The RS-232 standard defines multiple key electrical characteristics. RS-232 is a single ended point-to-point communication protocol – which means each data signal travels along one wire (as opposed to differential which uses two wires for one data signal) and there are only two nodes at each end of the communication bus. The nature of this bus is inherently unbalanced. The bus operates in a full duplex manner – which means that the bus can transmit and receive data simultaneously. Older versions of the standard required a data-rate of 19.2Kbps at a cable length of 50 feet; however, since there is variation between many cables with respect to their electrical properties modern revisions of the RS-232 standard now requires a minimum data-rate of 19.2Kbps with a capacitive load of 2500 pF – where the bus length is limited by capacitance per unit length. RS-232 transceivers can go up to 1Mbps. The signal levels are defined as such: a logic one from a transmitter is between –5 V and –15 V and logic 0 from the transmitter is between 5 V and 15 V. For a transceiver to be compliant, the device must recognize a voltage from –3 V to –15 V as logic 1 and voltages between 3 V and 15 V to be a logic 0 – the receivers can be more sensitive depending on the specific device chosen. The maximum output current can be up to 500 mA by standard, but generally most ICs are designed well below this level. Finally, the input impedance of a receiver is nominally 5 kΩ but can be as low as 3 kΩ.