Abstract: A device for testing line connections at a telephone exchange includes a combined generator and comparator for generating a bit sequence of pseudo-random type and for comparing a received bit sequence to a generated bit sequence. A generated bit sequence is through a line connection forwarded to a telephone set and back again to the generator and comparator, where the received sequence is compared to the forwarded one. The number of bit errors occurring in the transmission is a measure of the transmission quality of the connection. A selector device selects one of several line connections which is to be tested. Only one testing device is required for testing a number of line connections which through the same pair of one serial output line and one serial input line are connected to a telephone exchange.