Siemens S5-135U/155U appendix Bytes, Order numbers are given in the ordering information

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 928B additionally has an integrated DB RAM of 46 x
	210 bytes.
	A description of the memory submodules can be found in Sections 5.9 and
	5.10; order numbers are given in the ordering information.
PG Interface SI1	There are two independent interfaces on the front of the CPU 928B.
	You can connect programmers and OPs to the first interface. This PG
	interface SI1 is permanently installed on the CPU.
	You can use PG interface SI1 either via the front connector of the CPU 928B
	or via the front connector of the 923C coordinator module.
	Note
	Simultaneous operation of PG interface SI1 via the front connector of the
	CPU 928B and of the coordinator is not possible. Electrically, it is merely a
	PG interface which can be operated via two different terminals. Switching
	the programmer online without a job request is already sufficient to operate
	the interface. If you wish to utilize a second PG interface for connection of a
	PG or OP, the connection must be made via the second PG interface with the
	PG submodule.
	Apart from PG interface SI1, the second interface SI2 can also be used as a
	PG interface in parallel with the first one. You can only use the second PG
	interface via the front connector of the PG submodule on the CPU 928B.
	A detailed description of operations with the two PG interfaces can be found
	in the CPU 928B Programming Guide.
	The connection to the PG can be established with the CPU in any operational
	state.

System Manual	5-43
C79000-G8576-C199-06