Description:
This entry gives you an overview of the system limits of S7 Communication.

The following figures show the basic configuration of S7 Communication between F CPUs via Industrial Ethernet. Bidirectional data communication is via an S7 connection.

Fig. 01

Alternatively, bidirectional data communication can be via two separate S7 connections. In this way, you can structurally separate the send and receive channels, for example.

Fig. 02

The system limit of S7 Communication is determined by the following parameters:

• Max. number of connections supported by the CPU.
• Max. number of S7 connections that can be configured per interface.
• Max. number of instances supported by the CPU.

Max. number of connections supported by the CPU
The following table shows the maximum number of connections supported by the F CPUs.
 

F CPU	Max. number of connections
IM 151-8F CPU	12
CPU 315F-2 PN/DP	16
CPU 317F-2 PN/DP	32
CPU 319F-3 PN/DP	32
CPU 416F-2 DP	64
CPU 416F-3 PN/DP	64
WinAC RTX F 2009	64

Max. number of S7 connections that can be configured
The following table shows the maximum number of S7 connections supported by the F CPUs.
 

F CPU	Max. number of S7 connections that can be configured
IM 151-8F CPU	10
CPU 315F-2 PN/DP	14
CPU 317F-2 PN/DP	16
CPU 319F-3 PN/DP	16
CPU 416F-2 DP with CP443-1 Adv.	62
CPU 416F-3 PN/DP	30
WinAC RTX F 2009	Via CP5611: 6 Via CP5613: 48 Via CP1616: 30 Via IE general: 14

Max. number of instances
The following table shows the maximum number of instances supported by the F CPUs.
 

F CPU	Max. number of instances
IM 151-8F CPU	32
CPU 315F-2 PN/DP	32
CPU 317F-2 PN/DP	32
CPU 319F-3 PN/DP	32
CPU 416F-2 DP with CP443-1 Adv.	Firmware version < V5.2: 1800 (can be configured: 600 preset) Firmware version V5.2 onwards: 4000 (can be configured: 600 preset)
CPU 416F-3 PN/DP	600 (internal interface)
CPU 416F-3 PN/DP with CP443-1 Adv.	Firmware version < V5.2: 1800 (can be configured: 600 preset) Firmware version V5.2 onwards: 4000 (can be configured: 600 preset)
WinAC RTX F 2009	600 (can be configured: 300 preset)

Example:
In a CPU 319F-3 PN/DP, you select S7 Communication via TCP/IP for safe bidirectional data communication. Depending on whether the data communication is via one or two configured S7 connections, you can configure another 15 or 14 S7 connections.

The fail-safe communication blocks "F_SENDS7" and "F_RCVS7" are called in the CPU user program for safe bidirectional data communication via S7 connections. These blocks internally call the system function blocks SFB8 "USEND" and SFB9 "URCV" respectively. In this way, the user data and associated acknowledgments are sent and received. An instance data block is assigned to each SFB8 "USEND" and SFB9 "URCV" system function block. Thus, the number of instance data blocks (= instances) is identical to the number of communication jobs.

This means that in the case of safe bidirectional data communication, at least four communication jobs are executed and four instances are needed. In this case, with the CPU 319F-3 PN/DP there remain 28 free instances.

In the user program of the CPU 319F-3 PN/DP, you can call a maximum of 16 fail-safe communication blocks "F_SENDS7" or F_RCVS7", because the maximum of instances is limited to 32.
In the case of safe bidirectional data communication, the CPU 319F-3 PN/DP can communicate with a maximum of 8 F CPUs.

Calculation for safe bidirectional data communication in the CPU 319F-3 PN/DP:
8  "F_SENDS7" + 8 "F_RCVS7" = 16 fail-safe communication blocks
8*("USEND" + "URCV") + 8*("USEND" + "URCV")
= 16 "USEND" + 16 "URCV" = 32 communication jobs or instances

Note:
The safety function is foremost with F CPUs. Therefore, the system limit of S7 Communication is determined not only by the number of communication connections, but also by the response times achieved. If the required response times are not achieved because of the number of communication connections, the remedy is to

• Reduce the number of communication connections.
• Use a larger and faster CPU.