The paper is advanced by a 2-phase stepper motor that is directly coupled
to the platen roller. One step of this motor corresponds to one dot row of
paper movement. The motor control circuit consists of a circuit to
generate a clock pulse to advance the paper, a 4-state counter to generate
the 2 signals in phase quadrature and buffer circuits to amplify these 2
signals to actually drive the motor windings. The first 2 sections are 
located on the {\bf Control} PCB, the last is contained on the {\bf Motor 
Driver} PCB (along with its power supply circuit). These 3  sections will 
now be described separately.

The motor clock signal can come from 2 sources. For automatic paper
advance during printing, the state machine output decoder, U13, assets the
{\bf AutoFeed/} signal. For manual paper advance, the {\em Feed} button on
the front panel is debounced by the SR flip-flop U28c and U28d to provide
the {\bf Feed} signal. When this is asserted, and providing the printer is
not i nthe middle of printing a line -- that is that {\bf RotEn/} is high
(not asserted, NAND gate U12a is enabled and passes the 100Hz signal from
the oscillator u23a, U23e, U23d. The (active-low) output of this gate is
logically ORed with the active-low {\bf AutoFeed/} signal by U15a to
provide the {\bf MotorClk} signal.  Thus, when the state machine outputs
the {\bf AutoFeed/} signal, a single motor clock pulse is produced and the
paper is advanced by one dot row.  And when the {\em Feed} button is
pressed, a continuous stream of motor clock pulses is produced and the
motor runs continuously. 

The motor counter consists of the 2 coupled D -type flip-flops U21a and 
U21b. They count in the sequence 00, 01, 11, 10, 00\ldots The outputs of 
these flip-flops are thus the 2 required drive signals in phase 
quadrature .They, and their inverses, are buffered by the open-collector 
inverters U27a, U72b, U27d, U27e and fed via the logic backplane and 
chassis wiring to the {\bf Motor Driver} PCB.

The {\bf Motor Driver} PCB contains 2 identical driver stages (one for 
each motor winding) and a simple unregulated power supply. The latter 
produces $\pm$17V by rectifying and smoothing the output of a 
centre-tapped 14-0-14VAC transformer secondary winding, 

Since the 2 driver stages are identical, only one will be described, that
associated with the {\bf $\phi$A} signal which originates from the
flip-flop U21a on the {\bf Control} PCB, When this flip-flop is clear,
{\bf $\phi$A} is high and {\bf $\phi$A/} is low. The former causes Q5 and
thus Q1 on the {\bf Motor Control} PCB to be cut off, while the latter
turns on Q9 and thus supplies base current to Q6 and Q2, turning them on
too. Therefore the {\bf MA} signal is driven to the negative supply rail. 

When U21a on the {\bf Control} PCB is set, {\bf $\phi$A} is low and {\bf
$\phi$A/} is high. The former now provides base current to the PNP
transistor Q1 on the {\bf Motor Driver} PCB which in turn provides base
current to Q1, turning it on. Transistor Q9 receives no base current and
this provides no base current to Q6 which in turn provides no base
current to Q2. Q2 is therefore cut off. The {\bf MA} signal is thus driven
to the positive supply rail. 

The motor winding is connected between {\bf MA} and ground and thus the 
polarity of the current through it depends on the state of the flip-flop 
U21a on the {\bf Control} PCB as required..

