Fresenius Medical Care 4008 E Technical Manual page 579

Watchdog
The watchdog circuit consists of the watchdog timer IC 17 with integrated voltage regulator, a
dual optoelectronic coupler IC 16 and the transistors T 6 and T 9. The time-determining resistor
is R 51. The time window is set to a basis of 500 ms. The open trigger window (TOW ) of the
component internally ranges from -20% to + 20% (400...600 ms). Each time three successive
pulses arrive from IC8, pin 60, via T 6 and IC 16b on pin 3 of the watchdog timer within this time
window, the output pin 1 goes to L level and switches the 24 V_SW on via IC 16a and T 9. When
a pulse is missing or triggering is too low or too fast, i.e. outside the time window, pin 1 of IC 17
goes to H level and the 24 V_WD is switched off.
Peripherals
T 1 to T 5 are the power drivers for the respective actuators. The current status of the respective
actuator is read back via P 0.0 to P 1.1 via the voltage dividers R67 to R86 connected to the
outputs.
T11a represents a hardware lock of the air pump and the test valve when V 43 F (G54) is closed.
T 11a enables the supply voltage for the AIR_PUMP and the V_TEST only when valve 43 is
open. Acknowledgement for V43 status is via P1.4 (G19)
IC 4 is an instrument amplifier for the pressure transducers with series connected voltage follower
IC 2a. IC 2b is used for offset setting. The circuits associated with IC 19 and IC 21 have a
corresponding set-up and serve for later extensions.
IC 1a generates the processor and CAN controller reset.
IC 1b detects power failure via a control line from the CAN distributor.
IC 12 is a bidirectional CAN bus driver.
IC 28 is a serial dual-channel 8-bit DA converter. Channel 1, in combination with IC 27b, is used
to calibrate the temperature measuring circuit associated with IC 31a (not used at present).
Channel 2 and IC 27a are available for options.
Level detection
The circuit with the Opamps 32 to 34 serves for level detection in the air separation chambers of
the acetate and bicarbonate inlet line. IC 32a decouples the circuit from the 2KHz sinus signal.
The measuring branch with IC 33b and IC 34b is applied at the analog input AN6, the reference
branch with IC 33a and IC 34a is applied at the analog input AN7 of the controller.
The activation threshold can be set by changing R137 in the measuring branch or via the
software.
8-282
Fresenius Medical Care 4008 4/09.03 (TM)