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寄稿のタイプ: FAQ 寄稿のID: 97649464, 寄稿の日付: 16.07.2014

How are the communication resources assigned in the S7-300?

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In the S7-300 there are different types of communication resources which must be regarded separately, but which affect each other to a certain extent.

You must consider the different properties of the CPU and the CP to calculate the maximum communication quantity framework. The communication resources are as follows:

  • Number of S7 CPU connections
  • Number of instances in the CPU
  • Number of connections for CPU open communication (blocks TCON, TSEND etc.)
  • Number of CP connections
  • Number of instances via the CP

The number of S7 connections and open communication connections in the CPU and in the CP restricts the maximum possible number of usable connections.
The T blocks TCON, TSEND, TRCV and TDISCON, for example, are used for the open communication via the CPU. The open communication connections are established only at runtime. They are not configured. If you use more connections than permitted, this is signalized only at runtime with an error on the TCON block.
The restriction for S7 connections is valid only for S7 connections. But these also include PG connections and OP/HMI connections as well as S7 standard communication connections.
The connections via the CP can be either S7 connections or open communication connections. The open communication connections via the CP are configured in NetPro. The AG_SEND and AG_RECV blocks are used respectively for sending and receiving the data.
The number of instances restricts the possibility of running parallel send and receive jobs over S7 connections, in other words how many instances of PUT, GET, BSEND, BRCV, USEND and URCV can run.

For the following three cases the properties of the CPU 317-2 PN/DP (article no.: 6ES7317-2EK14-0AB0) and of the CP343-1 (article no.: 6GK7343-1EX30-0XE0) are used.

  • S7 communication
  • Open communication
  • Instances

The following table shows the properties of the CPU 317-2 PN/DP and of the CP343-1.

Technical data CPU 317-2 PN/DP
Number of open communication connections 16 16
Number of S7 connections 32 16
Restrictions for S7 connections Max. 16 for S7 communication
Max. 30 for S7 basic communication
Max. 31 each for PG/OP connections
No restrictions
Number of instances 32 16

Table 1

S7 communication
You can configure a maximum of 16 S7 connections via the CPU. The other 16 resources are reserved for PG, OP and S7 Standard Communication.
You can also establish 16 S7 connections via the CP. However, the CP uses one S7 connection of the CPU as soon as at least one S7 connection or PG/OP connection runs over the CP. Here it does not matter whether 1 or 16 S7 connections run over the CP. Just one S7 connection of the CPU is used. The S7 connection of the CPU used is one of the configured S7 connections, not one of the additional 16 resources for PG, OP and S7 Standard Communication.

The following table shows the maximum number of S7 connections.

Number of CPs Configurable per CP Configurable per CPU Other (PG, OP, S7 Standard Communication) configurable per CPU Total
0 (only CPU) - 16 16 32
1 16 15 16 471)
2 16 14 16 621)
3 16 13 16 771)
4 16 12 16 921)

Table 2

1) Multiplexing on the CP is possible only with Resource 02 (OP communication).

Open communication
You can use the TCON block to establish up to 16 connections (TCP, ISO-on-TCP, UDP) in the CPU. This number is not restricted by other communication services and itself does not affect any other types of communication.
You can configure up to 16 open communication connections via the CP. These resources are shared with the S7 connections. The CP establish a maximum of 16 connections (mixed S7 and open communication connections).
Open communication in the CP does not affect the communication resources of the CPU. If you configure only open communication connections via the CP, no S7 connection of the CPU is used. The AG_SEND/AG_RECV blocks work with the read/write data record functionality. In this way they do not use any resources of the CPU. The maximum number of function calls is unlimited.

In the CPU you can run up to 32 instances in parallel for S7 communication jobs. More instances are rejected with an error. You can then only continue with further S7 communication jobs when an instance has completed a job on an S7 connection and you can then use the instance on another S7 connection.
A maximum of 16 instances can run over the CP. You can, for example, send and receive on 8 S7 connections or just send or just receive on 16 S7 connections. It is not possible to send and receive on all the 16 possible S7 connections. This applies even if you wait until a job has been completed to then work on a different S7 connection. The CP rejects this. 16 instances are the absolute maximum.

S7 Communication and Open Communication in parallel
It is possible to run S7 communication and open communication in parallel. These resources do not affect each other in the CPU. The two communication services share the resources in the CP. A maximum of 16 instances can run via the CP.

Further Information

  • More information about quantity frameworks of the CPUs and CPs is available in the technical data of the CPUs and the CPs.
  • Entry ID 16767769 includes information about the quantity frameworks of the Industrial Ethernet CPs.
  • Entry ID 18909487 includes information about the quantity frameworks of the S7-300 CPUs with integrated PROFINET interface.
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