Single Shots; Pen Solenoid Control Circuit - IBM 1627 Manual

As shown in Figure 3-3, either a positive or a
negative going signal may be used to trigger the
drum-down Single Shot. However, when the 1627 is
used with the system only the negative going pulse is
used. In the absence of an input signal from the sys-
tem, transistor Ql is normally conducting and Q2
is cut off. The regenerative cycle is initiated by
applying a negative pulse to the base of Ql, driving
this stage out of conduction. Conventional cross-
coupling holds Ql cut off and Q2 in conduction for a
period of time determined by the RC time constant
of C2 and R5. This period is set to a nominal value
of 1. 5 ms. When the base potential of Ql decays to
a value equal to the emitter potential, this stage
again conducts and its negative-going collector
voltage cuts off Q2. The circuit then remains in its
original state until another negative pulse is received.
When the Single Shot is fired by a negative input
to pin Y from the system, Ql is cut off for a period
of 1. 5 ms. This causes the voltage at the junction
of R7 and R8 to rise from approximately -7 volts to
essentially ground potential for 1. 5 ms. This com-
prises the clock drum-down signal output, which is
connected from pin Z through an external jumper
to pin R at the input to the ring counter circuit.
Manual drum step signals are applied to the
single shot circuit through the diode CR5. The
manual input signal may consist of either a single
positive going pulse, or a continuous series of 120
cps positive going pulses. (See Drum and Carriage
Manual Control Circuits.)
The operation of the carriage left and right Sin-
gle Shots is identical to that of the drum-up and down
Single Shots. However, the Single Shot output (clock)
signals are connected to the carriage ring counter
through left and right limit switches (Figure 3-5).
The limit switches disconnect the associated single
shot output whenever the carriage has reached the
left or right limit of travel, and thus prevent further
stepping action in that direction.
Pen Solenoid Control Circuit
The schematic diagram for the pen solenoid control
circuit is shown in Figure 3-4. In this circuit, a
trigger is used for current control instead of the
Single Shots used in the step motor control circuits.
The trigger, consisting of transistors Q19 and Q20,
remains in the state to which it was set by the last
signal pulse, until another signal pulse causes it to
3.6
change state. The triggering circuit is designed so
that pulses of either polarity can be used to set the
trigger to either state. However, when the 1627 is
used with the system, only the negative input is used.
In the normal, or pen down state, Q19 is con-
ducting and Q20 is cut off. The trigger is switched
to the pen up state by applying a negative pulse at the
base of Q19. Q19 is then held cut off by the negative
voltage at the collector of Q20, and Q20 is held in
conduction by the positive voltage at the collector of
QI9~. The trigger then remains in the pen-up state
until a negative trigger pulse is applied to the base
of Q20.
Manual pen-up and pen-down signals are applied
to the trigger through diodes CR30 and CR26, re-
spectively. Both the manual inputs consist of a
single positive going pulse. (See Pen Manual Control
Circuit. )
The output voltage from the pen control trigger,
obtained from the junction of resistors, R73 and
R74, is applied to the base of the driver transistor
Q18. When the trigger is in the pen-down state,
Q20 is cut off and Q18 is biased off. When the
trigger changes state, Q20 conducts and the base of
Q18 becomes negative. Q18 is driven into conduction
and supplies current to the pen solenoid.
The driver transistor Q18 performs a dual func-
tion in the pen control circuit. It supplies current
to actuate transistor Q14, and it supplies a contin-
uous holding current to keep the solenoid retracted
as long as the trigger remains in the pen-up state.
When Q18 is first driven into conduction, its collec-
tor voltage changes very rapidly from a negative
potential to ground potential. This causes a large
positive pulse to be coupled through capacitor C13
to the cathode of diode CR20. Transistor Q14 is
normally biased on by the voltage divider action of
R54, CR20, and R53, which maintains a negative
potential at the base. When the positive pulse appears
at the cathode of CR20, the diode is cut off and the
base of Q14 then becomes positive, because resistor "-----
R54 is returned to +3 volts. Transistor Q14 is
therefore cut off, and its collector voltage goes
negative. Because the collector is tied to the base
of Q16, this negative voltage now causes Q16 to
conduct. Q16 supplies a surge of current to the
pen solenoid through pin J of PI. This initial surge
is sufficient to overcome inertia and lift the pen-
from the paper. Q14 remains cut off and Q16 con-
ducts for a period determined by the RC time con-
stant of C13 and R53. The voltage at the cathode of