Development of a Low-Power RS485 Repeater

Introduction:

　　RS485 transceivers are widely used when the communication distance is from tens of meters to thousands of meters. RS485 transceivers use balanced transmission and differential reception, so they have the ability to suppress common-mode interference. In addition, the receiver has high sensitivity and can detect voltages as low as 200 mV, so the transmission signal can be recovered thousands of meters away.

　　Using the RS485 bus, a pair of twisted pairs can realize multi-station networking and form a distributed system. Its equipment is simple, low-cost, and can communicate over long distances, so it has been widely used in engineering projects. However, if the project requires a longer communication distance, beyond the reliable data transmission range that the RS485 interface can provide, a single RS485 communication control chip pair cannot be completed. At this time, a repeater must be added to the transmission line.

　　Based on long-term practice, the author has designed a micro-power RS485 repeater. After field testing, the communication distance can reach 1.8 times the original.

1 Repeater principle

　　The schematic diagram of the repeater is shown in Figure 1. Among them, U1 and U2 are the transceiver chip pair of the repeater, which are responsible for sending and receiving data. They use Sipex's 3 V low-power chip SP3485, with a single-chip standby current of 10 μA. Other logic circuits all use HC type with a standby current of 2 μA, which greatly reduces the system power consumption.
　　

Figure 1 Schematic diagram

　　　　　　　　　　　
　　Low standby current and true fail-safe are two key features of this application. RS485 is a half-duplex communication standard, and the bus's transmit and receive states must be controlled. When the RS485 input is open, or has been terminated and not driven, U1 and U2 will make its receiving end output (RO) high. At the differential input terminals A1 and B1, if there is input byte data, a voltage jump will be generated at the RO terminal of U1, and the state machine composed of the trigger 74HC74 and the NAND gate 74HC00 will be locked to the ON state at the falling edge of RO. The state machine pulls the driver enable pin (DE) of U2 to a high level, so that the input data byte is forwarded from U2 at the RS485 level.

　　The state machine keeps monitoring the voltage jump of the RO pin. When a data byte is transmitted (when no falling edge is added to the state machine within a predetermined time interval, it means that the byte is transmitted), the state machine automatically resets and waits for the next data byte on either side of the interface.

　　After a frame of data arrives at U1, it is forwarded to the A2 port and B2 port of U2 for output. U2 releases its output 700 μs after the last jump. Other time delays can be obtained by adjusting R1/C1 and R2/C2 in Figure 1.

　　Similarly, if a frame of data arrives at U2, it will be forwarded to the A1 port and B1 port of U1 for output. In this way, bidirectional data transmission is realized, and due to the relay, the communication distance will theoretically increase to twice the original.

2 Test Results

　　Use Tektronix oscilloscope TDS2012 to observe the waveforms of the receiving and transmitting ends, and store and analyze them, as shown in Figure 2.

　　Among them, the vertical scale is 1 V/grid, and the horizontal scale is 400 μs/grid; the upper waveform indicates the arrival of a frame of data, and the middle and lower waveforms indicate that they are forwarded to B2 and A2 respectively. As can be seen from Figure 2, the rising and falling edges of the waveform are very steep, indicating that the data level is relatively standardized and the distortion is very small. And because of the state machine structure, it can automatically identify the direction of data transmission, which is more convenient and reliable than using software to control the direction, and meets the design requirements.

　　In addition, if the system allows, the baud rate should be as low as possible, because too high a baud rate will limit the transmission distance. Due to the loss of ohmic impedance and skin effect of the transmission line, the signal distortion causes the communication distance to be limited. From the experimental results, the data transmission baud rate with a repeater should not exceed 14400. After adding a repeater, the communication distance is 1.8 times the original.

　　In terms of power consumption, in the standby state without data transmission, the power consumption current of the whole machine is measured by a microammeter to be about 22 μA (power supply voltage 3 V).
　　　　　

Figure 2 Waveform

Conclusion
　　Based on the in-depth analysis of RS485 bus theory and combined with practical applications, a RS485 repeater composed entirely of hardware is designed. After field testing, good results are achieved; at the same time, the use of 3 V low-power chip makes it very suitable for working in battery-powered environments. This has a certain reference value for long-distance communication of single-chip microcomputers and other systems.