CN201434881Y - A kind of RS485 interface circuit and electric energy meter using the circuit - Google Patents

Abstract

The utility model relates to an RS485 interface circuit and an electric energy meter using the circuit. The RS485 interface circuit includes an RS485 communication interface chip and an inversion circuit. The transmitter input of the RS485 communication interface chip is connected to the inversion circuit input. Its transmitter enable and receiver enable are connected to the output of the inverting circuit together, and its transmitter output and receiver input are connected to a pull-up resistor and a pull-down resistor correspondingly, and its transmitter enable terminal is connected to the ground. There is a delay circuit, the electric energy meter includes a single-chip microcomputer and an RS485 interface circuit, the receiver output and the transmitter input of the RS485 interface circuit are respectively connected to two I/O ports of the single-chip microcomputer through an optical coupling, the utility model uses To solve the problem that the existing RS485 interface circuit and the electric energy meter using the circuit cannot communicate because the RS485 communication chip cannot always maintain the sending mode when sending data.

Description

A kind of RS485 interface circuit and electric energy meter using the circuit

technical field

The utility model relates to an RS485 interface circuit and an electric energy meter using the circuit, belonging to the field of electricity.

Background technique

RS485 communication is a method of data information transmission using differential balanced signals, which has the characteristics of long transmission distance, strong anti-interference ability, and high communication speed. Therefore, RS485 communication mode has been widely used in the communication circuit of electric energy meters. The core of the RS485 communication circuit is an integrated chip, of which there are 4 TTL level interfaces, which are signal receiving, signal receiving enable, signal sending, and signal sending enabling, because the effective levels of the two enabling signals are opposite and RS485 communication is Half-duplex mode, so in practical applications, two enable signals are connected together and controlled by one signal, so in fact, there are only three interfaces between the RS485 interface chip and the host MCU, namely receiving, sending, and enabling; The RS485 communication interface on the energy meter and the interface of the host MCU must be electrically isolated, so the interface between the RS485 interface chip and the host MCU is connected through a photocoupler; due to the limitations of cost, MCU resources, and line layout on the energy meter, In many cases, the circuit connection method shown in Figure 1 will be used, that is, the enabling end of the receiving and sending modes is connected to an inverting circuit composed of a transistor Q1, a resistor R3 and a resistor R9, so that only the receiving circuit is connected between the MCU and the RS485 chip. 1. Two interfaces for sending signals. The control of the enabling terminal of the RS485 chip is realized by sending the signal itself; The following problem: For example, the host MCU is sending a byte of data to the bus. When sending the signal "0", the RS485 chip works in the sending mode, and the signal "0" can be sent to the bus normally. When sending the signal "1", The RS485 chip is switched to receive mode, and the bus is released. At this time, if there is no matching resistor or other low-impedance load on the bus, the bus is in A due to the influence of the A-line pull-up resistor and the B-line pull-down resistor on the circuit. The state of positive B and negative is equivalent to sending the signal "1", that is to say, when the load on the RS485 communication bus is very light, this circuit mode can be used to realize the communication function. If there is a matching resistor connected to the RS485 communication bus, its A typical application is two 120-ohm resistors connected in parallel to both ends of the RS485 bus at a distance. Due to the series connection of the pull-up resistor, the pull-down resistor and the matching load resistor, the voltage obtained on the bus is very small, for example, in the pull-up resistor 1. When the value of the pull-down resistor is 10 kohms, the voltage value obtained on the bus is about 20mV, which is far less than the requirement of 200mV for the positive logic level of the RS485 bus, which leads to the failure of normal communication.

Utility model content

The purpose of this utility model is to provide an RS485 controller that can ensure normal communication and control data transmission and reception by sending signals and an electric energy meter using the controller, so as to solve the existing problem of using the sending signal itself to control data sending and receiving. The RS485 controller and the energy meter using the controller have the problem that the RS485 communication chip is not kept in the sending mode during the data sending process, which leads to the problem that the normal communication cannot be performed.

In order to achieve the above object, the utility model adopts the following technical scheme: a kind of RS485 interface circuit, this circuit comprises an RS485 communication interface chip and an inverting circuit, the transmitter input of the described RS485 communication interface chip is connected to the inverting circuit input, The transmitter enabling terminal and the receiver enabling terminal of the RS485 communication interface chip are connected to the output of the inverting circuit together, and its transmitter output and receiver input are correspondingly connected with a pull-up resistor and a pull-down resistor, which is characterized in that: A delay circuit is connected between the transmitter enabling terminal of the RS485 communication interface chip and the ground.

The delay circuit is composed of a resistor and a capacitor connected in series.

A kind of electric energy meter, it comprises a single-chip microcomputer and an RS485 interface circuit, the receiver output and transmitter input of this RS485 interface circuit are respectively connected with two of them I/O ports of described single-chip microcomputer through an optocoupler, and described RS485 The interface circuit includes an RS485 communication interface chip and an inverter circuit, the transmitter input of the RS485 communication interface chip is connected to the inverter circuit input, and the transmitter enabling end and the receiver enabling end of the RS485 communication interface chip are connected together The output of the inverting circuit is connected with a pull-up resistor and a pull-down resistor correspondingly between its transmitter output and receiver input, and a delay circuit is connected between the transmitter enabling terminal of the RS485 communication interface chip and the ground.

The delay circuit is composed of a resistor and a capacitor connected in series.

The beneficial effects of the utility model are: the reason why the normal communication cannot be performed is that the enable end of the RS485 communication chip is 0 when the sending signal is "1", the receiver is enabled, and the RS485 communication chip does not continue to maintain the sending mode. Therefore, the RS485 controller of the present invention adds a delay circuit composed of a resistor and a capacitor connected in series from the enable end of the RS485 communication chip to the ground to perform a "0" level start bit that must be sent before data transmission. Delay (because the communication is carried out in bytes, a "0" level start bit must be sent before the data is formally transmitted and the transmitter is enabled at the "0" level), that is, when the start bit "0" is sent At the same time as the level, the capacitor in the delay circuit begins to charge, and the capacitor is fully charged when the start bit is sent; after the start of sending data, due to the discharge of the capacitor, the transmitter enable terminal of the RS485 chip will It is always in the enabled state, that is to say, the RS485 communication chip is always kept in the sending mode, so that normal communication can be effectively guaranteed; at the same time, the electric energy meter using the RS485 controller described in the utility model can not only ensure the reliable transmission of communication data but also The relatively small increase in cost does not require any changes to the MCU's control program.

Description of drawings

Fig. 1 is a kind of circuit schematic diagram of existing electric energy meter;

Fig. 2 is the circuit principle diagram of electric energy meter described in the utility model;

Fig. 3 is a kind of oscillogram on the 485 bus when the existing electric energy meter sends data 55H;

Fig. 4 is a waveform diagram on the 485 bus when the energy meter of the present invention sends data 55H.

Detailed ways

Fig. 1 shows a kind of existing RS485 interface circuit and the watt-hour meter using this circuit, described watt-hour meter comprises host computer MCU, and the receiver input RXD of host computer MCU and transmitter output TXD are correspondingly connected with the transmission of an RS485 interface circuit device input D and receiver output R, the RS485 interface circuit includes a RS485 communication interface chip U3, the transmitter input D of the RS485 communication interface chip U3 is connected to the collector of the triode in the optocoupler U2, and the transmitter input D is also connected to There is a pull-up resistor R2, the emitter of the triode in the optocoupler U1 is grounded to GND485, the anode of its diode is connected to another pull-up resistor R5, and its cathode is connected to a transmitter output terminal of the host MCU, RS485 communication interface chip U3 The receiver output R of the receiver is connected to the cathode of the diode in the optocoupler U2, and the anode of the diode in the optocoupler U2 is connected to the third pull-up resistor R4, and its triode collector is connected to the receiver input terminal of the host MCU and the fourth pull-up resistor R1 is connected, its triode emitter is grounded to GND, the transmitter enable DE and receiver enable RE of the RS485 communication interface chip U3 are connected to the pull-down resistor R9 together, and their enable terminals are also connected to the collector of the triode Q1 together , the emitter of the triode Q1 is connected to a power supply VCC485, the base of the triode Q1 is connected to the transmitter input D of the RS485 communication interface chip U3 through the resistor R3, wherein the triode Q1, the resistor R3 and the resistor R9 form an inverting circuit, and the RS485 The transmitter output A and receiver input B of the communication interface chip U3 are correspondingly connected with the fifth pull-up resistor R6 and the second pull-down resistor R7, and a transient suppression diode TVS1 is connected between the transmitter output A and receiver input B .

Figure 3 shows the waveform diagrams at @1, @2 and @3 of the energy meter represented in Figure 1 when its MCU sends data "55H". According to the waveform diagram obtained from the test, @ on the RS485 bus in Figure 1 2 The positive level value at @3 is far less than the minimum logic level requirement of 200mV for the RS485 bus, thus resulting in failure of normal communication.

In conjunction with Fig. 2, the improved RS485 interface circuit of the present invention and the electric energy meter using the circuit are described. The electric energy meter includes a host MCU, and the receiver input RXD of the host MCU and the transmitter output TXD are correspondingly connected to an RS485 controller. Transmitter input D and receiver output R, described RS485 interface circuit comprises a RS485 communication interface chip U3, the transmitter input D of described RS485 communication interface chip U3 connects the collector of the triode in optocoupler U1, and transmitter input D also There is a pull-up resistor R2 connected, the emitter of the triode in the optocoupler U1 is grounded to GND485, the anode of its diode is connected to another pull-up resistor R5, and its cathode is connected to a transmitter output terminal TXD of the host MCU, RS485 communication interface The receiver output R of the chip U3 is connected to the cathode of the diode in the optocoupler U1, and the anode of the diode in the optocoupler U1 is connected to the third pull-up resistor R4, and its transistor collector is connected to the receiver input terminal RXD of the host MCU and the fourth pull-up resistor R4. The pull-up resistor R1 is connected, and its triode emitter is grounded to GND. The transmitter enable DE and receiver enable RE of the RS485 communication interface chip U3 are connected together with a pull-down resistor R9, and their enable terminals are also connected to the triode together. The collector of Q1, the emitter of the triode Q1 are connected to the power supply VCC485, the base of the triode Q1 is connected to the transmitter input D of the RS485 communication interface chip U3 through the resistor R3, and R8 and C1 are connected in series between the transmitter input D and the ground GND485. The delay circuit, wherein the triode Q1, resistor R3 and resistor R9 form an inverting circuit, the transmitter output A and receiver input B of the RS485 chip are correspondingly connected with the fifth pull-up resistor R6 and the second pull-down resistor R7, Transient suppression diode TVS1 is connected between transmitter output A and receiver input B.

The working principle of the electric energy meter circuit shown in Figure 2 is: refer to the DL/T645-1997 multi-function electric energy meter communication protocol section 5.1 serial communication byte transmission format specification, when the electric energy meter MCU sends data, the first data bit is The start bit of "0" level, this input level makes TXD1 in Figure 2 change from a high level "1" to a low level "0", at this time the transistor Q1 is turned on, and the high level directly controls the RS485 communication chip On the enable end of U3, the RS485 communication chip U3 changes from the receiving state to the sending state, and the "0" level at TXD1 is sent to the RS485 bus. Capacitor C1 is charged, and the charging constant is designed to be 100μS. This is based on the fact that the highest standard communication rate in the energy meter is 9600bps, and the time to send a data bit is 104μS. Therefore, no matter whether the data after the start bit is "0" or "1" , within the start bit duration, the capacitor C1 can be fully charged. After the start bit of the data ends, if the follow-up data is "0", the RS485 communication chip U3 is still at high level, and the RS485 communication chip U3 continues to keep in the sending state, sending the data "0" to the RS485 bus; if the follow-up data If it is "1", then the transistor Q1 is cut off. At this time, the capacitor C1 is discharged through the loop formed by the resistor R8 and the resistor R9, so that the enable terminal of the RS485 communication chip U3 keeps a high level for a certain period of time. This time is controlled by the capacitor C1 and the resistor R8. , R9, input impedance of the RS485 chip, etc., the high-level hold time is designed to be 10mS, because the standard default communication rate is 1200bps, the time to send a data bit is 833μS, and the time for 8 data bits is about 6.7mS, so 10mS can ensure the effective transmission of one byte of data, that is to say, in the 10mS time after the end of the data start bit of "0" level, no matter whether the subsequent data sent is "1" or " 0", the RS485 chip will keep sending until a byte of data is sent.

Fig. 4 has shown the electric energy meter described in the utility model when sending data " 55H " by MCU, the waveform diagram of @1, @2 and @3 place, according to the waveform diagram obtained by testing, @2 and @2 on the RS485 bus Both the positive and negative levels at @3 can meet the minimum requirement of 200mV for RS485 bus logic level, thus ensuring reliable communication.

Claims (4)

1. a kind of RS485 interface circuit, this circuit comprises an RS485 communication interface chip and an inverting circuit, the transmitter input of described RS485 communication interface chip connects inverting circuit input, the transmitter of this RS485 communication interface chip enables end Connect the output of the inverting circuit together with the receiver enable terminal, and its transmitter output and receiver input are connected with a pull-up resistor and a pull-down resistor correspondingly, and it is characterized in that: the transmitter of the RS485 communication interface chip uses There is a delay circuit connected between the energy terminal and the ground. 2. The RS485 interface circuit according to claim 1, characterized in that: said delay circuit is composed of a resistor and a capacitor connected in series. 3. A kind of electric energy meter, it comprises a single-chip microcomputer and a RS485 interface circuit, the receiver output and transmitter input of this RS485 interface circuit are respectively connected with two of them I/O ports of described single-chip microcomputer by an optocoupler, its characteristic In that: the RS485 interface circuit includes an RS485 communication interface chip and an inverting circuit, the transmitter input of the RS485 communication interface chip is connected to the inverting circuit input, the transmitter enabling terminal and the receiving end of the RS485 communication interface chip The transmitter enable terminal is connected to the output of the inverting circuit together, and its transmitter output and receiver input are connected with a pull-up resistor and a pull-down resistor correspondingly, and the transmitter enable terminal of the RS485 communication interface chip is connected to the ground. There is a delay circuit. 4. An electric energy meter according to claim 3, characterized in that: said delay circuit is composed of a resistor and a capacitor connected in series.

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