Can CPU Act as a Modbus Master and Slave at the Same Time?

Yes, the CPU can act as both a Modbus Master and Slave at the same time but ONLY if they are on different communication ports.
For example you may connect the CPU via RS232 programming lead to one of our HMI’s, and set it to act as a Modbus RTU Slave.
At the same time, you could have an RS485 expansion module attached to the CPU acting as a Modbus RTU Master to other CPU’s or Modbus Slaves