J.,

The dummy struct and macro are only required to preserve naming.

typedef struct h
{
    struct sd {
    U8    SDser[8]  ; // serial number
    U8    SDaddr    ; // address assigned to sign
    U8    SDdips    ; // dipswitch setting
    U8    SDtype    ; // type of sign
    U8    SDwidt    ; // width
    U8    SDhgth    ; // height
    } sd;
    U8    SDfill[256-sizeof(struct sd)-1] ;  // fill struct to be 256 long

    // put this one last
    U8    SDerno    ; // here store error number if FRAM match/update failed.
} SIGN_DESC;