1140 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



might, in fact be two or more such controls, forming a standard and 

 gate. The second control shown is the inhibit control. It is so biased 

 that the diode is in the high impedance, cut off condition. If let alone, it 

 has negligible influence on the behavior of the remainder of the system. 

 If a large negative pulse is impressed on it, overcoming the bias {C2) the 

 diode becomes conducting and the load remains shunted by the low 

 control circuit impedance. Whatever the condition of the other control 

 (or controls) very little current can get to the load and an output is 

 inhibited. 



Diode AND gate. 



There are several possible demands that might be put on a two control 

 gate which can be shown in the following tabulation. In this tabulation 

 a (1) means the presence of a pulse; a (0) means no pulse. 



These, and the corresponding relations when the two controls are 

 interchanged, are all the available combinations of input and output. 

 The first two are trivial — satisfied by a lack of any connection between 

 input and output. The third demands an or circuit. The fourth demands 

 an AND gate. The final case requires a new configuration but can be 

 satisfied by a combination of gates by adding the inhibiting controls. 

 Fig. 4 shows such a gate. The boxes show gates of the type of Fig. 3, 

 with the functional symbolism which is commonly used. A line with an 



