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Demand Data 2408
f
& 2404
f
PROFIBUS Communications Handbook
7-2 2408
f
& 2404
f
PROFIBUS Communications Handbook
Demand Data uses the first 8 bytes in both the request and response message of the cyclic Data Exchange.
DEMAND DATA STRUCTURE
Read Request (from PLC)
PLC Output Register
Number
Output Data
The first four registers are reserved for demand data.
The control program is responsible for writing values into
these first four registers to make requests.
1 Command Code and Parameter
Tag
2 Extended Parameter Tag
3 Reserved
4 Anything
The registers that follow are used for the fixed output
data defined by the GSD file
5 Value or State
6 Value or State
7 Value or State
etc. Value or State
Write Request (from PLC)
PLC Output Register
Number
Output Data
The first four registers are reserved for demand data.
The control program is responsible for writing values into
these first four registers to make requests.
1 Command Code and Parameter
Tag
2 Extended Parameter Tag
3 Reserved
4 Value or State to be written
The registers that follow are used for the fixed output
data defined by the GSD file
5 Value or State
6 Value or State
7 Value or State
etc. Value or State
PLC Input Register Number Input Data
The first four registers are reserved for responses to
demand data.
1 Command Code and Parameter
Tag
2 Extended Parameter Tag
3 Reserved
4 Returned value
The registers that follow are used for the fixed input data
defined by the GSD file
5 Value or State
6 Value or State
7 Value or State
etc. Value or State
Response to Read Request (from Controller)
PLC Output Register
Number
Output Data
The first four registers are reserved for responses to
demand data.
1 Command Code and Parameter
Tag
2 Extended Parameter Tag
3 Reserved
4 Write error code
The registers that follow are used for the fixed output
data defined by the GSD file
5 Value or State
6 Value or State
7 Value or State
etc. Value or State
Response to Write Request (from Controller)