Fresenius Medical Care 4008 E Technical Manual page 354

Real-time clock with battery buffering
The integrated module IC M 7170 or RTC 6591 represents a real-time clock (RTC), which is
required for the machine to switch on automatically and which serves as a reference time base for
time-critical functions during processing of a program.
Since the modules are buffered by lithium batteries, the system time continues to run correctly
even after the machine has been switched off, thus preserving the programmed switch-on time.
As soon as the programmed switch-on moment has been reached, the -INT output of the IC M
7170 emits a signal, which generates the -AUTO_ON signal via a switching transistor. This signal
is conducted to the POWER-LOGIC P.C.B. (LP 639), causing the machine to be switched on.
A variable capacitor permits the fine adjustment of the clock module (required with IC M 7170
only).
Watchdog
The watchdog monitors the correct function of the CPU 1 board.
Should the watchdog detect a malfunction, the 24-V supply voltage for system slide-in modules
and hydraulics is switched off via a relay.
Before the 24-V supply voltage is switched on, an initial test (functional check) is performed.
During this check, the correctness of the time window, the communication with CPU 2 and the
switch-off possibility by CPU 2 are tested.
Should one or more of these prerequisties not be fulfilled, switch-on of the 24-V supply voltage is
inhibited, with the system emitting the message "watchdog error" and remaining in the "safe
state".
In order to further increase operating safety, the hardware watchdog is equipped with its own
supply voltage; in addition, all signals transmitted to the hardware watchdog are optically
decoupled. Thus, a resistance against external voltages of >35 V is achieved.
Structure:
The hardware watchdog comprises two one-shot multivibrators of type CD4538, which are
responsible for the generation of the time window, as well as a D-flip-flop of type 74HC74 which,
in conjunction with a driver transistor, establishes the output stage. The multivibrator combination
is triggered by pulses (every 500 ms), which are generated by the software on CPU 1.
Control of the output stage is realized by means of two signals emitted by CPU 2 (WD_SET,
WD_RESET).
8-57
Fresenius Medical Care 4008 4/09.03 (TM)