sylvac PLC Mode D'emploi page 58

A.2 Frame description
Query frame format
Address field (8 bits)
Where :
Address field : The slave number (1...247, 0=reserved for broadcast)
Function code : See following table for supported functions
Error check (CRC) : Based on polynome 1 + X2 + X15 + X16
Function code (8 bits)
ADU (Application Data Unit)
PDU (Protocole Data Unit)
Data (any bits) Error check (CRC) (16 bits)