Serial communication is a method of transmitting data between devices. It is known for its simplicity, minimal wiring, and long-distance capability, which has made it widely used in information transmission, especially for long-range applications. Common serial communication standards include RS-232C, RS-422A, RS-423A, and RS-485A. RS-232C is a universal serial interface that operates using voltage levels. It allows communication between two devices with just three wires, but its maximum transfer rate is limited to 20 Kb/s, and the effective distance is only up to 15 meters. To overcome these limitations, the EIA introduced improved standards such as RS-422A, RS-423A, and RS-485A.
RS-422A uses differential signaling, where each channel employs two signal lines. It supports one transmitter and multiple receivers, offering a maximum transmission rate of 10 Mb/s over a distance of 120 meters. If the baud rate is reduced to 90 Kb/s, the transmission distance can extend up to 1,200 meters. RS-423A, on the other hand, uses an unbalanced differential format, while RS-485A allows for multiple transmitters, making it more flexible for multi-drop configurations. All these standards share common traits: they have lower transmission rates, their speed and distance are interdependent, and they require high-quality transmission media, often shielded cables. As a result, longer distances or more connected devices tend to increase system costs.
With the advancement of computer applications, there is a growing need for high-speed, long-distance signal transmission at a low cost. This section focuses on serial communication between microcontrollers and introduces a device designed to achieve both speed and range effectively.
First, let's explore the characteristics of a single-chip microcontroller’s serial port. Most microcontrollers, such as the 51 series and MCS-96 series, come with full-duplex serial interfaces, allowing simultaneous data reception and transmission. The 51 MCU, for example, supports four different serial communication modes. These modes can be configured via software, and the baud rate is typically generated by an internal timer. Both receiving and transmitting can operate in either interrupt or polling mode, offering greater flexibility. The serial interface usually works in either RS-232C or RS-422A mode through an interface circuit, thereby inheriting the features of those standards.
The four working modes are as follows:
1. **Mode 0**: A synchronous shift register mode with a fixed baud rate of fosc/12. Data is input via the RxD pin and shifted out via TxD. It is commonly used for expanding parallel interfaces, such as for keyboards or displays.
2. **Mode 1**: A standard asynchronous serial mode, where each frame consists of 10 bits—1 start bit, 8 data bits, and 1 stop bit. The baud rate is determined by the formula: (SMOD + 1) * 2^(SMOD) * (fosc / (12 * (256 - N))). SMOD is set by software, and N is the timer reload value.
3. **Mode 2**: A 11-bit serial mode, including 1 start bit, 8 data bits, 1 programmable bit, and 1 stop bit. The programmable bit can be set to 0 or 1 depending on the application. The baud rate is either fosc/32 (when SMOD=1) or fosc/64 (when SMOD=0).
4. **Mode 3**: Similar to Mode 2 in terms of data frame structure, but the baud rate is the same as Mode 1, offering the highest possible rate of fosc/192 and the lowest of fosc/98304.
Among these modes, Mode 2 provides the highest baud rate. For a 51 MCU with a crystal frequency of 12 MHz, this mode is particularly useful. These rates significantly exceed those of standard serial ports, so when communicating between MCUs, Mode 2 is typically the best choice for maximizing speed.
To support long-distance communication, additional circuitry is required. Therefore, we introduce a circuit designed to enable reliable high-speed, long-distance signal transmission.
**Second, the block diagram of the circuit**
1. **Signal Representation**
(1) **Representation of '1'**: When the TxD output is high, the bus appears in a high-impedance state after the transmit/receive circuit. The receiver converts this high-impedance signal back to a logic '1' at the RxD terminal. When the serial port is inactive, TxD remains high, and the bus is in a high-impedance state.
(2) **Representation of '0'**: A rectangular wave representing '0' is sent across the bus. The period of the wave is 32 or 64 times the oscillator cycle. After passing through the transmit/receive circuit, the signal is converted back to '0' at the RxD terminal.
2. **Block Diagram of the Transceiver Circuit**
The transceiver circuit, as shown in Figure 1-17, consists of a control circuit, frequency divider, output driver, differential input stage, coupling transformer, and other components. The control circuit may use a GAL or gate array, while the output driver employs a three-state buffer. The differential input stage uses three operational amplifiers to form a two-stage comparator. The frequency divider provides timing references for the control circuit, enabling accurate signal transmission and reception.
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