Serial communication is a method of transmitting data between devices. It is simple to implement, requires minimal wiring, and can support long-distance transmission, which has made it widely used in information transfer, especially for long-range applications. Common serial communication standards include RS-232C, RS-422A, RS-423A, and RS-485A. Among these, RS-232C is a standard serial interface that uses voltage levels for signal transmission. It allows communication between two devices using just three wires, with a maximum data rate of 20 Kb/s and a transmission distance of up to 15 meters. However, due to its limited speed and short range, 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, with a maximum data rate of 10 Mb/s over 120 meters. If the baud rate is reduced to 90 Kb/s, the transmission distance can be extended to 1,200 meters. RS-423A, on the other hand, uses an unbalanced differential configuration, while RS-485A allows for multiple transmitters on the same bus. All these standards have similar characteristics: low transmission rates, a trade-off between speed and distance, and the need for high-quality transmission media, typically shielded cables. As the number of connected devices increases or the transmission distance grows, the system cost also rises.
With the continuous advancement of computer technology, there is a growing demand for high-speed, long-distance signal transmission at a lower cost. To address this, this section focuses on serial communication between microcontrollers, introducing a device capable of achieving both high speed and long-distance transmission.
First, let's explore the characteristics of a single-chip microcontroller's serial port. Most modern microcontrollers come equipped with a serial interface. For example, the 51-series and MCS-96 series microcontrollers feature a full-duplex serial interface, allowing simultaneous data reception and transmission. Taking the 51-series microcontroller as an example, it offers four different serial operating modes. The baud rate can be set via software and generated using an internal timer. Both receiving and transmitting can operate in either interrupt or polling mode, offering greater flexibility. The interface typically operates in RS-232C or RS-422A mode through an external circuit, thus inheriting the properties of those standards.
The four working modes are as follows:
1. **Mode 0**
This is a synchronous shift register mode with a baud rate of fosc/12. Data is input through the RxD pin, while the TxD pin outputs the synchronous shift pulse. Each frame consists of 8 bits of data. This mode is commonly used for expanding parallel interfaces, such as for keyboards or displays.
2. **Mode 1**
This is a serial communication mode where each frame includes 1 start bit, 8 data bits, and 1 stop bit, totaling 10 bits. The baud rate is determined by the formula:
$$ \text{Baud Rate} = \frac{f_{\text{osc}}}{(32 \times (2^{\text{SMOD}})) \times (256 - N)} $$
Where SMOD is set by software (either 0 or 1), and N is a timer reload value (0–255). When SMOD=1 and N=255, the baud rate is highest at $ f_{\text{osc}}/192 $. When SMOD=0 and N=0, the lowest baud rate is $ f_{\text{osc}}/98304 $.
3. **Mode 2**
This mode also uses 11 bits per frame: 1 start bit, 8 data bits, 1 programmable bit, and 1 stop bit. The programmable bit can be set to 0 or 1 as needed. The baud rate is calculated as:
- When SMOD=1, the maximum baud rate is $ f_{\text{osc}}/32 $.
- When SMOD=0, the minimum baud rate is $ f_{\text{osc}}/64 $.
4. **Mode 3**
Similar to Mode 2, but with the same baud rate as Mode 1. Therefore, the maximum baud rate achievable is $ f_{\text{osc}}/192 $, and the minimum is $ f_{\text{osc}}/98304 $.
Comparing these modes, Mode 2 offers the highest baud rate. For the 51 MCU, when the crystal oscillator frequency is 12 MHz (the maximum for this series), Mode 2 is the best choice to achieve the fastest communication. These speeds are significantly higher than those of traditional serial ports. However, for long-distance signal transmission, additional circuitry is necessary. To address this, a specialized circuit is introduced to enable reliable long-distance communication.
Second, the block diagram of the receiving/transmitting circuit includes several key components: signal representation, control circuits, frequency dividers, output drivers, differential inputs, and coupling transformers. The signal representation defines how binary 1 and 0 are transmitted. A binary 1 is represented as a high-impedance state on the bus, while a binary 0 is represented as a rectangular wave with a period of 32 or 64 times the oscillator cycle. The block diagram (as shown in Figure 1-17) illustrates the structure of the circuit, which includes a control circuit (often implemented using a GAL or gate logic), a frequency divider, an output driver, and a differential input stage using operational amplifiers. This design ensures stable and accurate signal transmission over long distances.
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