A Simple Character Generator

E. P. Borovik, M. S. Ostrast, V. M. Tarasov

(Presented by the scientific seminar of the AFE sector)

The widespread use of computers in experimental physics imposes ever new requirements on the means of human–computer communication, oriented toward the use of cathode-ray tubes as display devices. Recently, in a number of physics institutes in the country, displays have appeared that are capable of presenting to the operator graphical as well as alphanumeric information.

A display based on the use of the BESM-4 and ELT 31L033V as the display processor was created and has been operated during the past year at the Research Institute of Nuclear Physics at TPI. It should be noted that the created display has found application in the following range of problems:

a) processing spectra obtained as a result of experiments in low-energy physics;

b) use of an oscilloscope when calculating elements of accelerator technology.

The solution of the first class of problems is based on the use of elements of the POFI-2 system [1], as well as a set of geometric programs [2].

In solving problems of both classes, there is an acute need to increase the screen filling coefficient \(k_{\mathrm{scr}}\)

\[ k_{\mathrm{scr}}=\frac{j_{\max}}{j_0}; \tag{1} \]

where \(j_0\) is the information capacity of the ELT screen for the selected method of image formation;

\(j_{\max}\) is the maximum amount of information that can be displayed on the screen during a time equal to the regeneration period before flicker appears on the ELT screen.

The maximum amount of graphical information \(j_{\max}\) in display systems is determined mainly by the characteristics of the display processor

\[ j_{\max}=t_{\mathrm{char}}\cdot N + t_{m\mathrm{gr}}\cdot n_m, \tag{2} \]

where \(t_{\mathrm{char}}\) is the time for forming a service character;

\(N\) is the number of characters;

\(t_{m\mathrm{gr}}\) is the time for forming one point of graphical information;

\(n_m\) is the number of points.

From formula (2) it is evident that, as the time for forming characters decreases, the time required for outputting graphical information increases. The tendency toward maximal use of the computer regeneration time for outputting graphical information leads to a hardware implementation of the character generator.

The functional diagram of the character generator developed by the authors is shown in Fig. 1.

reset pulse

\(RV\) — computer output register
\(DSH\) — matrix decoder
\(K_{1-i}\) — matrix selection keys
Start Gen. — triggering generator
\(DU\) — pulse distributor
\(M_{1-i}\) — diode matrix
\(\Phi_x\) — former of voltage \(U_x(r)\)
\(\Phi_y\) — former of voltage \(U_y(r)\)
\(U_z\) — modulator

Fig. 1. Functional diagram of the character generator

\(\rho U\)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

to the former \(U_x(t)\)

to the former \(U_y(t)\)

to the body-code \(T_2\)

to the beam brightness modulation circuit

\(U_x(t)\)

\(U_y(t)\)

Fig. 2. Method of wiring the diode matrix and obtaining stepped voltage.

The character code from the output register (OR) of the BESM-4 computer is fed to a matrix decoder (D), which produces the control signal for selecting the required character key \(K_i\). Simultaneously with the arrival of the character code from the computer, the character attribute is received; it is fed to the character-generator start circuit (start gen.).

After \(12\ \mu\text{s}\) (the time for presetting the beam to the initial point of the screen), a pulse from the start circuit is fed to the pulse distributor (PD), which issues a sequence of pulses uniformly distributed in time, each \(2\ \mu\text{s}\) long. To form a complex character with a \(5 \times 6\) coordinate grid, 18–20 pulses from the PD are required (there is a fundamental possibility of using, as the pulses obtained from the PD, the computer clock pulses and of eliminating the PD from the functional circuit of the character generator). The sequence of pulses from the PD passes through the selected diode matrix \(M_i\) to the inputs of the step-voltage shapers \(\Phi_x\), \(\Phi_y\), and to the modulator for illuminating the points of the character.

Fig. 3. Oscillograms of the voltages \(U_x(t)\) and \(U_y(t)\) for the digit “2”

In Fig. 2 the method of wiring the matrix and obtaining a stepped voltage for forming the digit 2 on the screen is explained. The beam moves along the contour of the character in the order of increasing point numbers. Points 14 and 15 are not illuminated. Figure 3 presents oscillograms of the voltages \(U_x(t)\) and \(U_y(t)\) for the digit 2.

The step-voltage shaper (Fig. 4) is an accumulator of electrical energy on a capacitor [4]. In this circuit, capacitor \(C_2\) is the energy accumulator, while capacitors \(C_1\) and \(C_3\), which have relatively small capacitance, serve to add individual portions of energy. In order to make each voltage increment \(\Delta U\) on capacitor \(C_2\) equal, emitter followers on transistors T1 and T2 are introduced into the circuit. To discharge capacitor \(C_2\) after completion of the forma-

Fig. 4. Schematic circuit of the step-voltage shaper.

... the discharge transistor T3 serves to form the stepped voltage. When a pulse of positive polarity is applied to input 1, capacitor \(C_2\) is charged to a certain value \(\Delta U\), and when a pulse of negative polarity is applied to input 2, it is discharged by the amount \(\Delta U\). By applying combinations of pulses to inputs 1 and 2, we obtain at the circuit output the stepped voltage required for forming the character.

The character generator implemented according to this circuit has a character output time on the order of 20 \(\mu\)sec, which agrees well with the time for outputting a single point to the oscilloscope screen by the BESM-4 computer (40 \(\mu\)sec).

REFERENCES

1. V. B. Zlokazov, L. S. Nefed'eva. Mathematical software for a light-pen oscilloscope in the POFI-2 system. JINR Preprint 10-5966, 1971.

2. I. Tomik, V. R. Trubnikov. A light-pen oscilloscope as a means of human communication with a computer. JINR Preprint 10—4211, 1968.

3. V. S. Govorov, Character generator. Pribory i tekhnika eksperimenta, 1970, No. 6.

4. V. A. Sorokin. Byull. izobret. No. 30, 1970.