S7-200 Communication possibilities
Which network configuration is possible with the S7-200 and what needs to be observed?
The S7-200 offers a multitude of communication possibilities:
- PCs or special programming devices (PGs) can be connected.
- The textdisplay TD 200 or panels like the PP 7, OP7, TP 170A, TP 170B TP 070 can be used for process visualization. It is also possible to exchange data between the various S7-200 CPUs in the PPI protocol.
It is also possible to connect certain non-system devices like printers, bar code readers or central computers. However, in a S7-200 communication network, it cannot be used simultaneously.
- The S7-200 CPUs use a RS485 interface for communication.
To connect to RS232 interfaces, for example the serial ports COM1 or COM2 to PCs, the so-called PC/PPI may be used. First, attention has to be paid, which communication protocol is used:
- On one hand, there is the system specific S7-200 protocol which is called PPI protocol (PPI = Point to Point Interface). The communication with the programming device, with the TD 200, the OPs or between S7-200 CPUs takes place via PPI. The PPI baud rate is 9600, 19200 or 187500 baud.
- On the other hand, there is the possibility to communicate via
an available ASCII protocol, which on the S7-200 is called freeport
mode. In this case, a start, a stop and seven or eight data bits
are transferred and optionally a Paritybit.
The baudrate can be set to a value between 1200 and 115.200 baud. This means that a user specific protocol can be freely programmed to transfer data between CPUs or between central computers. This communications takes place unidirectional, which meand that the sender cannot receive and the receiver cannot send at the same time.
In a S7-200 network, these two communication protocols cannot be used simultaneously due to technical and physical restrictions. That means for example that a TD 200, that communicates in PPI mode, and a printer that communicates via a free ASCII protocol, cannot be networked with a CPU interface.
However, even in a pure PPI net, not all PPI devices can be used in any given number or mode since certain rules have to be observed. Networking CPUs with each other for the purpose of transferring data is trouble-free. With the exception of the old CPU 210 and the 212 from first-generation S7-200s, all other CPUs can act as master in the network. TD200 can be used also, and several TDs (starting with version 1.1) can be connected to a CPU.
The possibility to use an additional programming device in this network to load programs or monitor operating modi depends on the interface configured on the PG. If a Siemens PG with MPI interface or a PC with MPI card is used, there will be no problems. However, if a serial port of the PG is used, it will not work with Windows NT. Before changes are made to the program, the CPU concerned needs to be removed from the network.
Regarding the HMI components, the following applies:
- An OP in a PPI network with several CPUs that can be used to transfer date between each other is possible.
- An OP3 can be conntected to a maximum of two CPUs at the same time; OP 7/17 and OP 27/37 to a maximum of 4 CPUs.
- A listing of HMI compontent that can be connected to the S7-200 and items to be considered can be found in entry-ID 14188898 .
In general, a maximum of 32 users is possible in a PPI network, and this number can be increased to up to 128 uses with repeaters. The maximum cable lenght is 1000 m.
Examples for the use of communication with the S7-200 can be found in the STEP 7-Micro/Win Tips&Tricks in entry-ID 1232912 in the menu item Communication.
A MPI master may not access a PPI master!
System configuration, distributed control systems, network