Method for electing an active master device from two redundant master devices

Method for electing an active master device from two redundant primary and secondary master devices, said primary and secondary master devices being directly connected to a fieldbus and linked together by a terminal cable and each having an active operating mode and a standby mode with active monitoring, and also having one and the same interface,

• wherein each of said primary and secondary master devices also has one and the same state machine comprising means for designating one of said primary and secondary master devices as the active master device, and the other master device as the standby master device with active monitoring,
• wherein the terminal cable comprises intrinsic means for establishing a directional relation of the primary master device to the secondary master device,
• wherein the primary master device and the secondary master device each comprise a primary input, a primary output, a secondary input and a secondary output and in that a terminal cable links the primary output of the primary master device to the secondary input of the secondary master device and the secondary output of the secondary master device to the primary input of the primary master device,
• wherein, when they start up, the two, primary and secondary, master devices signal, in an identical manner, that they are capable of operating on the interface by transmitting a nominal signal on their primary output and secondary output, the directional relation established by the terminal cable being configured such that the secondary master device (10, 20) detects the presence of the primary master device, if it is capable of operating, on its secondary input, and the primary master device detects the secondary master device, if it is capable of operating, on its primary input;
• and wherein, for the primary master device having detected on its primary input the secondary master device, the state machine of the primary master device proposes the active state, and for the secondary master device having detected on its secondary input the primary master device, the state machine of the secondary master device proposes the standby state with active monitoring.