Siemens S5-135U/155U appendix Fault Detection, Data Bus Faults, Address Bus

Monitoring Module

10.1.4Fault 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.

Data Bus Faults	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.
Address Bus	The addresses which have been set at switches S3 and S4 are only ªlistened
Faults	toº 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.

10-4	System Manual
	C79000-G8576-C199-06