That makes any input-output device compatible with the ZX-81 but any memory-mapped devices would have to be rearranged. The Spectrum has a 28-way double-sided edge connector of the same style as the ZX-81, with the keyway on pin 5. There is no RAM CS line, so that the extra RAM can be turned-off if required on the edge connector and the edge connector address lines have been moved to the outer edges so that it is incompatible with the ZX-81. There would be no difficulty in adding extra ports to the memory map, as on the ZX-81, above 32K on the basic version - but for two things. The 48K version will have the board fitted but to add it later it will cost £60, which I think is expensive. There is provision for an extra 32K board to be plugged in to IC sockets at the back of the printed circuit board.
The memory-mapped addressing of the RAM/ROM occupies 0-16K and the RAM 16K-32K on the basic 16K model. They give an instruction IN A(c) or OUT A(c) where registers BC give an address from 0 to 65535. The input-output map access has been improved greatly, however, with the addition to the Basic commands of IN and OUT. That means that you must make all of the lower five address lines a binary 1 to use your own devices. The keyboard is an input-output mapped device, as on the ZX-81, and along with the ZX printer, which is the same for the ZX-81 and the Spectrum network/RS232 interface, discs, loudspeaker, tape interface and border colours require only one address line to work. It would have been a better idea to put both on the left-hand side, as they often need to be used together and could be pressed with one hand while the other searches for the appropriate key. They are at opposite ends of the keyboard and as they are often used one after another, it tends to slow the input speed as you are constantly swapping hands.įor instance, RUBOUT and the cursor movements use CAPS SHIFT and +-* are SYMBOL SHIFT. It also means that two SHIFT keys are required to reach all the functions one is called CAPS SHIFT and the other SYMBOL SHIFT. The single keyword system has been retained and that saves memory, as all the Basic words can be stored as one byte. That and the fact that the keys repeat if held down for longer than one second, even when using SHIFT, makes the keyboard much easier to use. The sheet is suspended over the flat keys - which on the ZX Spectrum are bigger than the ZX-81 - so that when a key is pressed it bends to give some feel to the keyboard. Sinclair has kept the original keyboard to save space but has provided a rubber sheet with moulded keys on it which fits over the top. THE ZX SPECTRUM is very similar in shape and style to the ZX-81 and there are many similarities. Stephen Adams looks inside Sinclair's latest machine and finds it has a lot in common with the ZX-81 Taking the lid off the Spectrum
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