and the survey tool responds by transmitting the commanded data words upwardly to the surface equipment 3 both command and data words are transmitted as serial digital words in a bit by bit serial form using the standard rs232 format for serial digital data 4 the serial digital bit stream is encoded onto the wireline by frequency shift keying fsk such that a digital one bit is represented by one carrier frequency and a digital zero bit is represented by another carrier frequency referring now to fig1 analog voltages from the tool sensors and electronics are supplied on leads 112 to the analog data converter board 103 for multiplexed analog to digital conversion also the analog output signals of the angular rate sensor g 16 and the first acceleration sensor a1 17 are supplied on leads 113 to the v f voltage to frequency converter board 104 for conversion to digital representations of the time integral of each signal the integration and conversion of signals within board 104 are carried out by well known means by using a voltage to frequency converter and a digital counter within board 103 the analog signals are multiplexed in time sequence and converted to digital output by a well known successive approximation register parallel output analog to digital converter the outputs at boards 103 and 104 are available to the digital tool data bus 110 and are placed on the bus and presented to the communications board 102 at the times that that board wishes to receive such data also the communications board 102 has a digital command bus 111 by which it can transmit command.