 Before large-scale integration (LSI) UART integrated circuits were common, a minicomputer would have a serial port made of multiple small-scale integrated circuits to implement shift registers, logic gates, counters, and all the other logic for a serial port.Early home computers often had proprietary serial ports with pinouts and voltage levels incompatible with RS-232.Inter-operation with RS-232 devices may be impossible as the serial port cannot withstand the voltage levels produced and may have other differences that "lock in" the user to products of a particular manufacturer.