Abstract: A circuit for sensing network speed in a test instrument for local area networks (LANs) is provided. After coupling the test instrument to the LAN of unknown speed, a link pulse is found among the twisted-wire pairs so that the test instrument is configured for the transmit and receive pairs. The link pulse is then coupled to a comparator that compares the received voltage against a predetermined reference voltage level to develop a trigger signal. The trigger signal is received by a timer circuit which measures time values from the link pulse. The time values are collected by a microprocessor are then compared against a decision value to reach a decision whether the test instrument is coupled to a link running 10BASE-T protocol, 100BASE-T protocol, or a 10/100 dual-speed device. After the decision has been determined, the appropriate chipset is then powered up and coupled to the LAN, thereby minimizing battery drain and preventing errors from being introduced in the LAN by the test instrument.