The 7 and 8 pins of max3088 are connected with the bus after passing through the protection circuit. D12, D14 and D16 are lightning protection diodes, which can eliminate sharp pulse and high voltage in the circuit and have the function of protecting electronic devices. One end of D12 and D16 is grounded to prevent differential mode interference and D14 to prevent common mode interference. Power resistors R28 and R41 have the function of current limiting and protecting circuit. Voltage clamped diode D13 and d17 can clamp the interference signal below 6.8v. Resistor R29 is used as bus matching resistor for standby.