Siemens S5-135U/155U appendix Process Interrupt, Processing

CPUs, Memory Cards, Memory Submodules, Interface Submodules

User Memory	You require a user memory submodule (RAM or EPROM) for storage of your
	program. You can store up to 64 x 210 bytes of code and data blocks here. For
	data blocks, the CPU 928 additionally has an integrated DB RAM of 46 x 210
	bytes.
	Note
	The DB RAM is loaded with DB/DX blocks when the RAM submodule is
	full or an EPROM submodule is plugged in.
	A description of the memory submodules can be found in Sections 5.9 and
	5.10; order numbers are given in the ordering information.
Process Interrupt	There is an interrupt line (IR) in the PLC for each CPU. It can be used when
Processing	the reaction to one or more events must occur with higher priority than the
	reaction to other events.
	o process an interrupt, cyclic program processing is interrupted and the
	program stored in OB 2 (OB for interrupt processing) is inserted.
	(Refer to the CPU 928 Programming Guide for further details.)
	This interrupt-driven program processing is only possible using an
	interrupt-capable digital input module (e.g. 6ES5 432-...) or a CP/IP module
	which operates in this way.

System Manual	5-63
C79000-G8576-C199-06