The term "keyboard" is used herein for simplicity of expression but it is to be understood as including not only keyboards consisting of an array of individual switch members, each of which normally bears an identifying indicium or more than one identifying indicium corresponding to alphabetic letters in the Roman or other alphabets, numerals and/or other signs, graphic symbols and the like, but also any other arrangement consisting of an array of .individually identified locations, for example touch sensitive individual areas.

Keyboards are widely used as input devices to microprocessor based systems. Such systems include wordprocessing systems, computer terminal systems and control systems for, e.g. numerically controlled machine tools. In many of these applications it is ^' convenient and, in some applications it may be imperative to site the keyboard as a separate unit from the micro¬ processor. The separation may be relatively small, e.g. to allow a keyboard unit to be moved around a desk, or may be substantial, e.g. to enable a keyboard to be operated in a hostile environment but to preserve the microprocessor system away from such environment.' A normal keyboard contains at least 26 keys corresponding to the 26 letters of the Roman alphabet plus some function or shift keys and conveniently some

other keys, e.g. for punctuation marks. Separate keys may be provided for numerals and it is not unusual therefor for an operative keyboard to include more than 40 keys. Simply connecting these via conventional wires would accordingly require a substantial ulti-conductor cable and a large number of physical inter-connections e.g. a plug and socket arrangement having more than 40 pins. This is expensive and can be unreliable in practice. One solution to the problem which has been adopted in the past is .to provide the keyboard with encoder circuitry which converts the state of the keyboard into a suitable serial output signal which can be transmitted to a central microprocessor unit. ^" We have now found that a simple and accordingly very reliable keyboard may be produced which requires only a few inter-connections to the main micro¬ processor unit by using a keyboard matrix scanning system driven using clock pulses from the main icro- processor unit.

Thus in accordance with a first feature of the present invention there is provided a keyboard consisting of an array of individually identifiable key elements, a row and column matrix of connectors, each key element being uniquely associated with one intersection between a connector element in a row and a connector element in a column, means for altering the electrical characteristics between the connector in the respective row and the connector in the respective column when a key element is actuated and means for sequentially scanning all of the inter¬ sections between all of the rows and columns of connectors to determine the electrical condition of each intersection and for feeding a signal derived from such scanning to an output terminal.

The signal at the output terminal from such a

keyboard accordingly consists of a series of blocks each one of which may vary from the previous one or may be the same if nothing has been actuated or de-actuated on the keyboard between one scan and the next. Electronically it is possible to scan all of the intersections of the matrix very fast and appropriate microprocessor electronics may sort out the output signal to give an indication of whether any is actuated and if so, which. The microprocessor electronics may also sort out when two keys are actuated in a meaningful way, e.g. a shift key and a normal key or it may also sort out an invalid entry, for example when three keys are simultaneously depressed. The microprocessor circuitry may be suitably programmed to convert the keyboard output to any desirable useful form and such programming forms no part of the present invention. The microprocessor unit is however used normally to clock or drive the keyboard so as to impart an appropriate timing on the keyboard scanning.

The size of the row and column matrix of connectors may vary depending upon the use in question. A useful size is 16 units by 8 giving 128 intersections thus allowing a full alphabetic and numerical keyboard as on a typewriterplus a standard numerical 0 to 9 array plus the standard mathematical function keys and leaving space for several specialised function keys which in any particular system may be dedicated or may be "user programmable". Using this size of matrix, the intersections each associated with a key element may be scanned using simple integrated circuits and the results of the scan fed as successive series each of 128 pulses, each pulse being conveniently thought of as a "0" or "1". If actuation of a key substitutes a "1" for a "0" then the data signal appearing at the output of the keyboard which is processed by the main

microprocessor unit will consist of a string of O's with one or a few 1's interposed in it. The main microprocessor can work out which key or keys is/ are actuated depending on how many and the position of the 1's in the string of O's it receives. The scanning may take place very rapidly, for example, 250 timesper second and this enables reliable interpretation of the state of the keyboard even if data is being entered into the keyboard very fast, i.e a very fast typist is operating it. Scanning at this sort of repetition rate also enables the microprocessor circuitry to deal reliably with the problem of "bounce" which occurs with mechanical contact switches which are the generally most convenient and preferred key elements. When a mechanical contact switch is actuated by pressing a key, it is unusual for immediate perfect electrical contact to be made. Instead, there is a tendency for the contacts to bounce together and apart a few times which could lead to false data entry. Microprocessor electronics however may sort out whether a key has properly been depressed by, for example, only accepting as valid a keyboard configuration which remains the same for e.g. two or three repeated scan cycles. Programming of the microprocessor may also provide functions such as "two-key rollover" i.e. the typist may press the next key before the previous has actually released without falsifying the desired data output stream. A further useful ability is that of the microprocessor to recognise when several keys are actuated and ignoreall signals until all keys are released.

The keyboards of the present invention are particularly useful in connection with wordprocessing devices and analogous office machines where a movable keyboard unit connected by a highly flexible

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connector to a main microprocessor and display unit can be much more convenient to operate than a large fixed piece of equipment.

By way of example, the invention is illustrated with reference to a keyboard unit suitable for use in office wordprocessor apparatus.. In such application, it is desirable to indicate, e.g. using a light emitting diode whether the keyboard is being operated in a normal or in a "shifted" position, e.g. corresponding to upper case when printing. Typists are used to a shift key but require some indication of when the shift key is "depressed". In any electronic system, this is most conveniently effected by using light emitting diode in a key which is pressed once to shift, whereon the diode lights, and pressed again to release the shift.

The accompanying drawing shows the circuit diagram for such a keyboard.

Referring to this drawing, it will be seen that the basic keyboard consists of a number of key switches (not shown) each of which is associated with the intersection between two conductors. Each of the two conductors is part of a parallel array of - such conductors, the two parallel arrays being crossed to form an 8 by 16 matrix. At the intersection between the conductors the key switches are located. Each key switch is a standard type of momentary action key which has associated with it a silicon diode connected in series with the switch contacts. The presence of the diode in each key unit allows the decoding system, i.e. the main microprocessor unit to which the keyboard is connected, to perform n- key roll-over by allowing the state of every key in the array to be determined unambiguously, irrespective of whether other keys in the array are already depressed. The diode also has the function of protecting the

outputs of one of the integrated circuits from damage if two keys in .the same row but di erent columns are simultaneously depressed. In the absence of such diodes, the two column outputs in such a case would be effectively shorted together leading to excessive current flow and possible damage to the integrated circuits.

As shown on the drawing, the basic ciruitry consists of three CMOS integrated circuits being a dual four bit binary counter type 4520, a four to sixteen line decoder type 4514 and an eight to one line encoder type 4512. As can be seen from the diagram, there are also a few discrete components being C1 and C2 across the five volt power supply, and an array of two resistors, an LEDand a 2N3704 transistor for the shift indication and a set of eight 47K pull-down resistors connected to each of the data input lines to the 4512 encoder in order to discharge the appreciable capacitance of these input lines so as to prevent "ghosting" of keys in adjacent rows. As can be seen,apart from the power supply terminals and the shift indication terminal, the only other external connections required are clock and reset input connection and a data output connection. This enables the keyboard to be connected to a main microprocessor unit via a six-core screened spiral cord, for example, which may be connected using, e.-g. a standard seven-pin DIN plug.

The operation of the keyboard shown in the drawing is as follows: incoming clock pulses from the microprocessor unit increment the dual four-bit counter by one count per pulse. ^' The count sequence is straight binary and is eight-bits long giving a range of output values from the counter running from 0 to 255. The counter automatically resets thereafter and the count sequence restarts from 0. The most

significant output from the counter is not used so the effective count sequence is from 0 to 127.

The three least significant outputs from the counter are fed to the address outputs of the eight to one line encoder 4512 while the next foremost significant outputs address the four to 16 line encoder type 4514. The 16 outputs from the 4514 are fed to the 16 column connectors as shown on the diagram and the eight row connectors are connected to the eight inputs of the eight to one line encoder type 4512.

Each column line is accordingly set to logic high for eight clock pulses while the encoder IC 4512 scans the eight rows in sequence to see if any of the keys at the intersection are depressed. The output data from IC 4512 thus corresponds to a string of logical O's and 1's at the same frequency as the clock pulses and the depression of a particular key is uniquely represented in the output at data output 6 shown on the drawing by the position of a 1 in the set of 128 1's or 0's.

The output data stream may be suitably decoded using a microprocessor in any convenient fashion. One convenient way of effecting it is to clock the data received from the data output into an eight-bit serial to parallel shift register the output of which is then read in parallel by a 6801 microprocessor. ^• That same microprocessor may generate the clock pulses fed to the keyboard and to the clock input of the shift register. The microprocessor may read the shift register output every eight clock pulses and repeat this 16 times to scan the entire keyboard. The microprocessor may be prgra med to generate after the complete of each scanning sequence a reset signal which is sent to the keyboard via the reset input thereof. The microprocessor unit is also

programmed to detect a depression of the shift key on the keyboard and to generate a signal to actuate the shift indicator LED via input 1 to the keyboard.