20 the linear motors 45 48 and 56 and the rotary motors 32 in 52 by appropriate operation of these valve the operator can control yaw of the casing with respect to the body the position of the cutter head in three dimensions and the torque under load or speed no load of the cutter head there are two types of visual display in the control unit a video display and an analog bargraph display the video display is a conventional television monitor connected directly to the video camera by means of the conductors the bargraph display circuit shown in fig7 comprises an analog to digital converter a ttl decorder a simple clock circuit and an input amplifier the circuit board used to drive the bar graph display also contains interface circuitry to connect a digital panel meter the a d converter is a national semiconductor adc0820 which provides eight bit binary output the reference voltage for the converter is variable allowing the operator to set the range of the bargraph the output of the a d converter is decoded by a 74154 chip which drives the 16 front panel led 39 s the clock is used to trigger the a d converter and to activate the decoder when the data is valid a 555 timer chip is used in an astable configuration with a frequency of 83 hz and a duty cycle of about 50 the input circuit has a gain of 2 which inverts the signal and compensates for losses in the cable.