Service manual

PN 4237242C
2.4-9
INSTRUMENT DESCRIPTION
SYSTEM CONTROL
2
Motor Driver
All motors use the same Motor Driver circuit, which uses a UDN2878W driver device. Four
phase signals are input from the DRA card(s). The driver outputs four +24 V lines to the
motor in the phases set by the DRA card(s).
An Overload Timer circuit on the Motor/Solenoid Driver card offers protection to the motors
and drivers. The +24 V, which is received by the Motor Driver circuit’s 25-W resistors, is
delivered through Q3, a 10-A transistor. The base of Q3 is controlled by the output of an
LM339 comparator. When +24 V is directed to a motor, it is also presented to an RC timer (R1
and C1). C1 is input to the comparator along with +16 V, and when C1 charges higher (about
2 minutes), Q3 stops conducting. This cuts off the +24 V power supply from the motor
drivers. The +24 V overload latch is also set, which allows the instrument software to detect
that an overload occurred.
Solenoid Driver
The command to energize a solenoid reaches the Motor/Solenoid Driver card by way of a DRA
card. These signals are received by the card and latched using a 74HCT540 device. The
output of the latch for most solenoids is fed directly into a ULN2823 driver device that
applies +24 V to the designated solenoid.
The lyse pump is one exception. The LYSE PUMP signal from the latch is first passed through
a programmable logic device before entering the ULN2823 driver device. This logical circuit
enables a timer that de-energizes the lyse pump after 1 second.
Four solenoids, LV6, LV7, LV10, and LV11 use a UDN2878W driver device. This is a motor
driver that was used to energize larger solenoids in earlier versions of the instrument.
Sensor Control
All the system sensors are tied to the Motor/Solenoid Driver card. They are provided with a
current source for the LED and +5 V for the detector. The detector output is received and
made available to the proper DRA card. Most sensor signals are sent directly to the DRA card,
but the traverse sensors are treated differently. The traverse has two motors which STOP on
sensor and five sensors. The motor driver logic on the DRA card can take input from only one
sensor for each motor. A multiplexer is used to select the proper sensor signal for output to
the DRA card. The three horizontal position sensors are multiplexed for use by the traverse
motor and the two vertical sensors are multiplexed for use by the probe motor. The
Multiplexer circuit is implemented as part of the programmable logic device.
+24 V POWER ON Signal
The +24 V power supply has a POWER ON signal input. When this signal is high or floating
(disconnected), the power supply is turned on. When the input is low or grounded, the
power supply will not output +24 V. The MD II Series uses this feature and connects a
POWER ON signal from the Motor/Solenoid Driver card.
When the software loads and begins executing, a command is sent from the DRA card to the
Motor/Solenoid Driver card to turn on the +24 V power supply. This command is processed
by a programmable logic device, U21, which sets an output latch. The output of the latch is
inverted (low when the latch is set) and applied to the base of transistor Q4. If the base of Q4
is low, Q4 does not conduct, which causes the line to float and turn on the +24 V. There is a
jumper, X1 (Figure 2.4-4), in this line as well. If it is removed, the +24 V power supply always
turns on.
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