Siemens Industry Online Support
Siemens AG
Entry type: Application example Entry ID: 48955385, Entry date: 09/18/2018

SIMOTION, SIMATIC: Communication Library LCom for Ethernet Communication

  • Entry
  • Associated product(s)
In the field of automation, the TCP protocol based on Industrial Ethernet is typically deployed for safe, time non-critical (> 25 ms) and multi-system communication.

Standard mechanisms can be used for networks that include devices from different system families. The widespread and versatile Industrial Ethernet and the standard protocol TCP/IP in use worldwide, are suitable as a bus system.


The LCom library implements a point-to-point connection in both directions (full duplex) between two devices.

The function block can be used for standard TCP communication. The widespread deployment of the TCP standard allows data exchange to be implemented between two different devices.

All types of controllers and devices that have an Ethernet interface and support the TCP protocol can be connected, e.g. cameras, printers, scanners and PCs.

Since the functional scope of TCP is not sufficient for many applications, the LCom library defines a separate transport protocol (the LCom protocol). The LCom protocol can be activated if the communication partner supports the LCom protocol too. If not, the native TCP communication can be used. The following additional functions are supported by LCom protocol:

  • Data records with defined length up to 16 MB (with V1 up to 64 kB possible)
  • Different transmission modes. (cyclic, at data change or once)
  • Monitoring of the connection through cyclic transmission of a sign-of-life (faster response times on connection failure). In the case of a pure TCP communication, this is typically in the range of seconds.
  • Prevention of a full working memory on the system level through frequent sending of data via integrated flow control on the application level.
  • Feedback from receiver on successful data transmission on application level.
  • Simple time synchronization.
  • Data transport in both transmission directions can be started and stopped by both communication peers.

The function block can be instantiated and thereby used for several communication relationships (e.g. in a production line both to the upstream and to the downstream machine).


SIMATIC S7-300/400
  Manual SIMATIC 02/2013 (1,9 MB)
   ReleaseNotes 02/2015 (8,4 KB) 

      STEP7 V5.5
        Registrierung notwendig  LCom V1.2.8 (1,1 MB) 

      STEP 7 Professional/Basic V14 / V15
 Registrierung notwendig  LCom V1.2.8 (3,1 MB)

SIMATIC S7-1200/S7-1500
 Manual SIMATIC 10/2016 (1,2 MB) 

STEP 7 Professional/Basic V14 / V15
 Registrierung notwendig LCom V2.0.5 (3,1 MB)

STEP 7 Professional/Basic V13 SP1 
 Registrierung notwendig LCom V2.0.4 (1,5 MB)
 Registrierung notwendig LCom Example V2.0.4 (3,7 MB)



Version 2
  Manual SIMOTION 04/2017 (1001,5 KB)
 Registrierung notwendig  LCom V2.0.0 (345,8 KB)

The LCom version 1.2.x is obsoleted. For future projects the LCom version 2 is to be used!
The successor version LCom version 2 is easier to parameterize and data records with a defined length up to
16 MB can be transferred. Version 2 is compatible with existing systems protocol version 1 (SIMOTION <= V1.2.6, S7-300/400).

Additional Keywords
motioncontrol_apc_applications, CP, WinAC RTX, WinLC
Security information
In order to protect technical infrastructures, systems, machines and networks against cyber threats, it is necessary to implement – and continuously maintain – a holistic, state-of-the-art IT security concept. Siemens’ products and solutions constitute one element of such a concept. For more information about cyber security, please visit