Method for encoding/decoding a binary signal state in a fault tolerant environment

Abstract

A method for use in a fault tolerant environment for assuring that devices within the environment switch between primary and back-up systems in response to remotely generated control signals. In one embodiment, the inventive system uses a binary code in the form of a pair of different frequency signals, i.e., a binary zero is represented by one frequency and a binary one is represented by another frequency. The signals may be continuous or may be sent in timed bursts. At the individual devices, such as the aforementioned line cards, a receiver is provided to detect the presence of the signals. Since the line cards already have receivers to detect the binary signal, modification to detect a frequency signal requires the addition of minimal components. The receiver also includes circuitry for reporting the status of the card and such circuitry can be used to report to the remote controller whether the signals are reaching the line card. Alternately, the signals could be sent in burst format using a single frequency in which the number of bursts could indicate a binary one or a binary zero. For example, if N bursts are received in some unit of time, that could be indicative of one binary state. If 2N burst are received in the same unit of time, that could be indicative of another binary state. In another form, the receiver could be programmed to look for N changes of frequency per unit of time to indicate one binary state and 2N changes of frequency could be indicative of another binary state.

What is claimed is:  
     
         1 . In a fault tolerant system having a primary device and at least one redundant device, and further having a remote controller for monitoring the status of the devices and providing a control signal to switch from the primary device to the redundant device upon detection of a failure in the primary device, a method of determining validity of the control signal comprising: 
 continually sending a first signal having a predetermined varying characteristic to at least the primary device from the remote controller; and    sending a second signal having a different varying characteristic to at least the primary device from the remote controller upon detection of a failure of the primary device.    
     
     
         2 . The method of  claim 1  wherein the varying characteristic comprises a signal frequency.  
     
     
         3 . The method of  claim 2  wherein the signal frequency is a continuous wave frequency.  
     
     
         4 . The method of  claim 2  wherein the signal frequency is a selected number of signals per unit of time.  
     
     
         5 . The method of  claim 1  wherein the at least one device includes a receiver responsive to the to the first signal for acknowledging receipt thereof to the remote controller.