to increase the modbus clients number of S7-400_PN_?

Hello;

Where in the manual does it show that the Modbus server functions are limited to 4 clients?

First of all, the number of available communication connections dépends on the CPU technical data, please look them up in the search Box at the right of this page.

Second, Modbus TCPfunctions described in the manual  allow fore multiplexing of connections to port 502 (from the manual

SIMATIC Modbus/TCP Redundant Communication of PN H Systems.):

3.4 Multiple connections to port 502

General information

Some CPUs can multiplex TCP connections. In these cases, multiple MODBUS clients can connect to port 502 of the CPU (multiport). The CPU acts as the MODBUS server.

Information on which CPUs with which firmware versions allow multiple port 502 use is available here:

www.siemens.com/s7modbus

Requirements

For this function to be available, the following settings must be made in block selection and parameter assignment:

· CPU is the server

· Port 502 is the local port

· Unspecified TCP connection

· Passive connection establishment

Number of connections enabled

The number of connections that a CPU can accept at port 502 depends on the module. Please see the technical data of the CPU for details.

Configuration

One unique connection in the parameter DB is required for each client that is to connect to port 502 of the server.

Hope this helps,

Daniel Chartier

Hello,
you talking about Modbus TCP communication via CP, am I right?

CPs work with static connections. This means each local port can be used once. Some CPs have realised a special feature with allows up to 4 connections via port 502. But this feature is not usable with other port numbers. All other port numbers can only be used once.

In your case I recommend to use ModbusTCP communication via the integrated PN interface of the CPU. The limitation there is the number of "open communication connections" which is defined in the technical data of the CPU.

Regards,
Kaulquappe

Hello,
I think you have a general problem with understanding. This Modbus blocks are for redundant communication. One redundant device has got two IP addresses and 2 or 4 logical connections - depending on single sided or double sided redundancy.

When you want to connect more than one redundant device, you need two calls of the Modbus block with different instance DBs and different parameter DBs.

The Modbus communication in S7 is capable to use port numbers different to 502.

When you use port 502 for connetctions to different devices you don't know which client device connects to which Modbus FB call.

Regards,
Kaulquappe