A bus type RS485 isolation circuit, enhancement circuit and enhancement method

Abstract

This invention relates to the field of communication technology, specifically to a bus-type RS485 isolation circuit, enhancement circuit, and enhancement method. It includes a twisted-pair cable, node 1, and node N. Each node 1 and node N contains an RS485 chip. Each RS485 chip has a transmit driver A, a receiver B, and a GND terminal. The two ends of the twisted-pair cable are connected to the transmit driver A and receiver B terminals of the RS485 chips in nodes 1 and N, respectively, via a pulse transformer TC, for isolating nodes 1 and N. The GND terminals of the RS485 chips are all grounded. A thermistor RT and a transient voltage suppressor diode are connected in parallel between the transmit driver A and receiver B of the RS485 chips. This invention takes a different approach, abandoning the traditional converter-type RS485 isolator and adopting a fully digital isolation method. Striving for simplicity, it utilizes a pulse transformer to better solve the isolation problem of a bus-type RS485 bus.

Description

Technical Field

[0001] This invention relates to the field of communication technology, specifically to a bus-type RS485 isolation circuit, enhancement circuit, and enhancement method. Background Technology

[0002] The RS485 bus is a general-purpose serial communication bus that uses balanced transmission and differential reception to achieve communication. Due to its advantages such as strong anti-common-mode interference capability, low cost, strong noise resistance, long transmission distance, high transmission rate, and ability to connect up to 256 transceivers, it is widely used in various fields such as industrial intelligent instruments, medical and consumer products, and communication equipment.

[0003] Grounding is very important for electronic systems, but it is often overlooked. Improper grounding can lead to unstable operation of electronic systems or even endanger system safety. As a common serial communication interface, RS-485 also requires very important grounding for its transmission network.

[0004] The 485 bus must be reliably grounded at a single point. A single point means that there can only be one grounding point on the entire 485 bus. Multiple grounding points are not allowed because the purpose of grounding is to keep the voltage on the ground wire (usually the shielded wire) consistent and prevent common-mode interference. Multiple grounding points would have the opposite effect. For reliable grounding, the ground wire of the entire 485 line must have good contact to ensure consistent voltage.

[0005] In many cases, connecting an RS-485 communication link simply involves connecting the "A" and "B" terminals of each interface with a twisted pair of cables, neglecting the connection of the signal ground. While this connection method may work normally in many situations, it introduces significant hidden dangers for the following two reasons:

[0006] 1. Common-mode interference problem: The RS-485 interface uses differential signal transmission, which does not require signal detection relative to a reference point. The system only needs to detect the potential difference between the two lines.

[0007] However, people often overlook the fact that transceivers have a certain common-mode voltage range. For example, the common-mode voltage range of RS-485 transceivers is -7 to +12V. Only when the above conditions are met can the entire network work normally. When the common-mode voltage of the network line exceeds this range, it will affect the stability and reliability of communication, and may even damage the interface.

[0008] For example, when transmitter driver A sends data to receiver B, the common-mode voltage output of transmitter driver A is VOS. Since the two systems have their own independent grounding systems, there is a ground potential difference VGPD (ground potential rise). Therefore, the common-mode voltage VCM at the receiver input will reach VCM = VOS + VGPD. The RS-485 standard specifies that VOS ≤ 3V, but VGPD may be very large (tens of volts or even hundreds of volts) and may be accompanied by strong interference signals, causing the receiver's common-mode input VCM to exceed the normal range and generate interference current on the transmission line. This can affect normal communication or even damage the communication interface circuit.

[0009] 2. EMI problem: The common-mode part of the output signal of the transmit driver needs a return path. If there is no low-impedance return path (signal ground), it will return to the source in the form of radiation, and the entire bus will radiate electromagnetic waves outward like a giant antenna.

[0010] In summary, ordinary optocoupler isolation chips are only suitable for low communication rates. In high-speed signal transmission circuits, the RS485 enable signal can continue to be isolated using optocoupler isolation devices, while the data signal path can be implemented using high-speed digital isolation chips. Compared with traditional optocoupler circuits, the system transmission rate is increased and the system complexity is reduced. However, as long as optocoupler isolation exists, there will be disadvantages such as short lifespan, weak common-mode rejection, and high power consumption, which still greatly limits the application scenarios of the circuit. Using digital isolators for the entire isolation circuit can effectively avoid these problems. Summary of the Invention

[0011] This invention addresses the technical problems existing in the prior art by providing a bus-type RS485 isolation circuit, an enhancement circuit, and an enhancement method to solve the isolation problem of bus-type RS485 circuits.

[0012] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0013] A bus-type RS485 isolation circuit is provided, including a twisted pair, node 1, and node N. Each node 1 and node N contains an RS485 chip. Each RS485 chip has a transmit driver A, a receiver B, and a GND terminal. The two ends of the twisted pair are connected to the transmit driver A and receiver B terminals of the RS485 chips in nodes 1 and N, respectively, via a pulse transformer TC, to isolate nodes 1 and N. The GND terminals of the RS485 chips are all grounded. A thermistor RT and a transient voltage suppressor diode are connected in parallel between the transmit driver A and receiver B of the RS485 chips.

[0014] Furthermore, the transient voltage suppressor diode is reverse-clamped at 5.0V to protect the RS485 chip inside the node; the transient voltage suppressor diode is forward-clamped at 0.7V to maintain the weak forward bias voltage required between the RS485 chip transmitting driver A and receiver B when the bus is idle and when the level is 1, and to absorb the ringing of the pulse transformer TC.

[0015] Furthermore, an enhancement circuit for a bus-type RS485 isolation circuit is provided, wherein one wire of the twisted pair is connected in series with a capacitor CCA, and the other wire is connected in series with a capacitor CCB. A transformer TF is also provided between the RS485 chip transmitter driver A and receiver B. The transformer TF is located between the thermistor RT and the pulse transformer TC. A coupling capacitor CF is connected between the center tap connection point of the pulse transformer TC near the transformer TF and the GND terminal of the RS485 chip. A large-value resistor RB is connected in series with the center tap of the pulse transformer TC away from the transformer TF and the coupling capacitor CF.

[0016] Furthermore, a transient voltage suppression diode is connected in parallel between the transmit driver A and the receiver B terminals of the RS485 chip between the transformer TF and the pulse transformer TC.

[0017] Furthermore, a method for enhancing the enhancement circuit of a bus-type RS485 isolation circuit includes the following steps:

[0018] Step 1: By using the pulse transformer TC, the technical difficulties of designing a bus-type RS485 isolator can be solved well. It has the characteristics of being simple and practical, requiring no power supply, not needing to consider the data flow direction, having adaptive baud rate within a limited range, and being easy for the underlying user group to understand and control.

[0019] Step 2: The bus-type RS485 isolator based on pulse transformer is particularly suitable for upgrading and transforming a half-duplex two-wire RS485 communication network with a transmitter driver A and receiver B in industrial environments with complex spatial electromagnetic fields.

[0020] Step 3: After isolating all nodes, an electrostatic discharge channel should generally be provided for the communication lines in the floating state. At this time, shielded twisted-pair cables can be used as communication lines, and their shielding layer can be connected to the ground network.

[0021] Step 4: To further enhance the isolation level, the additional insulation requirements are transferred to the coupling capacitors, and the bias requirements are removed from the magnetic components. A seemingly complete isolation option can be provided solely by high-voltage capacitors or high-voltage capacitors in series. However, capacitors provide neither common-mode rejection nor the surge isolation characteristics of transformers. Furthermore, the capacitance value affects signal transmission. Experiments show that a suitable capacitor for RS485 should be between 10nF and 33nF, but high-voltage capacitors typically offer limited choice in capacitance, with 2kV 1nF being a common option. While series connection of high-voltage capacitors can improve isolation, the equivalent capacitance is smaller, clearly unsuitable for high-speed signal transmission. Therefore, the transformer-plus-capacitor method is actually the best approach. Using this method, the capacitors are charged to the nominal DC bias value, allowing the transformer to handle transient issues. Even ordinary transformers are well-suited for handling transient problems.

[0022] Step 5: The coupling capacitor CF is biased with a large-value resistor RB, and one end is connected to the center tap connection point of the pulse transformer TC. This has another advantage: if the DC current of the bias resistor RB is monitored, then any dielectric breakdown becomes a detectable fault. Therefore, the selected resistor RB is required to have a large resistance value, such as 10MΩ, so that the fault current is lower than the rated value of the transformer wire, while minimizing the impact damage to personnel.

[0023] The beneficial effects of this invention are:

[0024] 1. This invention takes a different approach, abandoning the traditional converter-type RS485 isolator and adopting a fully digital isolation method. In pursuit of simplicity, it instead utilizes a pulse transformer to better solve the isolation problem of bus-type RS485 buses.

[0025] 2. By utilizing pulse transformers, the technical challenges of complex design of bus-type RS485 isolators can be effectively solved. They are characterized by simplicity and practicality, no need for power supply, no need to consider data flow direction, baud rate self-adaptation within a limited range, and ease of understanding and control by the underlying user group. Bus-type RS485 isolators based on pulse transformers are particularly suitable for upgrading and transforming half-duplex A and B two-wire RS485 communication networks in industrial environments with complex spatial electromagnetic fields.

[0026] 3. The present invention further proposes a method of using a transformer plus capacitor coupling to alleviate the cost problem of RS485 high-voltage isolation. It eliminates the need for magnetic components to provide double insulation. In particular, it uses an inexpensive common mode choke (CMC) assembly wound on a coil to replace the dedicated helical transformer magnetic component, further reducing costs. Both the capacitor and the CMC are flat surface-mount chip components, making them competitively priced. Attached Figure Description

[0027] Figure 1 Schematic diagram of RS232 / RS485 converter;

[0028] Figure 2 This is a schematic diagram of the circuit structure of a bus-type RS485 isolation circuit according to the present invention;

[0029] Figure 3 This is a schematic diagram of the circuit structure of an enhancement circuit for a bus-type RS485 isolation circuit according to the present invention.

[0030] Figure 4 This is a schematic diagram of a transformer with a center tap constructed using two SMD CMC components.

[0031] Figure 5 This is a schematic diagram of the circuit structure of an enhancement circuit for another form of the bus-type RS485 isolation circuit of the present invention; Detailed Implementation

[0032] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0033] The present invention provides the following preferred embodiments:

[0034] refer to Figure 1 As shown, the terminal pattern of RS485 isolators is designed for automatic data flow identification and control. Common RS485 interface chips all use the same pin pattern: DI, RO, DE, RE on the MCU side and A, B on the bus side. Influenced by this, most RS485 isolators also follow this approach: T (transmit), R (receive), C (direction control) on the device interface side, and A, B on the bus side. These RS485 isolators are essentially RS232 / RS485 converters with isolation functionality. If we understand it as an RS485 isolator, we can call this type of converter with isolation functionality a switching RS485 isolator. Obviously, the terminals on both sides of a switching RS485 isolator are different, and the circuit is asymmetrical, therefore it has a fixed installation orientation and the two sides cannot be interchanged.

[0035] Many years ago, conversion-type isolators were very suitable for RS485 bus node isolation because PCs and other industrial equipment at that time still largely retained RS232 interfaces, making implementation both logical and easy. However, today, RS232 is rarely seen in new products; instead, the standard RS485 interface uses a two-wire A, B (or a three-wire A, B, GND) system. In this context, a new type of bus-type RS485 isolator, directly connected in series on the A, B two-wire bus, will be more popular with the broader RS485 user base.

[0036] The basic design requirements for a bus-type RS485 isolator are:

[0037] 1) The terminals on both sides are identical, and the internal circuitry is symmetrical;

[0038] 2) It can be directly connected to the A and B buses without any conversion;

[0039] 3) Simple structure and easy installation;

[0040] 4) Both ends are exactly the same, both are A, B, (GND), regardless of direction, and no transmit / receive control is required;

[0041] With the increasing adoption of A / B two-wire standard RS485 interfaces in equipment and instruments, the various new technologies and devices involved in the still-popular converter-type RS485 isolator mode objectively tend to increasingly cater to communication board designers, rather than considering the vast user base at the lower levels of the RS485 communication network. Furthermore, the unique characteristics of RS485, such as unipolar pulse drive, multi-node connection, simultaneous transmit and receive on the same line, and low-impedance loads, make the design of bus-type RS485 isolators far more complex and difficult than that of converter-type RS485 isolators.

[0042] This invention takes a different approach and strives for simplicity, aiming to solve this problem better.

[0043] Here we use a network transformer to achieve differential-to-ground isolation:

[0044] In this embodiment, reference Figure 2 As shown, a bus-type RS485 isolation circuit is provided, in which a pulse transformer is used for node isolation; a transient voltage suppressor diode is reverse clamped at 5.0V to protect the RS485 chip inside the node; and a forward clamp is clamped at 0.7V to maintain the weak forward bias voltage required between RS485 chips A and B of each node when the bus is idle and when the voltage is 1, and to absorb the ringing of the pulse transformer.

[0045] Reasons for using a pulse transformer:

[0046] 1) It embodies the creative philosophy of simplicity and returning to the basics;

[0047] 2) The grassroots user group is very easy to understand and control;

[0048] 3) Mature and continuously improving technology, materials, and processes;

[0049] 4) Convincing reliability;

[0050] 5) Nearly permanent lifespan;

[0051] 6) Successful precedents for use in Internet network cards;

[0052] 7) Particularly suitable for driving and transmitting RS485 unipolar pulses;

[0053] 8) It conforms to the current application and operation status of domestic RS485 communication networks;

[0054] 9) Passive devices, passive isolation — completely;

[0055] 10) Easily isolate all nodes, allowing the external bus to float completely;

[0056] 11) Relatively low signal loss and power loss;

[0057] 12) No need to consider data flow direction;

[0058] 13) (Within a limited range) Baud rate adaptive;

[0059] 14) Easy installation;

[0060] 15) Low cost.

[0061] To further enhance the isolation level, our second technical solution is to achieve a low-cost solution without using a transformer with reinforced insulation, namely:

[0062] like Figure 3As shown, by transferring the additional insulation requirements to the coupling capacitor, the bias requirements are removed from the magnetic components. While high-voltage capacitors or capacitors in series can provide seemingly complete isolation options, capacitors offer neither common-mode rejection nor the surge protection characteristics of transformers. Furthermore, the capacitance value affects signal transmission; research indicates that suitable capacitors for RS485 should be between 10nF and 33nF. However, high-voltage capacitors typically offer limited choice in capacitance, with 2kV 1nF being a common option. While series connection of high-voltage capacitors improves isolation, the effective capacitance is smaller, clearly unsuitable for high-speed signal transmission. Therefore, the transformer-plus-capacitor method is actually optimal. Using this method, the capacitor is charged to its nominal DC bias value, allowing the transformer to handle transients. Even ordinary transformers are well-suited for handling transients.

[0063] The coupling capacitor is biased with a very large resistor, one end of which is connected to the center tap of the transformer, as shown in the figure. This also has the advantage that if the DC current of the bias resistor is monitored, any dielectric breakdown becomes a detectable fault. The chosen resistor value is very large, for example, 10 MΩ, so that the fault current is below the transformer's wire rating while minimizing the impact damage to personnel.

[0064] Furthermore, to further enhance the isolation level, we propose a third technical solution:

[0065] refer to Figure 4 , Figure 5 As shown, we construct a center-tapped transformer using two SMD common-mode chokes (CMCs), utilizing existing flat magnetic components to reduce component height and weight (alleviating solder fatigue issues). Like any other component, this type of transformer can be assembled using automated surface-mount methods, thus reducing production costs. A good component choice with these characteristics is the discrete common-mode choke (CMC), which has a transformer structure and is typically used as a filtering component.

[0066] Applicable CMCs are inexpensive. CMCs are made by winding wire pairs on a chip-sized ferrite core using a machine, which can be produced quickly and easily. Sufficient inductance is obtained by using two chokes, which also brings the additional benefit of essentially forming a center tap connection, which is useful for common-mode bias and decoupling.

[0067] The beneficial effects of the present invention are specifically reflected in the fact that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A bus-type RS485 isolation circuit, comprising twisted pair, node 1, and node N, characterized in that, Both nodes 1 and N are equipped with RS485 chips. Each RS485 chip has a transmit driver A, a receiver B, and a GND terminal. The two ends of the twisted pair are connected to the transmit driver A and receiver B terminals of the RS485 chips in nodes 1 and N respectively through a pulse transformer TC to isolate nodes 1 and N. The GND terminals of the RS485 chips are all grounded. A thermistor RT and a transient voltage suppressor diode are connected in parallel between the transmit driver A and receiver B of the RS485 chips. The transient voltage suppressor diode is reverse-clamped at 5.0V to protect the RS485 chip inside the node; the transient voltage suppressor diode is forward-clamped at 0.7V to maintain the weak forward bias voltage required between the RS485 chip of each node when the bus is idle and when the level is 1. It also absorbs the ringing of the pulse transformer TC. One wire of the twisted pair is connected in series with capacitor CCA, and the other wire is connected in series with capacitor CCB. A transformer TF is also provided between the RS485 chip's transmitter driver A and receiver B. The transformer TF is located between the thermistor RT and the pulse transformer TC. A coupling capacitor CF is connected between the center tap connection point of the pulse transformer TC near the transformer TF and the GND terminal of the RS485 chip. A large-value resistor RB is connected in series with the center tap of the pulse transformer TC away from the transformer TF and the coupling capacitor CF. The large-value resistor RB is used to provide bias and to monitor the DC current flowing through the large-value resistor RB when the coupling capacitor CF breaks down, so as to detect the fault.

2. The enhancement circuit for a bus-type RS485 isolation circuit according to claim 1, characterized in that, A transient voltage suppression diode is connected in parallel between the transmit driver A and the receiver B of the RS485 chip between the transformer TF and the pulse transformer TC.

Images