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7.2
N o r m a l l y , the paper feeds through the printer as p r i n t i n g takes place.
However, a m a n u a l l y operated paper feed switch is fitted to enable the
paper to be fed through without p r i n t i n g , for example, to provide a
tear-off m a r g i n at the end of a print-out.
this switch protrudes through the top cover.
8.
LOGIC DESCRIPTION
8.1
The LED and the photodiode are positioned on opposite sides of an
apertured encoder disc.
the LED is interrupted, thus p r o v i d i n g a pulsed s i g n a l , the t i m i n g of
which is governed by the speed of rotation of the disc.
signal
is applied to one input of an AND-gate whose1 outPut 1S
connected, via a d i v i d e - b y - f o u r circuit, to a block input of a b u f f e r
store and decoder (see Figure 6 ) .
8.2
The b u f f e r store and decoder receives and stores information relating
to one l i n e of p r i n t , i.e. six scanned lines, from the computer; it
responds to clock pulses at its clock input to selectively enable the
high-voltage (50V)
s u p p l y to the conductor strip and thence, via the stylus to the paper.
The circuit is earthed via the conducting rubber roller of the paper
feed and a resistor.
paper,
the
undercoating exposed to provide the permanent image.
strip is also connected to the paper detect circuit which detects when
the stylus has run off the edge of the i n s u l a t i n g wafer and has
commenced tracking across the surface of the paper.
8.3
The encoder disc produces 1024 pulses across the p r i n t i n g width of the
paper;
these are produced to 256 by the divide-by-four circuit to
ensure that the register of each l i n e of dots w i t h i n the matrix is
m a i n t a i n e d to w i t h i n a quarter of the width of a dot.
9.
CIRCUIT DESCRIPTION
9.1
TR1 and TR2 form a self regulating 50V power supply.
measured conveniently on C4 (see Figure 7 ) .
9.2
When the stylus first makes contact with the paper at the left hand
m a r g i n , TR5 is switched on via R8.
the
ULA and results
associated micro computer.
momentarily and are critical
experienced with devices giving too much gain and TR5 which on some
early printers was a ZTX550 should be replaced by a ZTX551.
9.3
TR3
and TR4
p r i n t i n g .
As the disc rotates, the beam of light from
switch to permit current to flow from the power
Where the 50V is a p p l i e d to the surface of the
metallic
coating
in the encoder signal
C6 and R l l prevent contact being
switch the
50V to the stylus
2.3
The operating button for
is
burned
off,
This raises the level at pin 4 of
being passed to
in v a l u e .
Many problems have been
This, pulsed
leaving
the
The conductor
The 50V may be
in order to generate
dark
the
lost
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