A system for determining the presence or absence of data on one or more data lines including a flip-flop for each data line with the output of each flip-flop connected to a NAND gate. The NAND gate is connected through an OR gate to two output flip-flops in series. The logic of the system is such that the output of the second of the two output flip-flops provides a signal level which is high or low depending on whether data is present on all of the data lines or one or more lines has no data. <IMAGE> <IMAGE>
