In many schemes of medium and long-distance communication between industrial control equipment, RS485 bus is widely used in the fields of factory automation, industrial control, community monitoring, water conservancy automatic measurement and control and so on because of its simple hardware design, convenient control and low cost. RS485 bus is driven by balanced transmitting and differential receiving data transceiver. A pair of twisted pair is used as the transmission medium for the communication between nodes. Compared with RS232 interface, RS485 bus has longer transmission distance and stronger anti-interference ability. RS485 bus has the following characteristics:
① The interface adopts balanced driver and differential receiver, which has strong anti common mode interference ability, that is, good anti noise interference.
② The level between transceiver outputs a and B is + 2 ~ + 6 V, which is logic “1”; It is – 6 ~ – 2 V, which is logic “0”. The signal level is lower than RS232 and is not easy to damage the interface chip. Another “enable” control signal can make the transceiver in a high resistance state and cut off the connection with the transmission line.
③ The input sensitivity of the receiver is 200 mV. That is, when va-vb at the receiving end is + 200 mV, the output is logic “0”; When VA － VB ＜－ 200 mV, the output is logic “1”.
④ The maximum transmission rate is 10 Mbps and the standard value of the maximum transmission distance is 1200 m.
⑤ When the number of nodes is 32 and 120 Ω terminal resistance is configured, the driver can output at least 1.5 V voltage (the terminal resistance is related to the parameters of the twisted pair used).
⑥ It has multi site transmission capability, that is, multiple transceivers are allowed to be connected on the bus, and the equipment network can be established.
Based on the traditional RS485 bus node design, this paper is based on the enhanced RS485 transceiver ad-um2483 with isolation introduced by Adi company. The RS485 bus node designed by using magnetic coupling isolation and hardware Zero Delay technology has good anti-interference and high communication rate, and can be used in some industrial occasions with complex environment.
1 design idea
RS485 bus node is composed of single chip microcomputer STC12C5410AD, RS485 transceiver adum2483 with isolation, high-speed triode, etc. The application diagram of adum2483 is shown in Figure 1. STC12C5410AD is a single clock / machine cycle (1t) single chip microcomputer produced by macrocrystal technology. It is a new generation of 8051 single chip microcomputer with high speed / low power consumption / super anti-interference. The instruction code is fully compatible with the traditional 8051, but the speed is 8 ~ 12 times faster. Max810 special reset circuit, 4-way PWM and universal full duplex asynchronous serial port (UART) are integrated in it. Multiple serial ports can be realized by timer software; 8-way high-speed 10 bit a / D conversion, wide voltage, not afraid of power supply jitter, high anti-static (ESD protection), especially suitable for strong interference occasions.
In the complex field, there is a high common mode voltage between nodes. Although the RS485 bus is driven by the data transceiver of balanced transmission and differential reception, which has certain anti common mode interference ability, when the common mode voltage exceeds its limit receiving voltage, the RS485 transceiver will not work, and even burn the chip and instrument equipment in serious cases. Therefore, in order to adapt to strong interference environment and even more stringent performance requirements, each node of RS485 bus must be electrically isolated.
The traditional RS485 isolation bus node is composed of optocoupler and RS485 bus transceiver (such as MAX485). It uses light beam to isolate and protect the detection circuit, and provides a safe interface between high-voltage and low-voltage electrical environment. At present, 6n137 photoelectric isolation device is generally used, and MAX485 is used as RS485 bus transceiver. The 6n137 of Toshiba company has a working voltage of 5V, a maximum data transmission rate of 10 Mbps and a working temperature of 0 ~ 70 ℃; The isolation voltage is 2500 V (effective value), which is packaged in DIP8. Each chip provides only one isolation channel, with large volume, poor stability, high power consumption and easy aging of LED. These properties have limited the application of 6n137 in more demanding environments. This paper selects adum2483 of ADI company. It is a RS485 transceiver with isolation and a 3-channel Digital Isolator. It is a very cost-effective RS485 transceiver. Its function is equivalent to one MAX485 and three 6n137 optocoupler isolators, which can meet the requirements of RS485 bus nodes.
2 Introduction to adum2483
The icoupler technology adopted by adum2483 is based on the transformer of chip size, rather than the combination of LED and photodiode adopted by optocoupler. This technology cancels the photoelectric conversion process in the optocoupler, and uses the patent technology of icoupler transformer to integrate the transformer driving and receiving circuit, so as to realize the incomparable performance advantages of the optoelectronic isolator. Due to the use of wafer level manufacturing process to manufacture icoupler transformer directly on the chip, icoupler channel not only realizes the integration between channels more effectively than optocoupler, but also realizes other semiconductor functions more easily.
The icoupler Digital Isolator does not need an external circuit to drive the LED, and the power consumption is only 1 / 10 ~ 1 / 50 of that of the optocoupler. This new electromagnetic isolation method is much better than the optocoupler in resisting the influence of high temperature. The performance and reliability of icoupler Digital Isolator do not decline at 125 ℃, so it can be packaged with low-cost and small volume SOIC. In addition, the isolation channel of icoupler digital isolator has higher data transmission rate, timing accuracy and transient common mode suppression ability than optocoupler, so it is very suitable for various industrial applications (including data channel, data conversion interface and other multi-channel isolation applications).
The adum2483 includes a 3-channel isolator, a differential driver with three state output and a differential receiver with three state input. The receiver input impedance of 1 / 8 unit load allows up to 256 transceivers to access the bus, and the maximum transmission rate can reach 500 Kbps; The logic terminal is compatible with 3V / 5V working power supply, and the bus terminal is powered by 5V.
Adum2483 adopts limited slew rate driver, which reduces improper terminal matching and bit error caused by connector. The receiving input has the real failure protection function. When the receiver input is open circuit, short circuit or idle, the real failure protection can make the output logic of the receiver high. During power on or when a powered on backplane is connected, its hot plug circuit can eliminate the wrong data caused by the disturbance to the input of the enable and differential receiver. The driver of adum2483 has short-circuit current limit and can set the driver output to high resistance state through thermal shutdown protection circuit to prevent excessive power loss. The chip package adopts easy-to-use sow16 package, and the RS485 function can be realized without any discrete components in the industrial temperature range. Its main features are as follows:
◆ high transmission rate, up to 500 Kbps;
◆ with strong load capacity, 256 nodes can be connected on the bus at most;
◆ it has ± 2 kV ESD protection function and thermal shutdown protection function;
◆ industrial grade temperature range: – 40 ~ + 85 ℃;
◆ transient high common mode suppression capability, the receiving input terminal of the real fault prevention device;
◆ low power consumption, maximum working current of 2.5 mA;
◆ smaller volume, high integration, greatly reducing the area of printed circuit board.
3 RS485 bus node design
The two ends isolated by adum2483 have their own power supply and reference ground. Among them, the logic terminal voltage is 2.7 ~ 5.5 V, which can realize low-voltage power supply, so as to further reduce the system power consumption; The bus terminal is powered by 5 V, and the logic terminal voltage of the interface circuit designed in this paper is powered by 5 v. 0.1 shall be connected between the power supply and the reference ground μ F turtle capacity to filter out high-frequency interference. It should be noted that in the figure, GND1 and gnd2 are two different reference places, otherwise the isolation effect will not be achieved.
The unique PV (power valid) power monitoring function of adum2483 can disconnect the chip according to the stability of the power supply. To avoid slow power on / power off of GND1 (100) μ S / V), adum2483 designs PV pin. When the level is lower than 2.0 V, this pin is low and the chip does not work; Above 2.3 V, this pin is high and the chip works normally. D1 and D2 are used to prevent transient interference on the bus. RS485 bus interface circuit is shown in Figure 2.
B0505s isolation module of jinshengyang company is adopted to completely isolate the power supply at both ends of the transceiver from the ground. The isolation circuit is shown in Figure 3.
4. Realization of RS485 bus zero delay transceiver
In the design of RS485 bus node circuit, the receiving permission (RE) and transmitting permission (DE) pins of the transceiver are generally shorted, and the switching of transceiver is controlled by a signal line. In the control circuit, the MCU I / O pin is generally used to control the transceiver conversion of RS485 transceiver. In this way, the software needs to control the level of MCU I / O pin in order to control the transceiver conversion of RS485. RS485 transceiver is usually in the receiving state. When data is to be sent, re / De is changed to high level by program control, and the serial communication unit sends data; After waiting for sending, the program will convert the RS485 transceiver to the receiving state. The status indication of the sent flag is generally provided by the specific register of serial communication, and then queried by the program. This causes the following problems in RS485 communication:
◆ there is a certain delay between the data you want to send and the data you can actually send;
◆ if the conversion time from sending to receiving is improper, it will cause data loss;
◆ it is easy to introduce interference and receive redundant and disordered data during the conversion of received and transmitted data.
When designing the RS485 bus interface circuit, the bus delay problem is fully considered, and a high-speed NPN triode control is applied to realize the zero delay transceiver of RS485 bus, as shown in Figure 2.
When data is not transmitted, the TXD signal at the logic terminal is high. At this time, Q1 is turned on to make the re / de signal line low, and adum2483 is allowed to receive. When transmitting data, if TXD is low level, Q1 is turned off, so that the re / de signal line is high level. Transmission is allowed, and differential signals representing low level are generated on output terminals a and B of adum2483. When TXD is at high level, Q1 is turned on, so that the re / de signal line is at low level, and the A and B terminals of adum2483 are in high resistance state. At this time, the positive differential signal is generated on the bus by the pull-up and pull-down effects of resistors R1 and R2, so as to send the high-level signal of TXD.
It can be seen from the above analysis that when using this circuit, as long as the program can ensure that the receiving and transmitting operations are not carried out at the same time, that is, half duplex data transmission, it is not necessary to use software to control re / De to convert the receiving and transmitting, which can be completed by the hardware itself, so as to improve the high-speed rate and high reliability of RS485 bus receiving and transmitting data.
The design of this node adopts a new RS485 bus transceiver adum2483 with integrated isolation circuit, which reduces the power consumption of the system, simplifies the design structure and enhances the stability of the system. The zero delay technology of hardware is used to improve the transceiver efficiency and system performance. After testing, it has good effect and strong anti-interference ability. It is especially suitable for industrial control applications.