Supported Hardware

KL6xxx in 3-Byte Mode

The standard version, as supplied, of the Beckhoff serial bus terminal is operated in 3-byte mode. In other words, a bus telegram can transmit or receive 3 data bytes to or from the terminal. Since every data exchange between the PLC and the bus terminal requires 3 PLC cycles, the effective transfer rate is one byte per cycle.

The maximum effective data transfer rate in bits per second depends on the cycle time, T, of the PLC, and on the number of useful bits in each data byte transferred, LB:

Bps = LB / T

LB = 1 start bit  n data bits  p parity bits  m stop bits

The maximum effective data transmission rate is limited by the physical baud rate programmed into the terminal.

KL6xxx in 5-Byte Mode

The serial bus terminal can be reprogrammed offline by means of a configuration program (Beckhoff KS2000), so that, in 5-byte mode, 5 data bytes at a time can be transferred to or from the terminal. 3 PLC cycles are still necessary for each exchange. The effective data rate for a given cycle time is thus 5/3 greater than in 3-byte mode.

Bps = (LB * 5/3) / T

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. For this reason, the required re-programming means that 5-byte operation is only recommended for time-critical applications.

In order not to unnecessarily load the memory of the BCxxxx bus controller, a modified ComLibBC5B library is available for 5-byte mode.

Serial BC8x00 Bus Controller

The BC8x00 bus controller contains a serial interface that works in 5-byte mode as standard. The interface behaves like a serial KL6xxx bus terminal in 5-byte mode. The COMlibBC5B is therefore to be used for programming.