Siemens S5-135U/155U - Manual (Page 500)

10-4
System Manual
C79000-G8576-C199-06
10.1.4 Fault Detection
From an address which has been set at switch S1, the data (55H or AAH) is
read by the CPU from the monitoring module. This data is to be written back
by the CPU to the address set at switch S2. The module inverts the accurate
incoming data bit by bit (from 55H to AAH or vice versa) which is read
again in the next cycle from the address set at S1.
If the data which has been set on the monitoring module is not returned
within the set monitoring time, the module signals a data bus fault.
Detectable faults
Interruptions of the data lines, short circuits to ground and + 5V, short
circuits between adjacent data lines, e.g. DB0-DB1, DB1-DB2 etc.
Non-detectable faults
Short circuits between even data lines DB0-DB2-DB4-DB6 and between odd
data lines DB1-DB3-DB5-DB7.
The addresses which have been set at switches S3 and S4 are only “listened
to” by the monitoring module, i.e. they do not output an acknowledgement
signal (RDY/) and are not active on the data bus. If one or both addresses are
not accessed again within the monitoring time, the monitoring module
signals an address bus fault.
Detectable / non-detectable faults
The same as for the data bus, if the inverse addresses 85 (55H) and 170
(AAH) have been set at the addressing switches S3 and S4 of all monitoring
modules.
Data Bus Faults
Address Bus
Faults
Monitoring Module