Centronics 101AL Technical Manual page 41

The trailing edge of this pulse triggers the Delayed Strobe (DLYSTB) one-shot
which is adjusted to the same pulse width as STROBE. In normal character
printing, STROBE is used for full-step timing and DLYSTB for the half-step
timing.
4.6.2.1 Video Amplifier
The circuit used to amplify the video signal generated by the timing
fence is located on the video amplifier assembly board, contained on the print
head carriage.
Referring to schematic drawing #63002319 in Section 7, the video
amplifier consists of a high gain amplifier with positive feedback. When the
photo cell is dark, no current flows through it and the base of Q2 is held at
+5 volts through resistor Rl. When Ql is turned off, Q2 is turned on through
resistors R2 and R7. Q2 being on also turns on Q3 through resistor R4.
Because Q3 is on, the collector is held at approximately ground,
thereby allowing the current to flow through R7 and holding Q2 on through
the positive feedback. When the photo transistor detects light, current is
allowed to flow through it, thereby drawing current through transistor Q1
and resistor R2. Q1 then turns on and turns transistor Q2 off by shunting the
current away from the base of Q2. When Q2 turns off, Q3 also turns off and
the collector of Q3 is held to +5 volts through R6. R7 serves to drive Q2
further into the cut-off region. Capacitors C1 and C2 are used for proper
frequency response and noise suppression. Resistor R3 is used to prevent leak-
age by keeping Q2 from turning off.
4.6.2.2 Timing Signals
For normal character printing, five consecutive STROBE inputs to LSI
chip ME9, generate timing outputs DCW1-DCW5 as shown in Figure 4-5. These
timing intervals correspond to the five full-step columns in the character
matrix. Thequiescent state of this strobe counter is DCWO (internal to the
chip) which corresponds to the space interval between characters. During DCWO,
the STROBE input generates a CLKTB2 pulse which clocks the next character to
the output of the shift register. The DCW1-DCW5 timing outputs are used to
address the appropriate column in the "full-step" ROM (character generator).
4-}7