Fresenius Medical Care 4008 E Technical Manual page 353

Rules for communication:
Changes in value and/or status within the monitoring range of the respective computer result in
the transmission of a data record to the other computer.
Each data record transmission must be acknowledged by the receiver.
Data records are acknowledged positively only if the comparison of the identification, the check of
the value range and the comparison of the transmitted and computed checksum turned out to be
positive.
Any negative acknowledgement by the receiver causes the transmission of the data record to be
repeated.
Should a specific number of successive negative acknowledgements be exceeded, the system
enters the "safe state", i.e. either CPU 1 stops the periodical triggering of the hardware watchdog,
or CPU 2 activates the hardware watchdog by emitting a WD_SET signal.
Should changes in value not cause any data record to be transmitted within a specific period of
time, a data record is transmitted compulsorily with the reaction of the receiver being evaluated.
In order to prevent the regular program run from being inhibited by one computer permanently
transmitting to the other, at least 50 ms must elapse between two transmissions.
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Fresenius Medical Care 4008 4/09.03 (TM)