Tags Not Addresses. Siemens 7

Tags Not Addresses

Logix Features that May Not be Familiar to S7 Users Chapter 2

One of the first major differences that a S7 user will notice when starting to work with Logix is that data doesn't have addresses. Data items are created in a tag database, and RSLogix 5000 software allocates addresses “behind the scenes”. This makes it unnecessary for users to understand and manage memory addresses. This section describes data allocation in the two systems.

Data Areas in S7

Data Areas in S7 Controllers

Address Area

Process Image Input Table

IW

ID

Process Image Output Table Q

QB

S7 Notation

I

IB

Bit Memory

Timers

Counters

Data Block

T

C

MW

MD

QW

QD

M

MB

DBX

DBB

DBW

Unit Size

Input Bit

Input Byte

Input Word

Input Double Word

Output Bit

Output Byte

Output Word

Output Double Word

Memory Bit

Memory Byte

Memory Word

Memory Double Word

Data Bit

Data Byte

Data Word

The sections below say more about the two most commonly used areas in programming – bit memory and data blocks.

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Chapter 2 Logix Features that May Not be Familiar to S7 Users

Bit Memory

“Bit Memory” locations are denoted Mx where, for example:

• M5.3 is a bit.

• MB6 is a byte (BYTE).

• MW8 is a 16 bit word (WORD).

• MD10 is a 32 bit word (DWORD).

Bit memory locations can be labelled in the Symbol Table (similar to a PLC-5 or SLC Symbol Table), as shown in the following screen shot.

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Logix Features that May Not be Familiar to S7 Users Chapter 2

Data Blocks

Data Blocks have similar status to other blocks – Organization Blocks,

Function Blocks and Functions – except that they contain data rather than program code. The memory in Data Blocks is static – the data retains its value until it is changed.

Example of a Data Block

Publication LOGIX-AP008B-EN-P - June 2008

Data Block symbols do not appear in the Symbol Table, but the name of the

Data Block does.

Data Blocks can be assigned to hold the data used by Function Blocks. These are called Instance Data Blocks.

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Chapter 2 Logix Features that May Not be Familiar to S7 Users

Data in Logix

In the RSLogix 5000 programming environment, data is set up in a tag database. Memory addresses are hidden from the programmer, which makes things easier for the programmer.

Tag Database

Select a Tag from a Pull-down Menu While Programming

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In Logix, there is a controller-scope tag database and program-scope tag databases associated with each Program.

• Tags in the controller-scope database are global and can be accessed by routines in any part of the program.

• Program-scope tags can only be accessed by routines in that Program.

Publication LOGIX-AP008B-EN-P - June 2008