Function validate_read_discrete_inputs 

pub fn validate_read_discrete_inputs(
    address: u16,
    quantity: u16,
) -> Result<(), ExceptionCode>

Validate a read discrete inputs (FC 02) request.

Per spec §6.2 Figure 12 state diagram.

Errors

• IllegalDataValue if quantity is 0 or exceeds 2000
• IllegalDataAddress if address + quantity > 0xFFFF