a switch machine operates a track switch under radio control via a switch controller . the switch controller may be given an identifying address unique to it which distinguishes the switch machine from other switch machines in the railway territory . in addressing or programming the switch machine , a portable terminal is used . the controller is enabled by a manually actuable switch to accept identifying addresses which is stored in electrically erasable programmable read - only memory . whenever a message is received over the radio link , the identifying address is read out of memory . in order that the identifying address is vital and to guard against any errors which change the identifying address and which can cause the controller to effectuate operation of the track switch in front of an oncoming train or when the train is over the switch , the address is stored in the eeprom by encoding the identfying address into unique multi - bit check words . whenever a request to read the address is received by the switch controller , an attempt is made to write check value words , the value of which is equal to the difference between the value of the unique check words and the check words . a check value word replaces a unique check word in the event of a failure in the eeprom or its addressing logic . the check value is also read into a check word register . the content of the check word register and the location in eeprom for the check word under test is read into exclusive or gates . if there is a failure , the output of the array is a word having a value of zero , whereas if there was no failure the result of the exclusive or operation will be a word having the value of the check word . these check words are used in providing a vital output message for controlling the track switch and to provide and acknowledge message for transmission over the radio link to the central office of locomotive which communicates with the railway track switch controller . the switch is powered only when a vital output representing a desired switch operation is called for over the radio link .