3. An input stage according to claim 2, further characterized by a plurality of bistable means to selectively switched to a one stable condition or a different other stable condition including a keyword character bistable means, a spacer signal bistable means, and a terminal signal bistable means, one of each of said bistable means operatively associated with each of said stage means, said first stage means operable to set said keyword character bistable means to a one stable condition upon identifying a keyword character in said keyword character signal, said second stage means operable to check that said keyword character bistable means is in a one stable condition and said spacer signal bistable means is in a different other stable condition before setting said spacer signal bistable means to a one stable condition, and said third stage means operable to check that both said keyword character bistable means and said spacer signal bistable means are in a one stable condition before setting said terminal signal bistable means to a one stable condition.
3. An input stage according to claim 2, further characterized by a plurality of bistable means to selectively switched to a one stable condition or a different other stable condition including a keyword character bistable means, a spacer signal bistable means, and a terminal signal bistable means, one of each of said bistable means operatively associated with each of said stage means, said first stage means operable to set said keyword character bistable means to a one stable condition upon identifying a keyword character in said keyword character signal, said second stage means operable to check that said keyword character bistable means is in a one stable condition and said spacer signal bistable means is in a different other stable condition before setting said spacer signal bistable means to a one stable condition, and said third stage means operable to check that both said keyword character bistable means and said spacer signal bistable means are in a one stable condition before setting said terminal signal bistable means to a one stable condition. 5. An input stage according to claim 3, further characterized by said second stage, on setting said spacer signal bistable device to a one stable condition, being operable to reset said terminal signal bistable means and said keyword character bistable means to the other stable condition.