The document discusses Siemens memory structure and addressing. It describes three main memory areas - Load Memory for non-volatile storage, Work Memory for volatile storage during program execution, and Retentive Memory for limited non-volatile storage. The memory is divided into files for inputs, outputs, bits, temporary tags, and data blocks. Tags provide friendly names for memory addresses and are assigned data types. Projects in Siemens software consist of organizational blocks, functions, function blocks, and data blocks.

1. Supplement to Chapter 03
Siemens – S7-1200
Tags, Memory Structure and Addressing

2. Siemens Memory Structure
 Load Memory
 Memory area for the user
program, data storage and
configuration. This area is non-volatile
storage.
 Work Memory
 Memory area that stores some
elements of a user project that
are needed while the user
program is executing. This area
is volatile.
 Retentive Memory
 Memory area that stores a limited
quantity of work memory values.
This area is non-volatile storage.
Load
Memory
Work
Memory
Retentive
Memory

3. Memory Map
 The memory is
divided into
several data files.
 Each data file
consists of an
operand and tag,
a byte and
optionally a bit.
File Types
Input I
Output Q
Bit M
“Temp” L
Data Block DB

4. Memory Map
Memory Area Description
I
Process Image
Input
At the beginning of the scan cycle, the CPU records the status of the
input field devices to ‘I’ memory .
Q
Process Image
Output
At the third step in the scan cycle, the CPU copies the state that the
output field devices should be in to ‘Q’ memory.
M
Bit Memory
A user program can read or write the data stored in the ‘M’
memory. Any code block can access ‘M’ memory. ‘M’ memory can be
configured to retain the stored values after a power cycle.
L
“Temp” Memory
The CPU allocates the temporary, or local memory whenever a code
block is called to store data during the execution of the block. When
block execution has completed, the CPU reallocates the local
memory for the execution of other code blocks.
DB
Data Block
The DB memory is used to store various types of data, including
intermediate status of an operation or other information parameters
for Function Blocks (FB) and data structures required for many
instructions such as Timers and Counters. DB’s can be configured to
be read only, write only or read/write. The data can be accessed by
bit, bytes, words or double words.

5. Address Types
 The processor can
access address
types as shown in
the table.
 Following is an
example of an
output address.
Address Type
Inputs (I)
Outputs (Q)
Bit Memory (M)
Timers (DB)
Counters (DB)
Temporary (L)
Data Block (DB)
Peripheral Inputs (PI)
Peripheral Outputs (PQ)
Q1.2
Output Byte 1 Bit 2
Siemens uses Byte level addressing. More on this later.

6. Tag Based Memory
 Tag based memory structure
 Tag based memory structures are used in most
PLC/PAC platforms produced in the last 10-plus-years.
 A tag is a friendly name for a memory address. In
languages such as C/C++, VB.NET, java and
many others, the term ‘variable’ is used. The
control industry calls these ‘variable’s’, tags.
 As an example: Instead of addressing an input device
as %I5.5, a tag named ‘manualControl’ could be used
and then assigned the data type of BOOL.
 A tag is still assigned an address and in the Siemens
controllers the end user can either allow the address to
be automatically assigned or it can be change to suit
the needs of the program.

7. Tag Naming - Siemens
 Tag names should describe the function or purpose of
the tag. The name can be anything you want as long
as the name follows these rules:
 Tag names can contain numbers, letters, spaces and a
single underscore and are not case sensitive.
 Tag names can be hundreds of characters long; however,
the names should be kept short and to the point.
 Tag names must begin with a letter or a single underscore.
They can not end with an underscore or begin with a
number.
 Mixed case is used for ease of reading such as:
Conveyor_2 or Conveyor2 and not CONVEYOR_2 OR
CONVEYOR2.
 When viewing tags in the TIA-Portal software the tags are
displayed in a spreadsheet similar to Microsoft Excel. The
columns in the sheet can be sorted in any method the user
desires.

8. Invalid Tag Names
 The following tag names are invalid:
 Conveyor2_motor_
 This tag is invalid because a tag name cannot end
with an underscore.
 2Conveyor_motor
 This tag is invalid because a tag name cannot start
with a number.

9. Viewing/Sorting Tags
 When viewing tags in the TIA Portal software the tags
can be displayed in alphabetical order. Use this to your
advantage when naming tags. Using the same word to
start tag names from the same process areas will keep
them grouped together. As an example:
 Conveyor2_endOfConveyor
 Conveyor2_inputSensor
 Conveyor2_motor
Would keep all the tags associated with Conveyor2
grouped together.
 However, TIA Portal also allows the creation of more
than one tag table. Therefore, tag tables can be
created for the various sections and/or functions of a
machine or process.

10. Tag Data Types
 When a tag is created it must be assigned a
data type. The data type is assigned based on
the type of data that will be stored in the tag.
 There are many different data types. The five
basic data types are:
 Bool (Bit) Bit level data (0 or 1)
 Sint (Byte) 8-bit integer data (Single Integer)
 Int 16-bit integer data
 Dint 32-bit integer data (Double Integer)
 Real 32-bit floating point data

11. Basic Data Types
Data Type Bits
31 16 15 8 7 1 0
Bool (1-bit) Not used Not used Not used 0 or 1
Sint (Byte)
Not used Not used -128 to 127
(8-bits)
Int (16-bits) Not used -32,768 to 32,767
Dint (32-bits) -2,147,483,648 to 2,147,483,647
Real
-3.40282347E38 to -1.17549435E-38 (negative values)
0
1.17549435E-38 to 3.40282347E38 (positive values)

12. Memory/Project
Organization
 Memory and projects are organized in
code blocks:
 Organizational Blocks (OB)
 Functions (FC)
 Function Blocks (FB)
 Data Blocks (DB)

13. Organizational Block (OB)
 Organizational Blocks define the structure of
a program. There are several types of OB’s:
 Program Cycle – Repeatedly executes while the
processor is in Run Mode. OB1 is the default
block.
 Startup – Executes one time when the processor
mode is changed from Stop to Run; OB100.
 Time Delay – Executes at specific time intervals,
specified after an event is configured by the start
interrupt (SRT-DINT) instruction. (Will not be
covered).
 Cyclic Interrupt – Executes at user defined time
intervals that interrupt cyclic program execution.
(Will not be covered).

14. Function (FC)
 Functions are code blocks that do not
retain memory values after execution.
 Any data stored in tags during execution
will be lost when execution leaves the
function.

15. Function Block (FB)
 Function blocks are code blocks that retain
memory values after execution.
 Function blocks behave similar to
subroutines and can be “called” whenever
and where ever their functionality is
required.

16. Project Structure
 Siemens projects consist of:
 OB – Organizational Block
 FB – Function Block
 FC – Functions
 DB – Data Block

17. Siemens Portal Project
Project and Processor
Object Blocks (OB)
Function (FC)
Function Block (FB)
Data Block (DB)
PLC Tags

18. Intro Portal Step 7 Lab
 The next lab can be an instructor led or a
self-paced introduction to TIA Portal Step 7
and will cover:
 Starting a new project
 Configuring the hardware
 Configuring communications
 Entering ladder logic
 Creating tags, assigning addresses and data
types
 Downloading the project and running the
program.