Introduction to RS-232C Interface

The D-sub connector on the computer is mostly used in the RS-232C interface.

RS-232C basic characteristics

• Connector: DB25 and DB9 (D type) connectors are used. DB25 is mostly used for early devices, and DB9 is mostly used now.

• Cable length: The maximum length of the RS-232C cable is related to the cable type, communication speed, etc., and is generally limited to 15 meters.

• Communication rate: fixed optional speed • Connector: DB25 and DB9 (D type) connectors, DB25 is mostly used for early devices, and DB9 is mostly used now. Rates 110, 300, 600, 1200, 2400, 3600, 4800, 7200, 9600, 14400, 19200, 28800, 33600, 38400, 57600, 115200 bps (BitPerSecond).

• RS-232C signal level: Bipolar unbalanced mode, negative level (-3~-15V) stands for logic 1, positive level (+3~+15V) stands for logic 0. Generally use ± 5V or ± 12V. The interface circuit completes the conversion of the internal logic level (0~3/5V) and the interface signal level (-12~+12V).

• RS-232C internal waveform and interface signal waveform:

• RS-232-C interface connector: The two ends of the connection are DTE (Data Terminal Equipment) and DCE (Data Communication Equipment).

• Interface signal description (→: DTE to DCE, ←: DCE to DTE)

–TxD (TransmittedData→): Data transmission.

–RxD(ReceiveData←): Data reception

–RTS (RequestToSend→): Request to send, indicating that data is required to be sent to DCE.

–CTS (ClearToSend←): Clears the transmission (allows transmission), indicating that the DCE can receive data and respond to the RTS.

–DTR (DataTerminalReady→): The data terminal is ready, indicating that the DTE is ready.

–DSR(DataSetReady←): The data device is ready, indicating that the DEC is ready.

– RI (RingIndicator ←): Ring detection indication, used by MODEM.

– CD (Carrier Detect ←): Carrier detection indication, used by MODEM.

An application example of RS-232C

Operation example: DCE is MODEM and DTE is PC. The communication method is full-duplex with DTR/DSR contact (without RTS and CTS). (called party RS-232)

Implementation of RS-232C in PC

• In earlier PCs, the serial interface was implemented by a separate IC chip, such as the Intel 8250. The functional component for serial communication was called UART (Universal Asynchronous Receiver/Transmitter).

• In modern PCs, the UART is integrated with other standard interface circuits and is called a SuperI/O chip. The SuperI/O chip is integrated into the South Bridge chip (or ICH).

The general chipset integrates two serial interfaces, which can be added by plug-in or USB expansion. The system default port address is configured as an interrupt.

The PC allocates 8 register port addresses for the UART, and most of them use 7 of them.

Parallel Interface Purpose: An early parallel port was used to connect to the printer. Later developed into a standard multi-function interface for connecting external optical drives, external hard drives, and so on.

Standard: The IEEE 1284 standard is a parallel interface standard determined in 1994. The standard defines the physical characteristics, electrical specifications, and data transfer modes of the parallel port.

Features: 8 data lines are defined in the parallel interface, one byte at a time. The speed is much faster than the serial port, from 150KB to 2MB/s.

Connector: The DB25 connector is used on one end of the PC.

IEEE1284 interface signal

Signals in the DB25 parallel port connector: 8 data lines, 4 output control lines, and 5 status lines are defined. All signal lines are 5VTTL level.

IEEE1284 working mode

The earliest (the initial stage of the PC) parallel port only has no input function and is only suitable for connecting to the printer.

After the parallel port increase mode:

• Standard Parallel Port (SPP: Standard ParallelPort): Adds 4-bit input capability with an output rate of 150KB/s and an input rate of 50KB/s.

• Bidirectional parallel port (PS/2): 8-bit input with bidirectional transfer rate of 150KB/s.

• Enhanced Parallel Port (EPP: Enhanced ParallelPort): High-speed bidirectional, transfer rate 500KB~2MB/s. Suitable for two-way high-speed transmission.

• Enhanced Performance Parallel Port (ECP: Enhanced CapabilityPort): High-speed bidirectional, with a transfer rate of 500KB to 2MB/s. Support for DMA. Suitable for one-way high-speed transmission of batch data.