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Special Hardware Requirements:

This example shows how an S7-200 multiple master PPI network can be accessed by a PC or programming device over a PC/PPI cable by means of a modem connection and the DTU-3000 by PanelTec. (A network with up to three masters was tested.) This allows the user to program or monitor a remote PLC with MicroWin. The modem used must support a character string of 11 bits for communication.

DTU – 3000: Description and Technical Specifications

 

Communications:
300 baud to 19.2 kbaud
3 serial communication ports DB-25
RS-232, RS-422, RS-485

Power Requirements:
9V to 28V
3 W

Selecting Parameters for the DTU – 3000

Panel-Tec will provide support for DTU-3000 parameter selection for individual applications.

Bausch-Induline: Description and Technical Specifications

Communications: 2.4 to 33.6 kbaud RS-232 or RS-485 Connectors: 25-pin Sub-D for serial interface and 9-pin Sub-D for power requirements Power Requirements: 7 to 38 VDC or 6 to 26 VAC External power supply Miscellaneous: Direct mounting or DIN mounting rail Remote configuration

Setting Up the Communications Parameters When Using Modems

To set up communications parameters between your PC or programming device and the CPU when using modems, you must use the interface parameter set for the PC/PPI cable.

Refer to the S7-200 System Manual for a detailed description of this procedure.

About PPI Networks

PPI networks are "token ring networks," meaning that the masters pass control of the network by circulating a token around in the network station address sequence. The token grants permission to a station to transmit, and only one station at a time can hold the token. A slave can never hold the token and is therefore never granted permission to transmit. In other words, slaves cannot initiate messages, but must wait until a master transmits a request or prompts the slaves for a response.

All S7-200 CPUs act as slaves in the network; but they can be used as masters if the PPI master mode is activated in the user program. Once the PPI master mode has been activated, data can be read from other CPUs or written to other CPUs using the instructions net read (NETR) and net write (NETW).

In PPI communication, the number of masters that can communicate with any given slave-CPU is not restricted. However, only a total of 32 masters can be present in the network.

Communication with the S7-200:

a) Single-master PPI network:

A single-master PPI network consists of one master and one or more slaves. In this example, a programming device is used as the only master and is connected over a PC/PPI cable to a network made up of several S7-200 CPUs. The CPUs do not communicate with each other (no NETR/NETW), that is, they are configured as slaves.

The programming device with STEP7-Micro/Win can be used to program or monitor the CPUs.

b) Multiple master PPI network

 

Multiple master operation means that TD200s and OPs can be operated in the PPI network and that data can be exchanged among the S7-200s when a programming device using MicroWin is active in the PPI network.

A multiple master PPI network consists of one or more slaves and multiple masters. In this example, the S7-200 CPUs are used as slaves, that is, they do not communicate with one another. The TD200s and the OP3 are masters because they exchange data with the CPUs assigned to them. The programming device with STEP7 Micro/Win is connected to the network by a PC/PPI cable and is also a master that can access the individual CPUs.

c) Communication with a network of PPI slaves by means of 11-bit modems

When you use STEP7-Micro/Win 32 under Windows 95, Windows 98, or Windows NT on a programming device as the only master, you can set up a connection by means of 11-bit modems to one or more S7 200 CPUs (slaves). (If 10-bit modems are used, the connection can only be set up to one CPU.)

Only one master and the PPI protocol are permitted in this configuration. In order for communication to take place by means of the PPI interface, the modem must work with 11-bit strings (1 start bit, 8 data bits, 1 parity bit, 1 stop bit, asynchronous transmission rate, and 9600 baud).

d) Communication by means of an 11-bit modem and DTU-3000 with a multiple master PPI network.

A multiple master PPI network cannot be accessed by means of a modem with a PC/PPI cable because the transmission rate of the modem is not sufficient to ensure correct handling of the token. To set up the connection you need an additional device, such as the DTU-3000 by Panel-Tec, to maintain communication on the network side. On the modem side, the device must act like a PPI slave. In this way, the programming device is the only master in the network and does not have to pass the token within the network.

On the PPI network side, the DTU-3000 acts like a master and takes on the task of passing the token to the next station address designated as a master.

For communication by means of a PPI interface, the S7-200 requires a modem with a transmission format of 11 bits because S7-200 controllers require 1 start bit, 8 data bits, 1 parity bit (even parity), 1 stop bit, asynchronous communication, and 9600 baud for the PPI protocol.

The Bausch-Induline modem by KSI-Kommunikation is used in this example.

The supplied connection cable is used to connect the programming device to the modem. On the CPU side, the modem is connected to a port on the DTU-3000 by means of a serial interface cable. (A null modem adapter is not required). The DTU-3000 is connected to the PPI network by means of a PPI cable.

Note:  How the modem and PPI network are connected to the ports depends on how the DTU-3000 is configured. In this case, the DTU-3000 is configured so that Port 1 must be connected to the PPI network and Port 3 must be connected to the modem.