light pulses on said second optical conduction path, wherein the supervising unit is arranged with a third out-put where a status signal indicates whether the transmitter-receiver device is in said test mode, wherein the transmitter unit comprises a transmitter circuit comprising a light source and arranged to operate said light source to transmit optical communication signals in response to electric input signals from a first and a second circuit point between which circuit points a balanced electric input signal is intended to be present, wherein said transmitter circuit comprises a first circuit branch which extends from said first point via a third point to at least a fourth point and wherein said transmitter circuit comprises a second circuit branch which extends from said second point via a fifth point to at least a sixth point, wherein said light source is connected between said third and fifth points, wherein the components which are positioned on said first and second circuit branches are chosen such that the transmitter circuit is formed with a symmetry which is such that under normal operation conditions a balanced drive voltage is the case between said third and fifth points, which balanced drive voltage only depends on the voltage difference between said first and second points, wherein also the modulation current through the light source only depends on said voltage difference.