For a fault code 76, which table 17 says is an "accessory fault" (document 65-0249-07, S7810M Modbus Module), register 40001 decimal, (address 0000hex) will hold the 16 bit, unsigned integer 76 decimal.
The next sequential address, 0001hex (register 40002) will hold the 'string code', an unsigned 16 bit integer, in this case 11 decimal. **
Table 17 gives the string code values associated for each fault code.
Table 16 gives the text string displayed by the 7800 for that string code. *
The text string associated with code 11 is ACCESSORY FAULT
The text string itself is not available via Modbus RTU, only the numerical string code representing the text string *is available.
Fault code 76 decimal = 4C hex
The full 16 bit word in register 40001 would be 004C hex.