Distributed Peripheral (Dp) Standard Communications - Siemens Simatic S7-200 System Manual

9.5

Distributed Peripheral (DP) Standard Communications

The PROFIBUS-DP Standard
PROFIBUS-DP (or DP Standard) is a remote I/O communication 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 CPU 215 has implemented the DP Standard protocol as defined for slave devices in the
following communication protocol standards:
EN 50 170 (PROFIBUS) describes the bus access and transfer protocol and specifies
the properties of the data transfer medium.
EN 50 170 (DP Standard) describes the high-speed cyclic exchange of data between DP
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 now 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 CPU 215 as a DP Slave
The CPU 215 can be connected to a PROFIBUS-DP network, where it functions as a DP
slave device. Port 1 of the CPU 215 (labeled DP on the unit) is the DP port. This port
operates at any baud rate between 9600 baud and 12 Mbaud. As a DP slave device, the
CPU 215 accepts several different I/O configurations from the master to transfer different
amounts of data to and from the master. This feature allows you to tailor the amount of data
transferred to meet the requirements of the application. Unlike many DP devices, the
CPU 215 does not transfer only I/O data. The CPU 215 uses a block of variable memory to
transfer to and from the master. This allows you to exchange any type of data with the
master. 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 CPU 215. Likewise, data
from the master is stored in variable memory in the CPU 215 and can be moved to other
data areas.
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02
Network Communications and the S7-200 CPU
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