Function validate_read_registers 

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

Validate a read holding/input registers (FC 03/04) request.

Per spec §6.3 Figure 13 / §6.4 Figure 14 state diagrams.

Errors

• IllegalDataValue if quantity is 0 or exceeds 125
• IllegalDataAddress if address + quantity > 0xFFFF