Hello,
If you have used a one.Can you tell me when the PC is connected to the MODBUS will the PC be acting as a master/slave ?

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.

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.

As for the spec.It is NOT a hi-fi one.

With the MODBUS you know we can add as many I/O's as we need.But my requirement is,

1.Digital input : 32
2.Digital output : 32
3.Analog input : 10 bit 4 channels
4.Analog output : 8 bit 4 channels

5.Scan time : 10mS
6.Communication : 1.With PC on MODBUS RS485
2.With modem on RS232.

7.Analog input & output must have digital user calibration.

8.Proposed processor : AtMega128 as the master PCL controller

9.Keyboard & Graphic LCD controller : P89C51RD2

10.digital input & output : AT89C51


Thanks

S.Sheik mohamed