12-8 www. m oxa. com Multiport Serial Boards > The Basics of RS-232/422/485 Multiport Serial Boards 12 The general relationship between the UART, TTL signal, transceiver, and RS-232 signal is illustrated in the following figure. UART is short for “universal asynchronous receiver transmitter,” and TTL stands for “transistor-to-transistor logic.” The UART, which is located on the serial board and stands between the computer’s CPU and the transceiver, transmits signals at 0 and 5 volts. The RS-232 transceiver converts the signal voltage to +3V to +15V, and -3V to -15V. RS-232—the most common and easy-to-use communication interface The Basics of RS-232/422/485 Start bit: 1 bit Data bits: 5, 6, 7, or 8 bits Parity: None, Odd, Even, Space, Mark Stop bits: 1, 1.5 (if data bits = 5), or 2 bits The RS-232 serial interface was developed to connect a computer to common peripherals such as modems, overhead projectors, and the sensors and actuators used for industrial automation applications. Despite its limited 15 m transmission distance, RS-232 is low cost and easy-to-wire, making it the first choice for many applications. Standard Connection Type Operation Mode Drivers per Line Receivers per Line Max. Cable Length Max. Data Rate EIA RS-232 Point-to-point Single-ended 1 1 50 ft (15 m) 921.6 kbps D0 UART Stop Parity Data Start Stop Parity Data Start Stop Parity Data Start Transceiver Stop Parity Data Start D7 TTL 5V 0V Parallel RS-232 +3V to +15V -3V to -15V Tx Rx 5V 0V +3V to +15V -3V to -15V TxD Transmit Data RxD Receive Data RTS Request to Send CTS Clear to Send DTR Data Terminal Ready DSR Data Set Ready DCD Data Carrier Detect GND Ground Stop Parity Data Start 1 5, 6, 7, 8 1 1, 1.5, 2 Unit: bit RS-232 establishes full-duplex (2-way) communication, with signals represented by voltage levels measured with respect to a system common ground (power or logic ground). The “idle” state (MARK) is negative with respect to the common ground, and the “active” state (SPACE) is positive with respect to the common ground. RS-232 Data Format RS-232 Signal Definition RS-232 Specifications