RS232 link for 5V-only serial ports
The MAX251CSD+T: It handles the voltage-level translation between a 5V UART and the ±5V to ±15V RS232 bus, so no additional charge-pump or dual-rail supply is needed on the board.
What the 600 mV hysteresis means for noise margin
The receiver hysteresis is specified at 600 mV, which filters out noise on long or unshielded RS232 cable runs. A typical RS232 receiver without hysteresis might toggle on a 300 mV glitch; the 600 mV band means the input must cross a wider threshold before the output changes state, reducing false start-bit detection in electrically noisy environments like factory-floor serial links.
14-SOIC footprint and supply rails
The single 5V supply rail simplifies the power tree — no 12V or -12V rails are required on the board for the RS232 interface.