KL6031, KL6041 - Serial interface terminals

Operation with TwinCAT PLC software

Required TwinCAT version
These Bus Terminals are only supported from TwinCAT version 2.0 of the serial communication library (ComLibV2). In the case of Bus Terminals, the K-bus update time of the Bus Coupler must be considered when selecting the cycle time. Further information on configuration and operation with TwinCAT can be found in the Beckhoff Infosys at

KL6031 and KL6041 in 22-byte mode

The Beckhoff KL6031 und KL6041 serial Bus Terminals are operated in the delivered standard version with a 24-byte process image (22 bytes of user data + 1 Control/Status word), so that 22 data bytes can be transferred both from and to the terminal.
Three PLC cycles are required for the data exchange between the PLC and the Bus Terminal. The maximum effective data transfer rate (bps) depends on the cycle time T of the PLC and the number of bits necessary to transfer one data byte (LB):

bps = (LB x PA / 3) / T
LB = 1 start bit + n data bits + p parity bits + m stop bits
PA = number of user data in the process image

The maximum effective data transfer rate is upwardly limited by the physical bit/s rate configured in the Bus Terminal.

bps = (10 bits x 22/3) / 0.010 s ≈ 7333 bps


KL6031 and KL6041 in 5-byte mode

The serial Bus Terminal can also be reconfigured in such a way that 5 data bytes can be transferred both from and to the terminal in 5-byte mode (6-byte process image). 3 PLC cycles are still necessary for each exchange. The effective data rate for a given cycle time of the PLC is thus 5/3 greater than in 3-byte mode.

bps = (10 bits x 5/3) / 0.010 s ≈ 1666 bps

The Bus Terminals cannot be re-programmed while the PLC is running, since the 3-byte and 5-byte modes differ in the register mapping and in the TwinCAT System Manager configuration.


KL6031 and KL6041 in 3-byte mode

When operating the KL6031 + KL6041 in 3-byte mode, 3 data bytes (4-byte process image) can be transferred to and received by the terminal in a bus telegram. Since every data exchange between the PLC and the Bus Terminal requires 3 PLC cycles, the effective transfer rate is one byte per cycle.

bps = (10 bits x 3/3) / 0.010 s ≈ 1000 bps


Serial PC Interface

The serial PC interface (COM1, COM2 etc.) is handled by the TwinCAT system similarly to the serial bus terminal, but use larger data transfer buffers than the serial bus terminal. COMlib uses a 64-byte buffer, so up to 64 data bytes are transferred at once between the PLC and the interface driver. 3 PLC cycles are again needed for the exchange of a data block with the serial PC interface.

bps = (LB x 64/3) / T