what is the data stored in the registers and coils when used to communicate with a DCS and software to upload and downlload data from / to the DCS ?

 

The data stored in registers are highly device specific. Typically 16-bit Input Registers (IR's)contain read-only integer data like ADC counts or some other scaled analog data. Holding Registers (HR's) are similar to IR's only are read/write. HR's are good for holding configuration information that is usually maintained in EEPROM or some other non-volatile memory. IR's or HR's can also be concatenated to hold 32-bit floating point values. Input Status (or discrete inputs: DI's) and Coils are boolean (1/0) equivalents of IR's and HR's. Again this is all very device specific.

Hope this helps.
Bruce