Sinclair ZX Spectrum
General principle of operation
The printer functions in rather the same way as a TV, i.e. by scanning from left to right. A conductive stylus is pulled across the paper at high speed, and where a black dot is wanted a pulse of current is passed though the stylus. This evaporates the aluminium coating on the paper, and allows the black backing to show through. To avoid the need to return the stylus quickly to the left hand edge of the paper, there are in fact two styli, mounted on a moving belt, which follow each other in quick succession. The belt and the paper feed roller are both driven continuously whilst printing; so that when the next stylus comes round the paper has been moved up ready for the next line.

In order that the styli always stop off the paper, the BASIC commands slow the motor down for the last two scans before stopping - this makes sure that graphics always "join up" if done in several goes, and this feature should be put in any machine code program that requires the printer to stop whilst the computer thinks about the next line. At full speed, each pen is on the paper for about 32ms, and off it for about 16ms.

Since the speed of printing can vary, an encoder disc is used to give 256 pulses across a 92mm print width (allowing 4mm margins). These pulses are synchronised with the stylus hitting the paper so that the printing is always vertical.

There is about 60s between the starts of successive pulses at full speed.

Sinclair ZX Spectrum

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