RS485AC is the SOHARD designation for the ARCNET signals of the SMSC transceivers HYC4000 and HYC5000. As with RS-485 / EIA-485, logical ones are transmitted by square-wave pulses. However, this is done with alternating polarity, which means that the signal is DC-free (hence “AC”).
Other designations are “EIA-485 AC coupled”, “EIA-485 AC coupled” or “RS485 isolated”.