Technical restrictions

Activating/Enabling the communication

With newer Siemens S7 Controllers, the communication must be explicitly activated or enabled. Please consult the manual for your Siemens S7 Controller for more information about enabling the TCP/IP communication with the controller. The chapter Activating the S7 protocol access shows you some sample screenshots from the TIA portal.

Only communication with absolutely addressed variables

When communicating with variables from the relevant Siemens data area, only absolutely addressed variables can be used. A symbolic access is not possible. Caution is therefore required in case the Siemens control program changes, thereby possibly making memory addresses change.

Maximum frame size per request

Siemens S7 Controllers have a maximum frame size per request. This is reported by the controller when the communication is started up and is approximately 960 bytes for most S7 types. If more than 960 bytes are created in the process image, the TwinCAT S7 communication driver automatically splits the requests into several requests.

Maximum number of connections

The function TF6620 is based on the TwinCAT TCP/UDP RT stack, which allows a maximum of 32 connections (per stack) as standard.

Additionally, there may also be limitations on the part of the Siemens S7 Controller regarding the simultaneous number of TCP/IP connections.

Maximum number of requests per connection

A maximum of 64 messages per connection can be configured. However, this number can be increased by additional connections.

Maximum number of variables per request

A maximum of 255 variables can be used per request. However, the number can be increased by additional requests.