an address for reading the slot id the slot id thus read out is compared in a comparator 52 with the slot id of the current slot if the two slot ids do not match an interrupt generating circuit 54 causes an interrupt to the cpus on the lan control board and in the terminal equipment 10 at the same time one input of an or gate 56 is driven low when a lan control board is already installed in the slot the other input of the or gate 56 is low so that the output of the or gate is low which causes an and gate 58 to close as a result a buffer 60 provided in an outgoing path from the lan control board to the terminal equipment does not conduct even when a read write signal is output from the lan control board thus preventing the malfunctioning of the lan control board from causing an adverse effect on the terminal equipment if the slot id of the slot matches the slot id read out of the terminal equipment as a result of the comparison in the comparator 52 the output of the or gate 56 goes high and the and gate 58 is opened this allows data to flow from the lan control board to the terminal equipment if the board installed in the slot is not a lan control board the output of the or gate 56 is high regardless of the level of the.