Description
| Technical article Membrane keyboards: Ghost key detection | |
| How many keys can be pressed simultaneously on a keyboard without generating an error depends on the matrix technology used. Buzzwords like "full n-key rol- lover", "3-key rollover" or "2- key lockout" are mentioned here. But what is behind these terms? | |
| If more than three keys of a standard keyboard are pressed at the same time, the logic may appear to generate keystrokes that have not been pressed at all. These "ghost keys" lead to annoying incorrect entries or can even make the operation of a program impossible if complex key combinations are required. The best-known example of three simultaneously pressed keys is probably ALT-STRG-ENTF. | |
| The keys of a standard keyboard are usually arranged in a matrix. The control logic applies a signal to the rows of the matrix, e.g. one after the other. The columns are controlled by the logic. The keys are connected to one row and one column each. A key press thus connects a crossing point and the logic can "see" the signal of the row on the corresponding column. Figure 1 shows the arrangement and the associated current flow when the key is pressed. | |
| Due to the matrix arrangement and the sequential scanning, a large number of keys could actually be pressed and recognized, were it not for the pitfalls of electrical engineering. If at least two keys are pressed in a row and at the same time a second key is pressed in one of the two active columns, a "reverse current" is generated via the key in the crossover point to the second column. The key that forms a rectangle with the three keys pressed also appears as pressed (Fig. 2). | |
| How to prevent such ghost keys? | |
| The simplest and also most effective method is to use decoupling diodes on each key between row and column. Since the diode prevents reverse current, it is actually possible to press any number of keys simultaneously. A so-called "fulln-key rollover" is created. However, the use of diodes is expensive and also not easily possible in every keyboard technology, such as membrane keyboards. In order to prevent incorrect entries, a "2-key lockout" was usually used in the past. This type only allowed two keys to be pressed. Every other key was ignored. | |
| However, many industrial programs use key combinations of three keys to call program-specific functions. Function keys and modifier keys such as ALT or CTRL are mainly used for this. In order to be able to press at least three keys at the same time, i.e. to achieve a so-called "3-key rollover", the modifier keys were decoupled by means of diodes or the key combinations known for the application were taken into account in the matrix. | |
| Versatile GeBE keyboard controller GCK-990 | |
| The new GeBE keyboard controller GCK-990 (Fig. 3) has a "ghost key detection" consisting of two functional parts. The 18 x 8 matrix of the controller allows the connection of up to 144 keys. The 40 unused positions for the 104-key standard layout are cleverly placed in connection with the matrix design so that multiple combinations with modifier keys cannot lead to ghost keys in the first place, since empty positions in the matrix are "masked out". If ghost keys occur nevertheless - due to incorrect entries - these key entries are deleted according to the saying "Broom, been there!" and an error message is sent to the PC. | |
| The use of a "ghost key detection" and a complex construction of the matrix arrangement make it possible to operate standard membrane-based keyboards in applications with 3-key combinations. | |
| This technology has been implemented in the newly developed GFT- 104 industrial keyboard. In a housing only 10 mm thick, an extremely robust, IP 65 waterproof industrial keyboard with USB-PS/2 combo port could be realized, which does not have to fear the functionality comparison with short-stroke keyboards. |
Downloads
Fachartikel_Industrielle Folientastatur mit Ghost-Key-Detection.pdf ↓
Fachartikel Folientastaturen: Ghost-Key-Detection
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