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11. Details of Logical Control.

In this section we shall describe circuits for the logical control in terms of the circuit elements introduced in § 5. It is assumed that § 5, 6 are well understood.

The main components of LC are as follows:—

(1) A short storage (like a TS) called current data CD. This contains nothing but the appropriate instruction number IN, i.e. the position of the next instruction to be carried out.

(2) A short storage called current instructions CI. This contains the instruction being or about to be carried out.

(3) A tree for the selection of a particular delay line, with a view to finding a particular instruction.

(4) Timing system for the selection of a particular minor cycle from a delay line.

(5) Timing system for the selection of particular pulses from within a minor cycle.

(6) Arrangements for controlling CA, i.e. for passing instructions on to CA.

(7) Arrangements for the continual change of the contents of CD, CI.

(8) Timing arrangements for LC itself.

(9) Starting device.

Let us first describe the starting device. This merely emits pulses synchronously with the clock from a certain point onwards, on the closing of a switch manually. The switch causes a voltage to rise and this eventually operates a trigger limiter. This starting mechanism sets a pulse running round a ring of valve elements providing the timing within a minor cycle. (Fig. 12, 13).

In order to check that this circuit is behaving we compare P32 with a signal which should coincide with it and which is obtained in another way, stimulating an SOS signal when there is failure. This forms one of the monitoring devices. We are not showing many of them in the present circuits (Fig. 14).

The timing system for the selection of minor cycles is quite simple, consisting chiefly of a ‘slow counter’ SCA, which counts up to 255 in the scale of 2, keeping the total in a delay line of length 8. The pulses counted are restricted to appearing at intervals which are multiples of eight. As shown (Fig. 15) it is counting the pulses P 10. The suppression of the outputs at P 9 prevents undesirable carries, from the most significant digit to the least.

The information in CD and CI being in dynamic (time) form is not very convenient for control purposes. We therefore convert this information into static form, i.e. we transfer it on to trigger circuits (Fig. 16).

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