sdn this is checked by detecting whether or not the addressed device specific table addressed via line 62 includes all zeroes therein this condition may have also been checked by determining whether or not the sdn location in the addressed device specific table included all blanks in the former case such condition in the dt pointer location of the device specific table addressed is utilized in order to provide a signal from one of the and gates 60 in response to either a yes signal from detector 48 or timing signal t4 to or gate 64 the output of which is coupled to the zeroes detector 66 if the physical address dvnn is currently associated with a logical address sdn then the output from zeroes detector 66 will be a zero on line 68 and will correspond to the yes output on block 23 in fig2 thus having determined that the dvnn address is currently associated with an sdn and accordingly the peripheral device associated with such dvnn i e the physical address is active the logical address sdn is dialed out as busy at time t5 by blanking out the status information contained for such sdn in both the device table 18 and the respective one of the device specific tables block 29 this is accomplished by the enabling of and gate 70 which couples to enable and gate.