The EL6861 serial interface enables the connection of BACnet MS/TP devices (Master-Slave/Token-Passing) via an RS485 interface. Up to 32 MS/TP devices can be connected (up to 64 if ultra-low power transceivers are used). The EL6861 interface can only be used in combination with TwinCAT, then at least a CX9020 is required. When using TwinCAT 3, at least a CX81xx since the BACnet MS/TP driver integrated in TwinCAT implements the communication via the MS/TP protocol (BACnet Revision 12). The configuration is also done in TwinCAT.

The RS485 interface guarantees high immunity to interference through electrically isolated signal transmission. The EL6861 provides 1 x 5 V DC at 20 mA (electrically isolated, short-circuit proof) from the E-bus supply for use with the bias resistors (network bias).

The following devices can be typically connected to the EL6861:

• frequency converters
• pumps
• drives (especially valve drives)
• room control units
• compact controllers (for example volume flow controllers)

https://www.beckhoff.com/vi-vn/products/i-o/ethercat-terminals/el6xxx-communication/el6861.html