Patent GB-1148849-A (GB)
Title: Digital vector generator
IPC: G09G1/10
Filed: 1967-07-14
Abstract: 1,148,849. Cathode-ray tube displays. SPERRY RAND CORP. 14 July, 1967 [1 Aug., 1966], No. 32530/67. Heading H4T. In an arrangement for tracing vectors on the screen of a cathode-ray tube by simultaneous deflections of the beam along orthogonal X and Y axes the deflections are controlled by digital signals specifying the X and Y co-ordinates of the start point, the X or Y co-ordinate of the end point and the tangent or cotangent of the angle which the vector makes with a reference direction-e.g. the X axes. The vector is traced by deflecting the beam in unit steps in the X or Y direction, depending on which component is the greater and simultaneously in the Y or X direction in units steps each of which is multiplied by the ratio of the two components, i.e. multiplied by the tangent or cotangent of the vector angle, the trace being terminated when it reaches the end point specified by an X or Y value. The speed of tracing is increased for long vectors by producing deflections in multiples, e.g. X2, X4 ... X32 of the unit steps, the speed being reduced, however, as the end point is approached and to maintain a constant luminous intensity of trace the tube beam intensity is controlled in accordance with the variations in the tracing speed and additionally in accordance with the vector angle since the tracing speed varies from a minimum in the direction of the X and the Y axis to a maximum at an angle of 45 degrees. The trace may be in a continuous line or it may consist of a dashed line having dashes of three (or more) different lengths. The tube screen is deemed to be divided into a 1024 by 1024 point matrix termed "major position points" (see Fig. 4) and each of the 1024 matrix squares is deemed to be divided into an 8 by 8 point matrix termed "minor position points". The initial start point of a vector coincides with a major position point but for a succession of vectors joined together the end point of one is the start point of the next. The digital signals controlling the display are derived from a computer and in the case of an initial vector comprise two digital words each of 24 bits. The first, the Position Word (Fig. 3a), includes a first bit ("word flag") signifying that it is a position word, i.e. that it specifies the X and Y start co-ordinates, bits 2 to 4 indicating the "Octant" (see Fig. 1) in which the vector is to be drawn and bits 5 to 14 and 15 to 24 specifying the X and Y co-ordinates of the start point. The second, the Vector Word, includes a first bit ("word flag") signifying that it is a vector word, bits 2 to 3 indicating the line code, i.e. whether a continuous line or a dashed line, bits 4 to 14 indicating whether the tangent or cotangent is being employed (to ensure that the angular coefficient never exceeds unity) and bits 15 to 24 specifying the X or Y co-ordinate of the end point (final position). In operation the two words, which are received successively from a computer, are fed to an input buffer 5-2 (Fig. 5). The "word flag" in the first word, the Position Word, then operates via a circuit 5-10 to condition gates 5-18, 5-22 to pass the X and Y position bits to the ten most significant stages X u , Y u  of respective X and Y position registers 5-20, 5-24, each of twenty stages and to condition gate 5-14 to pass the "Octant" bits to a decoder 5-16 which supplies various gating pulses controlling, together with clock and other pulses supplied by the word flag circuit 5-10, the whole operation of the system. After the position bits have been set-up in registers 5-20, 5-24, the word flag in the second word, the Vector Word, operates via circuit 5-10 to condition gates 5-28, 5-34 and 5-38 to pass respectively, the tangent or cotangent bits to a register 5-30, the X or Y (either of which may be specified by the computer) end position bits to a register 5-36 and the line code bits to a circuit 5-40. The operation then consists of incrementing whichever is the larger of the X and Y start position values determined by the circuit 5-16 by unit (or multiples thereof) steps and by multiplying the other, i.e. Y or X values, by the numerical value of the tangent or cotangent. To effect this the contents of the registers 5-20 or 5-24, whichever is the larger, termed the main position register, are gated by gates 5-42 or 5-64 into a counter register 5-44 in which they are incremented by the unit or multiple unit steps fed back to the registers 5-20 or 5-24 via respective gates 5-48 or 5-68 and then incremented again in 5-44, the cycle repeating until the count in register 5-44 (and also in 5-20 or 5-24) which is fed to a comparator 5-54 becomes equal to the count in the final position register 5-36 (containing the end position bits) which is also supplied to the comparator 5-54. Simultaneously with this operation the contents of the Y or X register (that containing the lower count) are gated by gates 5-76 or 5-84 to a subtractive type adder 5-72 in which they are operated on (multiplied) by the output of the tangent/ cotangent register 5-30, supplied thereto via gate 5-116. The output of 5-72 is then fed back to 5-20 or 5-24 via gates 5-90 or 5-80 and the cycle is repeated in unit (or multiple) steps until the comparator 5-54 detects the equality of the incremented larger X or Y component with the final value as described above. When equality is reached an output from 5-54 produces, via a control circuit 5-60, a stop signal which closes gates 5-90, 5-48, 5-68 and 5-80 and arrests operation of the line code circuit 5-40. In incrementing the registers 5-20 or 5-24 only the upper ten (most significant) stages X u  or Y u  are involved when the register considered acts as the main position register, the lower ten stages X L  or Y L , all containing zero bits. When, however, either of the registers is incremented by the tangent/cotangent value the increments are supplied to the lower ten stages X L  or Y L . In practice the tangent/ cotangent value is specified by a three digit octal number, only the most significant digit (representing the three most significant binary bits) of which is actually used. The increments then occur in steps which, as the " accuracy bits " from the seven lower order stages of the tangent register " overflow " cause the steps to vary. This is depicted in the table in Fig. 4, for the case where the Y register is incremented by a tangent specified by 0À223 (octal). During the above operations the outputs from the upper ten stages and from the three most significant stages of the lower ten stages of the X and Y registers 5-20, 5-24 are supplied via respective buffers 5-96, 5-104 and D/A converters 5-98, 5-106 to the X and Y deflection windings of the display tube to trace the required vector, the brightness and type of trace (e.g. continuous or dashed line) being determined by circuits (not shown) under the control of signals derived from the circuits 5-10, 5-16 and 5-40. The Specification includes details of all the logic and other circuitry involved in carrying out the invention.