80 is not obtained by the fail safe module during the 480 revolutions of the main shaft the output line of the fail safe module remains de activated and the knitting machine remains energized in this case fail safe module is reset to zero counts by the runner length so as to continue its monitoring of the up down counter the fail safe module 120 and the rate multiplier 28 are initially reset to zero when 34 start 34 line 125 is activated causing a reset module 127 to generate a reset pulse on output lines 129 and 131 the start line is activated when automatic control by yarn runner length controller 20 is desired therefore any binary number previously stored in the fail safe module is undesired since it is unrelated to the current yarn control operation the present invention also includes an external error display incorporating an error counter decoder 126 that receives all electrical pulses on lines 68 and 72 and generates one electrical pulse on output line 128 or 130 for every 10 electrical pulses respectively received since every pulse on line 68 or 72 represents a deviation between the actual and desired runner length of one hundredth of one inch each output pulse generated by error counter decoder 126 represents a deviation in the actual runner length of one tenth of one inch these output lines are connected to an error counter 132 where they are algebraically summed during every 480 revolutions of the main shaft the binary number accumulated by the error counter after a reset signal is received from runner.