Rules for Configuring a Subnet

This chapter describes how to configure a subnet and provides examples for subnets.

You must observe the following rules when connecting the nodes of a subnet:

Before you interconnect the individual nodes of the subnet you must as- sign the MPI address and the highest MPI address or the “PROFIBUS address” and the “highest PROFIBUS address” to each node (except for RS 485 repeater).

Tip: Mark all nodes in a subnet with the address on their housings. In this way, you can always see which node has been assigned which address in your system. For this purpose, each CPU comes with an enclosed sheet of address labels.

Connect all nodes in the subnet “in a row”; that is, integrate the stationary programming devices and OPs direct in the subnet.

Note

From 3 Mbaud, use only bus connectors with order No. 6ES7-0B.10-0XA0 or 6ES7 972-OB.40-0XA0 to connect the nodes! (see also Section 7.2)

Connect only those programming devices/OPs that are required for startup or maintenance via spur lines to the subnet.

Note

From 3 Mbaud, use only the programming device connecting cable with order No. 6ES7 901-4BD00-0XA0 to connect the programming device! (see also Section 7.2)

If you operate more than 32 nodes on a network, you must connect the network segments via RS 485 repeaters.

All network segments in a PROFIBUS subnet must have at least one DP master and one DP slave between them.

You connect non-grounded bus segments and grounded bus segments via RS 485 repeaters (see the description of the RS 485 repeater in the Mod-

ule Specifications Reference Manual).

In this Chapter

Each RS 485 repeater that you use reduces the maximum number of nodes on each bus segment. That means if a RS 485 repeater is installed in one of the bus segments, only a further 31 nodes can be installed in that segment. The number of RS 485 repeaters has no impact on the maximum number of nodes on the bus, however.

Up to 10 segments can be installed in a row.

Switch the terminating resistor on at the first and last node of a segment.

Before you integrate a new node in the subnet, you must switch off its supply voltage.

Reserve the MPI address “0” for a service programming device and “1” for a service OP that will be connected temporarily to the MPI if required. This means, that you must assign different addresses to programming devices/OPs that are integrated in the MPI subnet.

Recommendation for MPI address of CPU in case of replacement or ser- vice:

Reserve the MPI address “2” for a CPU. You thus avoid that double MPI ad- dresses occur after connection of a CPU with default setting to the MPI sub- net (for example, when replacing a CPU). This means that you must assign an MPI address greater than “2” to the CPUs in the MPI subnet.

Reserve the PROFIBUS address “0” for a service programming device that can be connected temporarily to the PROFIBUS subnet if required. Allocate other PROFIBUS addresses to the programming devices integrated in the PROFIBUS subnet.

You connect the individual nodes via bus connectors and the PROFIBUS bus cable (see also Section 7.2). Make sure that the bus connector is provided with a programming device socket so that a programming device can be con- nected if required.

Use RS 485 repeaters to connect segments or extend the cable. Rules, Continued

Recommendations for MPI Addresses

Recommendation for PROFIBUS Ad- dresses

A cable must be terminated with its surge impedance. To do this, switch on the terminating resistor at the first and last node in a subnet.

The nodes with a terminating resistor switched on must have their power sup- ply switched on during power up and operation.

Fig. 7-1 shows you how to switch on the terminating resistor on the bus con- nector. Terminating resistor switched on Terminating resistor switched off on off on off

Figure 7-1 Terminating Resistor on the Bus Connector

Fig. 7-2 shows you where to switch on the terminating resistor on the RS 485 repeater. Terminating resistor bus segment 1 Terminating resistor bus segment 2 DC 24 V L+ M PE M 5.2 SIEMENS ON ON

Figure 7-2 Terminating Resistor on the RS 485 Repeater Terminating Resis-

tor

Terminating Resis- tor on the Bus Connector

Terminating Resis- tor on the RS 485 Repeater

Fig. 7-3 shows where you must connect the terminating resistor in a possible MPI subnet configuration.

RS 485 repeater

Terminating resistor switched on

S7-300 S7-300 S7-300 Spur line S7-300
OP 25 OP 25 PG PG*

* Connected via spur line for startup/maintenance only

Figure 7-3 Connecting a Terminating Resistor in an MPI Subnet Example: Termi-

nating Resistor in an MPI Subnet

Fig. 7-4 shows an MPI subnet that is configured in accordance with the above rules.

S7-300** S7-300 S7-300 S7-300

S7-300 S7-300

* Connected via spur line for startup/maintenance only (with default MPI address)

S7-300

** Connected to the MPI subnet later (with default MPI address)

0 13

0 ... x MPI addresses of the nodes

9 10 11 12 1 2 3 4 5 6 PG* OP 25 OP 25 PG OP 25**

Terminating resistor switched on

*** The CP also has a PROFIBUS address in addition to the MPI address (address 7 here) FM 8 CP 7 PROFI- BUS subnet***

Figure 7-4 Example of an MPI Subnet Example of an MPI

Fig. 7-5 shows a PROFIBUS subnet that is configured in accordance with the above rules.

** Connected to MPI via spur line for startup/maintenance only (with MPI address = 0)

0 ... x PROFIBUS addresses of the nodes

S7-300 with CPU 315-2 DP as DP master ET 200M 0 11 10 9 8 7 2 3 4 5 6 ET 200M S5-95U ET 200B ET 200B


Terminating resistor on ET 200M ET 200M ET 200M ET 200B ET 200B PG**

* 1 = Default PROFIBUS address for DP master

1* 3

0 ... x MPI addresses of the nodes

Figure 7-5 Example of a PROFIBUS Subnet Example of a PRO-

Fig. 7-6 shows an MPI subnet with an integrated CPU 315-2 DP, also operat- ing as DP master in a PROFIBUS subnet.

OP 25 S7-300 S7-300 with CPU 315-2 DP as DP master ET 200M S5-95U ET 200B RS 485 repeater PG* S7-300 S7-300 S7-300 OP 25 ET 200M S5-95U ET 200B
Terminating resistor on

* Connected via spur line for startup/maintenance only (with default MPI address)

S5-95U

ET 200B ET 200B 1 3 4 5 6 8 7 1 2 3 4 5 6 8 7 10 9

0 ... x MPI addresses of the nodes

   PROFIBUS addresses of the nodes 0

PROFIBUS subnet

MPI subnet

Figure 7-6 Example of a Configuration with the CPU 315-2 DP in an MPI and PROFIBUS Subnet Example with