Yes, if there is overvoltage on the RS485 line then it is possible that both the 75176 on the PLC and the Auto485 are fried. You can buy replacement ICs from Digikey.com or local electronic store rather easily. Alternatively you may want to buy a fault tolerant version of the RS85 driver made by Linear Technology. The part number is LT1785CN8 which can withstand up to +/-60V on the RS485 line. This chip is more robust if your RS485 line is long and has potential danger of experiencing high voltage spikes.