Smiths Graseby 3200, Technical & Service Manual

Page: 46 / 117

Smiths Medical International Ltd.
Issue 5 (August 2004)
3200 Service Manual 4 — 3
The stepper motor is controlled by a sequence of pulses supplied from the main processor.
A common ENABLE_MOTOR signal (see Figure 4.4 ) and one of the four motor control lines are
fed to each AND gate. When the ENABLE_MOTOR signal is high, a pulse on one of the motor
control lines will cause one of the four power mosfet’s to be switched on, and the resulting
output pulses are used to drive the stepper motor.
The ENABLE_MOTOR signal is pulled low by the microprocessor supervisor chip IC15, when
the chip detects a watchdog timeout. R35 and R36 enable the main processor to detect
current flow through Q19 or Q20 (MOTORSENSE1) and Q21 or Q22 (MOTORSENSE0).
C26 and C27 remove any voltage spikes which may occur.
R7C and R7D are current-limiting resistors.
The ‘T’ filters L6, L7, L8, and L9 are for electro static discharge protection of the motor
interface. The diodes provide a path for the charge stored in the motor coils when the
transistors are switched off.
Motor interface
Motor interface
IC2 (see Figure 4.5 ) acts a linear regulator in order to provide a 5 V DC supply (Vcc) for the
logic circuits.
C11, C12 and C13 are decoupling capacitors.
IC3B and IC3C form a set/reset latch to operate the power MOSFET Q4. The latch is
triggered by a high pulse from the main processor (OFF), causing Q4 to turn off and
disconnect the power. Once tripped, the latch cannot be reset by the processor, but must
be reset by a high pulse from the keyboard
ON button.
Q6, R28 and R29 form a logic level translator to shift 0 to 5 V logic levels into 0 to 7 V logic
levels for IC3, which has a permanent 7 V DC supply.
C14, C10, and R30 overcome the effects of any transients.
D5 prevents C10 from discharging into IC3B in the event of Vdd being removed.
The Vacuum Fluorescent Display (VFD) has its own independent voltage regulation circuit.
With Q7 switched on Vin will be applied to IC4, a high current 5V voltage regulator.
The output of IC4 is used to power the VFD.
The circuit comprising R23, R24, C9, bandgap references D4 and D30, and transistor Q5,
supplies a constant voltage (Vcc’) of approximately 3 V DC.
The supervisor chip transfers the power for the real time clock and RAM from the regulated
5 V DC supply, Vcc, to Vcc’ when the main power, and hence Vcc, is turned off. This
preserves any memory held in RAM and keeps the real time clock running.
LED D6 is illuminated when a voltage is present on Vac. This signifies that the AC supply is
connected to the pump.
Power control