Spectrum Keyboard Buffer

  
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Most people who want to modify and improve their Spectrum look first at the keyboard as a prime area for change. But straightforward replacement involves digging around inside the computer - something of a put-off, for two important reasons. To begin with it might nullify the guarantee from Sinclair and, frankly, there`s just no-one else around to undertake this kind of
Spectrum Keyboard Buffer - schematic

repair work (there`s no published circuit diagram and a lack of certain specialised parts). Secondly, it`s easy for damage to occur when fitting the keyboard to the computer or mounting the keyboard in a separate case (which requires the removal of the complete circuit board from the casing of the computer). So what`s outlined here is a keyboard buffer that fits on the back of the ZX Spectrum, connecting to the real expansion port. It can easily be fitted or unplugged for testing, and should your Spectrum develop a fault, it can be removed without trace for guarantee purposes. The Sinclair keyboard, like many others, works on a matrix of keys where each key connects up two wires. The combination of the incoming and outgoing wires is continually being tested by the computer to see which one has been pressed. On the ZX Spectrum the keyboard has eight incoming wires - the upper eight address lines. These are tested by holding only one of them to a Binary 0 (a LOW signal) and seeing what the result is on the incoming (data) lines. If a switch has been pressed on the address line being tested, then the LOW signal will be passed on to the data line to which it is connected. Until then the data line will be held to a Binary 1 by the resistors connected to the five data lines. In the keyboard layout diagram (Figure 1) the keys as you can see are arranged in a similar order to the keyboard (but not in the familiar QWERTY layout)....



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