8 bovf which is an 8 bit wide parallel by bit signal representing another component term of the signature of the block length portion of the candidate cell undergoing testing the output of translator 35 is applied to signal leads 36 1 through 36 8 translator 35 comprises eight exclusive or gates 35 1 through 35 8 signal leads connect the inputs of these gates to bus 19 for clarity of depiction these signals leads are indicated by numbers within parentheses thus the numbers within the parentheses shown below gate 35 1 indicate that its inputs are connected to signal leads 19 1 19 4 19 5 19 6 and 19 7 of bus 19 a gating arrangement 37 comprising exclusive or gates 37 1 through 37 8 combines the two above described parallel by bit signals with the youngest byte of the candidate cell being tested to generate an updated syndrome word on bus 38 a nor gate 40 generates the block boundary bb signal and causes it to equal 1 when the updated syndrome word is all zeroes a representative example of operation of this first embodiment will now be described in this representative example the record member stores in order a preliminary string of bytes that are each all zero followed by two identical cells each cell comprises 7 data.