Fresenius Medical Care 4008 E Technical Manual page 333

Microprocessor
The crystal Q2 between the processor connections 39 and 40 determines the pulse frequency of
the microprocessor. The crystal starts to oscillate by means of the capacitors C5 and C6.
The keyboard signals and the BSST, BPST signals are read in via port P4.
IC 9 is provided as intermediate storage of the addresses A0 to A7.
The ALE signal at pin 50 of the microprocessor represents the control line of the data-address
latch.
The signal from the revolution and rotational direction sensor (position sensor) in the pump bed is
read in via port T1.
When the hemodialysis machine is being switched off or if the voltage fails, the operating data of
the pump are stored in the NOVRAM IC 21 via ports 5.1 to 5.3. The comparator IC 23 recognizes
any low voltage (Power-Down).
The WR line is used to control the data acceptance for the display into the external data latch
IC 14.
Intermediate PLL circuit
Together with the counter IC 19, the PLL module IC 4 causes the frequency coming from the
processor to be multiplied by a factor of 4, to activate the stepper motor, because the processor
would be too slow for generating this frequency.
RISC processor
The RISC processor IC 5 is supplied with a pulse signal by the processor. Upon each pulse cycle,
it alternately reads an 8-bit dataword from a look-up table for each phase of the stepper motor.
This dataword contains both the current direction and the rated current value.
In addition, the RISC processor contains a watchdog for the CPU.
Should the pulse at pin 8/IC 5 be missing, the RISC processor releases a reset at the CPU via
pin 7.
Display control
The dataword for energization of the display is stored in IC 14. Multiplexed operation of the
individual digits is made possible by the decoder IC 18.
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Fresenius Medical Care 4008 4/09.03 (TM)