The generator is based on a Type 4006 quadruple shift register, IC1. Two of the shift registers are four bits long, the other two, five bits: a total of 18 bits. Only the clock is common to the registers: all inputs and outputs are separate. Of the four-bit registers, only the fourth bit is available as an output; of the five-bit ones, the fourth and fifth bit.
Feeding back the outputs of the four registers to the inputs results in a (pseudo) random generator. The outputs of the four XNOR gates serve as the outputs of the generator. The advantage of this design is that the four bits do not only form a random sequence of numbers but also that each of them has a different pattern. This is in contrast to the shift register itself, where the levels are shifted from one output to another. This generator may, therefore, be used to produce random four-bit numbers or four random digital signals that are not related. The BASIC program in Fig. 2 enables the Operation of the shift register to be simulated. Line 140 states how long the register is (MAX) and how many output bits there are (BMAX). Online 150 all bits of the ‘shift register’ (array A) is set to zero. If, as is usual in practice, a random starting state is required, this line will have to be rewritten. On lines 190-220 the XNOR functions may be seen. Any changes in the way these gates are connected should be indicated here. In that case, it may also be necessary to change lines 290 and 300. The program contains only 390 steps (the monitor screen is then full) of the 218 = 262 144 possible.
In the diagram, a clock generator in the form of a Schmitt trigger (IC3a) has been added. It is, however, possible to use any suitable clock at the clock input.