Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/10

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In the present section we shall only be concerned with what the circuit elements do. A discussion of how these effects can be obtained will be given in § 15. The circuit elements will be divided into valve-elements and delay elements.

(i) Delay line, with amplifier and clock gate. This is shown as a rectangle with an input and output lead

The arrow at the input end faces towards the rectangle and at the output end faces away. The name of the delay line, if any, will be written outside and the delay in pulse periods inside.

This circuit element delays the input by the appropriate number of pulse periods and also standardizes it, i.e. converts it into the nearest standard form by correcting amplitude shape and time.

(ii) The unit delay. This is represented by a triangle, thought of as a modified form of arrow

The input to output direction is indicated by the arrow. This delay element ideally provides a delay of one pulse period.

(iii) Limiting amplifier. Ideally this valve-element is intended to give no output for inputs of less than a certain standard value, and to give a standard pulse as output when the input exceeds a second standard value. Intermediate input values are supposed not to occur. If we combine this with a resistance network in which a number of input signals are combined the condition takes the form that if the input signals are s₁ s₂ . . . s_n there will be zero output unless $\alpha _{1}s1+...+\alpha _{n}s_{n}\geq \beta _{1}$ and a standard or unit output if $\alpha _{1}s1+...+\alpha _{n}s_{n}\geq \beta _{2}$. This may be simplified by assuming that the inputs $s_{1}...s_{n}$ are always either 0 or 1 and the coefficients α₁ . . . α_n either 1 or −∞ and also by requiring the integral parts of β₁ β₂ to be the same. We represent the valve element by a circle, and the inputs with a line and an arrow facing towards it, the outputs with lines and arrows facing away (Fig. 1). A coefficient −∞ (inhibitory coupling) is shown with a small circle cutting a large circle (Fig.2). The smallest total for which an output is obtained (i.e. integral part of β₁ or β₂ plus 1) is shown inside the circle, but is omitted if it is 1. This number we may call the threshold.

When we require coefficients α larger than 1 we may show more than one connection from one source. Negative coefficients may effectively be shown by means of the negation circuit which interchanges 0 and 1. Thus in the circuit of Fig.3 the valve element D will be stimulated (i.e. emit a standard pulse) if either A is stimulated or both B and C are not.

(iv) Trigger circuits. A trigger circuit, which is shown as an ellipse, differs from a limiting amplifier circuit in that once the inputs have reached the threshold so that it emits one pulse, it will continue to emit pulses until it receives an inhibitory stimulus. It is in fact equivalent to a limiting amplifier with a number of excitatory connections from itself with a delay of one unit. Thus

for instance the two circuits shown in Fig. 4 are equivalent. We show

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