Siemens SIMATIC S7-300 Hardware And Installation Manual page 242

Glossary
GD circuit
A GD circle encompasses a number of CPUs which exchange data by means of
global data communication and which are used as follows:
• One CPU broadcasts a GD packet to the other CPUs.
• One CPU sends and receives a GD packet from another CPU.
A GD circuit is identified by a GD circuit number.
GD packet
A GD packet can consist of one or more GD objects which are transmitted together
in a frame.
Global data
Shared data can be addressed from any --> code block (FC, FB, OB). In detail, this
refers to memories M, inputs I, outputs Q, timers, counters and data blocks DB.
Absolute or symbolic access can be made to shared data.
Global data communication
Global data communication is a procedure used to transfer --> global data between
CPUs (without CFBs).
GSD file (device master file)
The device master file (GSD file) stores all slave specific properties. The GSD file
format is specified in EN 50170,Volume 2, PROFIBUS.
Instance data block
A DB is automatically generated and assigned to every function block in the
STEP 7 user program. The values of the input, output and in/out parameters are
stored in the instance data block, together with local block data.
Communication processor
Communication processors are modules for point-to-point and bus communication.
Compress
The programming device online function "Compress" is used to align all valid
blocks contiguously in the RAM of the CPU at the start of the user memory. This
eliminates all gaps which arose when blocks were deleted or modified.
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S7-300 Programmable Controller Hardware and Installation
A5E00105492-02