Modbus serial communication requires that the protection relay variant is equipped with a serial interface card at slot A2. The serial interface card can contain one or two serial interfaces.
The Modbus link mode can be either Modbus RTU or Modbus ASCII.
Modbus serial communication can run on two separate serial ports simultaneously. The Modbus serial link characteristics can be different on the two ports. This applies also to the Modbus RTU and ASCII link modes and the unit address.
Note: Documentation on the Modbus serial link messages and the Modbus standard can be obtained from www.modbus.org.