Transmitter-receiver device and a communication system

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 output where a status signal indicates whether the transmitter-receiver device is in said test mode and wherein the supervising unit is arranged such that when the transmitter-receiver is in said test mode, the time between said light pulses is less than 1 s.