Siemens SIMATIC S7-200 System Manual page 454

S7-200 Programmable Controller System Manual
Distributed Peripheral (DP) Standard Communications
PROFIBUS--DP (or DP Standard) is a remote I/O communications protocol defined by the
European Standard EN 50170. Devices that adhere to this standard are compatible even though
they are manufactured by different companies. DP stands for distributed peripherals, that is,
remote I/O. PROFIBUS stands for Process Field Bus.
The EM 277 PROFIBUS--DP module has implemented the DP Standard protocol as defined for
slave devices in the following communications protocol standards:
EN 50 170 (PROFIBUS) describes the bus access and transfer protocol and specifies the
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properties of the data transfer medium.
EN 50 170 (DP Standard) describes the high-speed cyclic exchange of data between DP
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masters and DP slaves. This standard defines the procedures for configuration and
parameter assignment, explains how cyclic data exchange with distributed I/O functions,
and lists the diagnostic options which are supported.
A DP master is configured to know the addresses, slave device types, and any parameter
assignment information that the slaves require. The master is also told where to place data that is
read from the slaves (inputs) and where to get the data to write to the slaves (outputs). The DP
master establishes the network and then initializes its DP slave devices. The master writes the
parameter assignment information and I/O configuration to the slave. The master then reads the
diagnostics from the slave to verify that the DP slave accepted the parameters and the I/O
configuration. The master then begins to exchange I/O data with the slave. Each transaction with
the slave writes outputs and reads inputs. The data exchange mode continues indefinitely. The
slave devices can notify the master if there is an exception condition and the master then reads
the diagnostic information from the slave.
Once a DP master has written the parameters and I/O configuration to a DP slave, and the slave
has accepted the parameters and configuration from the master, the master owns that slave. The
slave only accepts write requests from the master that owns it. Other masters on the network can
read the slave's inputs and outputs, but they cannot write anything to the slave.
Using the EM 277 to Connect an S7-200 as a DP Slave
The S7-200 CPU can be connected to a PROFIBUS--DP network through the EM 277
PROFIBUS--DP expansion slave module. The EM 277 is connected to the S7-200 CPU through
the serial I/O bus. The PROFIBUS network is connected to the EM 277 PROFIBUS--DP module
through its DP communications port. This port operates at any PROFIBUS baud rate between
9600 baud and 12 Mbaud. See the Specifications for EM 277 PROFIBUS--DP Module for the
baud rates supported.
As a DP slave device, the EM 277 module accepts several different I/O configurations from the
master, allowing you to tailor the amount of data transferred to meet the requirements of the
application. Unlike many DP devices, the EM 277 module does not transfer only I/O data. Inputs,
counter values, timer values, or other calculated values can be transferred to the master by first
moving the data to the variable memory in the S7-200 CPU. Likewise, data from the master is
stored in variable memory in the S7-200 CPU and can be moved to other data areas.
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