Hi,
1.if acting as master then what happens when the PC is unplugged.And who will controll the MODBUS devices regardless of the status of the PC.

If PC needs data from the I/O cards continuously, Modbus Master needs to run in an infinite loop. Modbus Slave does nothing, except respond to a proper/improper modbus query (read modbus spec) from the modbus master. Hence if the slave does not receive any request from the modbus master, it sits quietly waiting to be queried with a valid request.


2.If actine as slave then how can the PC get the details from the PLC when ever it needs or will the PC be waiting in a loop for the PC to query it.

The PC cannot act as Modbus slave. A slave device can only report data and cannot request for any specific I/O points. If the PC is Modbus Master, it requests for digital and analog I/O at the specified address.

Modbus spec ... Analog input : 10 bit 4 channels
Modbus protocol does not define any 10 bit data type. So the PC (Modbus Master) will have to request 16 bit data and then mask the not required 6 bits.