Overvoltage protection for RS485 inputs
Rather than the brute-force method which is fraught with challenges &amp; compromise, I'd put a couple of PTC resistors (aka polyfuse) in series with the 2 RS485 lines, &amp; probably place them the last thing before they reach the connector to go off-board.

PTCs aka polyfuses have a near-zero resistance normally, but when you exceed their hold current &amp; approach their trip current, they go fairly high resistance, and stay that way until the 'fault current' is removed.  So if you apply 24V to the 485 pair, current will go through your termination resistor (120 ohms?) &amp; fairly quickly put them into trip.

For RS485, depending on what source &amp; line &amp; termination resistors &amp; impedance you've got, something around 50-100mA 'hold' current should be adequate for them to stay low-resistance at normal operating conditions.

PTC/polyfuse examples:
http://www.littelfuse.com/products/resettable-ptcs/radial-leaded.aspx