The test is conducted on a MAX485 device, and the test conditions are as shown below: The power supply input is left floating, a 0.1uF capacitor is connected between the power supply and GND, and B is connected to GND. When different voltages are applied to A, the results of power supply voltage and RO output voltage are monitored:
A=2.4V VCC=1.2V OUT=0.22V
A=5V VCC=1.7V OUT=1.73V
A=8V VCC=2V OUT=2V
A=10V VCC=2.2V OUT=2.2V
Based on the test results, when a voltage is applied between A and B before the device is powered, the power supply voltage and output voltage are both kept at relatively low voltage and since none of the absolute maximum ratings are violated , the device is not damaged. In addition, an RS485 device can withstand at least a common mode range of -7V to 12V.