With classic Serial circuits( like a serial port on a Pc and/or a modem etc) there are a bunch of control signals (up to 4 in and 5 out from memory) - which could be either on or off (i.e. 12 volts, or 0 volts) that were used for the sorts of complicated processing back in the days when we had dial telephones and the like, plus there are 1 or 2 'data' signal wires, which are where the data being communicated goes, in a on/off (serial) fashion at a certain frequency [baud rate] and in a specific format [parity, stop bits, # of bits, 5, 6 7 8 etc].


The headlight on/off and Cellphone mute inputs to the Empeg are treated as if they are Serial control signals (even though they are technically nothing to do with serial communications).

For historical reasons the devices that convert the 12 volts or more on/off to 'control signal on'/ 'control signal off' is called a RS232 Transceiver and they are generally pretty toughly designed to take all kinds of electrical conditions that would fry the average chip - which is why they are used to convert the voltages present in your cars electrical circuits for lights and cellphone mutes etc to to much lower voltages used inside the RioCars.