The computer serial input/output port is one method of communicating with other devices, i.e. external modem. Mostly it’s connected for two-way communication, which can also include provision for handshaking
signals between the connected devices. In this one-way example each key-press will generate an 8-bit word that is converted from its parallel form to serial data within an integrated circuit called a UART (Universal Asynchronous Receiver Transmitter). Within this ic, start and stop bits are added along with an optional parity bit. In this case set to ‘ODD’ as there will always be an output pulse, even if there is a NULL keyboard character. The RS232 driver inverts the data and raises the logic levels to approximately ± 12V prior to sending the data down an external cable to another device.
At the receiver end the incoming data is inverted back to its original state and converted to TTL binary data levels. Each character is decoded by looking for a logic pattern, defined at the system design time. In this example it’s a LOW start bit, which is a transition from a previous HIGH stop bit, followed ten bits later by a HIGH stop bit. At this stage it doesn’t care about the data or parity levels. Parity checks should indicate there is always an odd number of logic 1’s within the data word. If this is incorrect then it may be necessary to retransmit that character as the parity check has indicated a transmission error.