Siemens SIMATIC S5-100U System Manual page 30

Technical Description
Program Memory (EPROM/EEPROM)
In order to safely store the control program outside of your S5-100U, you must store it on an
EPROM or EEPROM memory submodule (see section 4.4).
Programs that are available on a memory submodule (EPROM or EEPROM) can be copied to the
internal program memory (see section 4.3). This internal program memory is a reserved area of the
CPU's internal RAM memory.
The internal RAM memory has the following characteristics:
• The memory contents can be changed quickly.
• Memory contents are lost when there is a supply voltage failure and there is no battery backup.
Operating System (ROM)
The operating system contains system programs that determine how the user program is executed,
how inputs and outputs are managed, how the memory is divided, and how data is managed.
The operating system is fixed and cannot be changed.
Process Image Tables (PII, PIQ)
Signal states of input and output modules are stored in the CPU in "process image tables". Process
image tables are reserved areas in the RAM of the CPU.
Input and output modules have the following separate image tables:
• Process image input table (PII)
• Process image output table (PIQ)
Serial Interface
You can connect programmers, operator panels, and monitors to the serial port (cable connector).
You can use the serial port to connect your S5-100U as a slave to the SINEC L1 local area network.
Timers, Counters, Flags
The CPU has timers, counters, and flags available internally that the control program can use.
The program can set, delete, start, and stop the timers and counters. The time and count values are
stored in reserved areas of the RAM memory.
There is another area in the RAM memory where information such as intermediate results can be
stored as flags. You can address the flags by bits, bytes, or words.
If battery backup is available, then some of the flags and counters remain in the internal RAM
memory even if the supply voltage fails or your S5-100U is switched off. These flags and counters
are retentive.
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S5-100U
EWA 4NEB 812 6120-02b