This third signal is the reference potential used by the transceiver to measure the A and B voltages. Along with the A and B connections, the EIA customary additionally specifies a 3rd interconnection level known as SC, which is the common sign reference floor. To address these limitations, the EIA developed the RS-485 standard. The origin of RS-485 can be traced again to its predecessor, RS-232, which was developed within the 1960s by the EIA.