Overview
To meet the different communication requirements at cell level (non-time-critical) and field level (time-critical) SIEMENS offers the following subnets.

MPI
The MPI subnet is designed for use at the cell level. MPI is the multipoint interface in SIMATIC® S7, and C7.
The MPI is basically a PG interface, that is, it is designed for the connection of PGs (for startup and testing) and OPs (human-machine interface). The MPI subnet can, however, also be used for networking a small number of CPUs.

Industrial Ethernet
Industrial Ethernet is the network for the plant management and cell levels in the SIMATIC® open, manufacturer-independent communication system.
Industrial Ethernet is designed for non-time-critical transmission of large quantities of data and uses gateways to provide facilities for connection to remote networks.

PROFIBUS
PROFIBUS is the network for the cell and field levels in the SIMATIC® open, manufacturer-independent communication system. There are two versions:
• PROFIBUS is for non-time-critical communication between equal, intelligent
nodes at cell level.
• PROFIBUS DP is the fieldbus for time-critical, cyclic data exchange
between intelligent masters and field devices.

Point-to-Point Connection
Point-to-point connections are primarily used for non-time-critical data exchange between two stations or for connecting devices such as OPs, printers, bar code scanners, magnetic stripe ID card readers, etc. to a station.

AS Interface
The Actuator-Sensor-Interface is a subnet for the lowest process level in an automation system. The AS Interface enables binary sensors and actuators to be networked.



Global Data

This communication method enables data to be exchanged between CPUs cyclically with the MPI interface without programming. Data is exchanged at the scan cycle checkpoint when the process image is updated. On the S7-400™, data exchange can also be initiated using SFCs. Global data can be inputs, outputs, bit memories, timers, counters and data block areas. Data communication is not programmed, but configured by means of a global data table. None of the connections on the CPU need to be used for global data communication.

Basic Communication

This communication method can be used with all S7-300™/400™ CPUs for transmitting data with the MPI subnet or within a station on its K bus.
System functions (SFCs), such as X_SEND at the Send end and X_RCV at the Receive end, are called in the user program. The maximum amount of user data is 76 bytes. When the system function is called, a connection to the communication partner is established and cleared dynamically. One free connection is required on the CPU.

Extended Communication

You can use this communication method with all S7-400™ CPUs. Up to 64KBytes of data can be transmitted with any subnet (MPI, Profibus, Industrial Ethernet). This transmission is done with system functions (SFBs), which also allow communication with acknowledgement. Data can also be read from or written to an S7-300™ (PUT/GET blocks). You can not only transfer data, but also perform control functions, like Stop or Start, on the communication partner. Configured connections (connection table) are required for communication by this method. These connections are established on a complete restart of the station and usually remain in force. Free connections on the CPU are necessary for this.



Introduction
Every programming device has an MPI interface. The MPI interface of the CPU enables all intelligent modules in a PLC to be accessed, for example, the function modules of a station.
Each MPI node needs its own MPI address (between 0 and 126, the default settings are PG=0, OP/TD=1, and CPUs=2).
In the S7-300™, the MPI bus is looped through on the K bus on a one-to-one basis. This means that every node on the K bus (FMs and CPs) in the S7-300™ rack is also an MPI node and needs to have its own MPI address.
In the S7-400™, communication frames are converted for the internal K bus
(10.5 Mbps) via the MPI (187.5 Kbps). In an S7-400™ rack, only the CPU has its own MPI address. The other intelligent modules, such as FMs and CPs, do not have a separate MPI number.
Connection Facilities
The main advantage is that several devices can establish a communication link with the CPU at the same time. This means, for example, that a programming device, an HMI device and a link with another PLC can be in operation at the same time.
The MPI interface also makes it possible to create a network in which a network administrator has central access with a PG to all the intelligent modules in the stations connected.
The number of channels for connection to other communication partners that can be used at the same time depends on the type of CPU. For example, the CPU 314 has four connection resources and the CPU 416 has sixty-four.
Features
Main features of the MPI interface:
• RS 485 physics
• Transmission rate 19.2 Kbps or 187.5 Kbps or 1.5 Mbps
• Distances up to 50 m (between 2 neighboring nodes) and with two repeaters,
1100 m and 23.8 km with optical fiber and star coupler.
• Profibus components (cables, connectors)
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