A half-duplex network using an RS485 interface is typically two-wired, often employing shielded twisted-pair cable. This wiring method uses a bus topology, allowing up to 32 nodes to be connected on the same bus. We know that initially, data was output as simple analog signals, representing simple process quantities. Later, instrument interfaces used RS232 interfaces, which enabled point-to-point communication but not networking. The subsequent development of RS485 solved this problem. Therefore, this article provides a detailed introduction to the RS485 interface through a question-and-answer format.
I. What is an RS-485 interface? What are its characteristics compared to an RS-232-C interface?
A: Because the RS-232-C interface standard appeared relatively early, it inevitably has some shortcomings, mainly the following four:
(1) The signal level of the interface is high, which can easily damage the chip of the interface circuit. Also, because it is incompatible with TTL level, a level conversion circuit is required to connect to the TTL circuit.
(2) The transmission rate is low. In asynchronous transmission, the baud rate is 20Kbps.
(3) The interface uses one signal line and one signal return line to form a common ground transmission. This common ground transmission is prone to common mode interference, so it has weak noise interference resistance.
(4) Limited transmission distance: The standard maximum transmission distance is 50 feet, but in practice it can only be used for about 50 meters. To address the shortcomings of RS-232-C, new interface standards have emerged, one of which is RS-485, which has the following characteristics:
1) Electrical characteristics of RS-485: Logic "1" is represented by a voltage difference of +(2-6)V between the two lines; logic "0" is represented by a voltage difference of -(2-6)V between the two lines. The interface signal level is lower than that of RS-232-C, which makes it less likely to damage the interface circuit chip. Moreover, this level is compatible with TTL levels, making it easy to connect to TTL circuits.
2) The maximum data transmission rate of RS-485 is 10Mbps.
3) The RS-485 interface uses a combination of balanced driver and differential receiver, which enhances the common-mode interference immunity, i.e., it has good noise immunity.
4) The standard maximum transmission distance for the RS-485 interface is 4000 feet, but in practice it can reach 3000 meters. Furthermore, the RS-232-C interface only allows one transceiver to be connected on the bus, i.e., single-station capability. In contrast, the RS-485 interface allows up to 128 transceivers to be connected on the bus, providing multi-station capability. This allows users to easily establish a device network using a single RS-485 interface.
5) Due to its excellent noise immunity, long transmission distance, and multi-station capability, the RS-485 interface has become the preferred serial interface. Because RS485 interfaces form a half-duplex network, generally only two wires are needed, so shielded twisted-pair cable is used for transmission. The RS485 interface connector uses a DB-9 9-pin plug; the RS485 interface for smart terminals uses a DB-9 (hole), and the RS485 interface for keyboard connections uses a DB-9 (pin).
II. RS-422 and RS-485 Serial Interface Standards
1. Balanced transmission
RS-422 and RS-485 differ from RS-232 in that they use differential transmission, also known as balanced transmission. This uses a twisted pair of wires, with one wire designated as A and the other as B.
Normally, the positive voltage level between transmitter drivers A and B is +2 to +6V, representing one logic state, while the negative voltage level is -2 to 6V, representing another logic state. There is also a signal ground C. In RS-485, there is an "enable" pin, while in RS-422, this is optional. The "enable" pin is used to control the disconnection and connection of the transmitter driver to the transmission line. When the "enable" pin is active, the transmitter driver is in a high-impedance state, called the "third state," which is distinct from logic "1" and "0."
The receiver also follows the same specifications as the transmitter. The transmitter and receiver are connected via a balanced twisted-pair cable, with AA and BB connected accordingly. When there is a voltage level greater than +200mV between AB at the receiver, a positive logic level is output; when it is less than -200mV, a negative logic level is output. The voltage level range received by the receiver on the balanced line is typically between 200mV and 6V.
2. RS-422 Electrical Specifications
The full name of the RS-422 standard is "Electrical Characteristics of Balanced Voltage Digital Interface Circuits," which defines the characteristics of the interface circuit. Figure 2 shows a typical RS-422 four-wire interface. In reality, there is also a signal ground wire, for a total of five wires. Figure 1 shows the pin definitions of its DB9 connector. Because the receiver uses high input impedance and the transmit driver has stronger driving capability than RS232, multiple receiving nodes can be connected on the same transmission line, up to a maximum of 10 nodes. That is, one master device and the rest are slave devices. Slave devices cannot communicate with each other, so RS-422 supports point-to-many bidirectional communication. The receiver input impedance is 4kΩ, so the maximum load capacity at the transmitting end is 10 × 4kΩ + 100Ω (termination resistor). Because the RS-422 four-wire interface uses separate transmit and receive channels, it is not necessary to control the data direction. Any necessary signal exchange between devices can be implemented in software (XON/XOFF handshake) or hardware (a separate twisted pair). The maximum transmission distance of RS-422 is 4000 feet (approximately 1219 meters), and the maximum transmission rate is 10 Mb/s. The length of its balanced twisted-pair cable is inversely proportional to the transmission rate; the maximum transmission distance can only be achieved at speeds below 100 kb/s. The highest transmission rate can only be obtained over very short distances. Typically, the maximum transmission rate achievable over a 100-meter twisted-pair cable is only 1 Mb/s.
RS-422 requires a terminating resistor, the value of which should be approximately equal to the characteristic impedance of the transmission cable. For short-distance transmission, a terminating resistor is not necessary, generally not required for distances under 300 meters. The terminating resistor is connected at the farthest end of the transmission cable.
3. RS-485 Electrical Specifications
Since RS-485 was developed from RS-422, many of its electrical specifications are similar to RS-422. For example, both use balanced transmission and require terminating resistors on the transmission lines. RS-485 can use two-wire or four-wire configurations; the two-wire system enables true multi-point bidirectional communication.
When using a four-wire connection, it can only achieve point-to-many communication, just like RS-422, meaning there can only be one master device and the rest are slave devices. However, it is an improvement over RS-422, as up to 32 more devices can be connected to the bus, regardless of whether it is a four-wire or two-wire connection.
The difference between RS-485 and RS-422 also lies in their common-mode output voltage. RS-485 is between -7V and +12V, while RS-422 is between -7V and +7V. The minimum input impedance of the RS-485 receiver is 12kΩ, while that of the RS-422 is 4kΩ. Since the RS-485 meets all RS-422 specifications, RS-485 drivers can be used in RS-422 networks.
Like RS-422, RS-485 has a maximum transmission distance of approximately 1219 meters and a maximum transmission rate of 10 Mb/s. The length of the balanced twisted-pair cable is inversely proportional to the transmission rate; the longest specified cable length can only be used at rates below 100 kb/s. The highest transmission rate can only be achieved over very short distances. Typically, a 100-meter twisted-pair cable has a maximum transmission rate of only 1 Mb/s.
RS-485 requires two terminating resistors, the resistance of which must be equal to the characteristic impedance of the transmission cable. Terminating resistors are not required for short-distance transmission, typically below 300 meters. The terminating resistors are connected at both ends of the transmission bus.
III. Key Points for RS-422 and RS-485 Network Installation
RS-422 supports 10 nodes, and RS-485 supports 32 nodes, thus multiple nodes can form a network. The network topology generally uses a bus structure with terminal matching; ring or star networks are not supported. When building a network, the following points should be noted:
1. Use a single twisted-pair cable as the bus to connect all nodes in series. The length of the lead-out wire from the bus to each node should be as short as possible to minimize the impact of reflected signals on the bus signal. The diagram shows some common incorrect connection methods (a, c, e) and correct connection methods (b, d, f) in practical applications. Although network connections a, c, and e are incorrect, they may still work normally over short distances and at low speeds. However, as the communication distance increases or the communication speed increases, their adverse effects become more and more serious. The main reason is that the signal is reflected at the end of each branch and superimposed on the original signal, causing a decrease in signal quality.
2. Attention should be paid to the continuity of the characteristic impedance of the bus, as signal reflection will occur at points of impedance discontinuity. The following situations are prone to causing this discontinuity: different sections of the bus use different cables, too many transceivers are installed close together on a certain section of the bus, or excessively long branch lines are led out to the bus.
In summary, a single, continuous signal path should be provided as the bus.
IV. Some instructions on matching RS-422 and RS-485 transmission lines
RS-422 and RS-485 bus networks generally require terminating resistors for matching. However, for short distances and low data rates, terminating matching may not be necessary. So, under what circumstances is terminating matching unnecessary? Theoretically, when sampling at the midpoint of each received data signal, matching is unnecessary as long as the reflected signal attenuates sufficiently at the start of sampling. However, this is difficult to determine in practice. An article from Maxim Integrated mentions an empirical principle that can be used to determine the data rate and cable length at which matching is required: when the signal transition time (rise or fall time) exceeds three times the time required for the electrical signal to travel unidirectionally along the bus, matching is unnecessary. For example, the MAX483 RS-485 interface with a limited slope characteristic has a minimum rise or fall time of 250ns. The typical signal transmission rate over twisted-pair cable is approximately 0.2m/ns (24AWG PVC cable). Therefore, as long as the data rate is within 250kb/s and the cable length does not exceed 16 meters, terminating matching is unnecessary when using the MAX483 as the RS-485 interface.
Generally, terminating resistors are used for matching. As mentioned earlier, for RS-422, a resistor is connected in parallel at the far end of the bus cable, while for RS-485, terminating resistors should be connected in parallel at both the beginning and end of the bus cable. The terminating resistor is typically 100Ω in RS-422 networks and 120Ω in RS-485 networks. This is equivalent to the characteristic impedance of the cable, as the characteristic impedance of most twisted-pair cables is approximately 100–120Ω. This matching method is simple and effective, but it has a drawback: the matching resistor consumes a significant amount of power, making it unsuitable for systems with strict power consumption limitations.
Another more energy-efficient matching method is RC matching, which uses a capacitor C to block the DC component, thus saving most of the power. However, choosing the value of capacitor C is a challenge, requiring a trade-off between power consumption and matching quality.
Another matching method uses diodes. Although this approach doesn't achieve true "matching," it utilizes the clamping effect of diodes to quickly weaken reflected signals, thereby improving signal quality. It offers significant energy savings.
V. Grounding issues for RS-422 and RS-485
Grounding is crucial for electronic systems, yet it's often overlooked. Improper grounding can lead to unstable operation and even jeopardize system safety. Grounding in RS-422 and RS-485 transmission networks is equally important, as an inadequate grounding system can affect the stability of the entire network, especially in harsh environments and over long distances, where grounding requirements are even more stringent. Otherwise, the interface damage rate is high. In many cases, connecting RS-422 and RS-485 communication links simply involves connecting the "A" and "B" terminals of each interface with a twisted pair of wires, neglecting the signal ground connection. While this method may work in many situations, it creates significant hidden dangers for the following two reasons:
1. Common-mode interference problem: As mentioned earlier, both RS-422 and RS-485 interfaces use differential signal transmission and do not require signal detection relative to a reference point; the system only needs to detect the potential difference between the two lines. However, people often overlook the fact that transceivers have a certain common-mode voltage range. For example, the common-mode voltage range of RS-422 is -7 to +7V, while that of RS-485 transceivers is -7 to +12V. Only when these conditions are met can the entire network function 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. Taking Figure 1 as an example, when the transmitter driver A sends data to the receiver B, the output common-mode voltage of the transmitter driver A is VOS. Since the two systems have their own independent grounding systems, there is a ground potential difference VGPD. Therefore, the common-mode voltage VCM at the receiver input will reach VCM = VOS + VGPD. Both RS-422 and RS-485 standards specify VOS ≤ 3V, but VGPD may have a large amplitude (tens of volts or even tens 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.
2. (EMI) problem: The common-mode part of the output signal of the transmitting 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.
For the reasons mentioned above, although RS-422 and RS-485 use differential balanced transmission, a low-impedance signal ground is required for the entire RS-422 or RS-485 network. This low-impedance signal ground connects the working grounds of the two interfaces, short-circuiting the common-mode interference voltage VGPD.
This signal ground can be an additional wire (unshielded twisted pair) or the shield of a shielded twisted pair. This is the most common grounding method.
It is worth noting that this approach is only effective against high-impedance common-mode interference. Because the interference source has a high internal resistance, short-circuiting it will not create a large ground loop current, thus having little impact on communication. However, when the common-mode interference source has a low internal resistance, it will create a large loop current on the grounding wire, affecting normal communication. The author believes the following three measures can be taken:
(1) If the internal resistance of the interference source is not very small, a current-limiting resistor can be added to the grounding wire to limit the interference current. The increase in grounding resistance may increase the common-mode voltage, but as long as it is controlled within an appropriate range, it will not affect normal communication.
(2) Use floating ground technology to break the grounding loop. This is a commonly used and very effective method. When the common-mode interference internal resistance is very small, the above method can no longer work. At this time, you can consider floating the node that introduces the interference (such as field equipment in a harsh working environment) (that is, the circuit ground of the system is isolated from the chassis or earth ground). This will break the grounding loop and will not form a large loop current.
(3) Use an isolation interface. In some cases, for safety or other reasons, the circuit ground must be connected to the chassis or earth and cannot be floating. In this case, an isolation interface can be used to isolate the grounding loop, but there should still be a ground wire to connect the common terminal of the isolation side to the working ground of other interfaces.
VI. Network Failure Protection for RS-422 and RS-485
Both RS-422 and RS-485 standards specify a receiver threshold of ±200mV. This specification provides relatively high noise suppression capability. As mentioned earlier, when the receiver A level is +200mV higher than the B level, the output is positive logic; conversely, the output is negative logic. However, due to the existence of the third state—that is, after the host finishes transmitting a data message, it puts the bus into the third state, meaning the bus is idle and there are no signals driving the bus—the voltage between A and B ranges from -200 to +200mV until it approaches 0V. This introduces a problem: the receiver output state is uncertain. If the receiver output is 0V, the slave devices in the network will interpret it as a new start bit and attempt to read subsequent bytes. Since there will never be a stop bit, a frame error result is generated, no device requests the bus, and the network becomes paralyzed. Besides the situation mentioned above where the bus idleness causes the voltage difference between the two lines to be less than 200mV, this situation can also occur when there is an open circuit or short circuit. Therefore, certain measures should be taken to avoid the receiver being in an uncertain state.
Typically, a bias is applied to the bus. When the bus is idle or open, a bias resistor is used to bias the bus to a specific state (differential voltage ≥ -200mV). As shown in Figure 1, A is pulled up to ground, and B is pulled down to 5V. The typical value of the resistor is 1kΩ, but the specific value varies depending on the capacitance of the cable.
The above method is a classic approach, but it still cannot solve the problem when the bus is short-circuited. Some manufacturers have moved the receive threshold to -200mV/-50mV, which can solve this problem.
VII. Transient Protection for RS-422 and RS-485
The signal grounding measures mentioned earlier only protect against low-frequency common-mode interference; they are ineffective against high-frequency transient interference. Since a transmission line is essentially an inductor for high-frequency signals, the grounding wire is practically an open circuit for high-frequency transient interference. Although such transient interference is short-lived, it can still generate hundreds or thousands of volts.
In real-world applications, high-frequency transient interference is still possible. High-amplitude transient interference is typically generated during the switching of high-power inductive loads such as motors, transformers, and relays, or during lightning strikes. Without proper protection, this can damage the RS-422 or RS-485 communication interface. Isolation or bypass methods can be used to protect against this type of transient interference.
1. Isolation Protection Method. This approach effectively transfers transient high voltage to the electrical isolation layer within the isolation interface. Due to the high insulation resistance of the isolation layer, no damaging surge current is generated, thus protecting the interface. Electrical isolation of the interface is typically achieved using components such as high-frequency transformers and optocouplers. Some device manufacturers have integrated all these components into a single IC, making it very convenient to use. The advantages of this approach are its ability to withstand high-voltage, long-duration transient interference and its relative ease of implementation; the disadvantage is its higher cost.
2. Bypass Protection Method. This method utilizes transient suppression elements (such as TVS, MOV, gas discharge tubes, etc.) to bypass harmful transient energy to ground. Its advantage is lower cost, but its disadvantages include limited protection capability, only protecting against transient interference within a certain energy range, and a short duration. Furthermore, it requires a reliable ground connection, making implementation difficult. In practical applications, the two methods mentioned above are combined flexibly, as shown in Figure 1. In this method, the isolation interface isolates large-amplitude transient interference, while the bypass element protects the isolation interface from being damaged by excessively high transient voltages.
8. When using an RS485 interface, how should the length of the transmission cable be considered ?
When using an RS485 interface, for a specific transmission line diameter, the maximum allowable cable length for data signal transmission from the generator to the load is a function of the data signal rate. This length is primarily limited by signal distortion and noise. The relationship between maximum cable length and signal rate was derived using 24AWG copper core twisted-pair telephone cable (0.51mm diameter), with an inter-line bypass capacitance of 52.5pF/m and a terminating load resistance of 100 ohms. When the data signal rate drops below 90Kbit/s, assuming a maximum allowable signal loss of 6dBV, the cable length is limited to 1200M. In practical applications, cable lengths exceeding this are entirely possible. The maximum cable length will differ depending on the cable diameter used.
9. How to implement RS-485/422 multi-point communication
Only one transmitter can transmit on an RS-485 bus at any given time. In half-duplex mode, only one transmitter can transmit at a time (master or slave). In full-duplex mode, the master can always transmit, but only one slave can transmit at a time.
10. Under what conditions is terminating matching required for RS-485/RS422 interface communication? How is the resistor value determined? How is the terminating matching resistor configured?
In long-distance signal transmission, a terminating resistor is typically connected at the receiving end to prevent signal reflection and echo. The value of the terminating resistor depends on the impedance characteristics of the cable and is independent of the cable length.
RS-485/RS-422 typically uses twisted-pair cable (shielded or unshielded) for connection, with terminating resistors generally between 100 and 140 Ω, typically 120 Ω. In actual configuration, a terminating resistor is connected to each of the two terminal nodes of the cable, i.e., the closest and the farthest ends. Nodes in the middle should not have terminating resistors connected, otherwise communication errors will occur.
11. If I don't know which station is the furthest in an RS-485 network, how should I connect the matching resistor?
This situation arises because users did not follow the principle of keeping the connections from stations to the bus as short as possible when setting up the RS-485 network. If the bus wiring followed this principle, there would be no issue of not knowing which station is the furthest. Furthermore, it should be noted that such wiring will result in poor system performance.
12. Why does the receiver continue to output data when communication is stopped in an RS-485/RS-422 interface?
Because RS-485/RS-422 requires all transmit enable control signals to be turned off and receive enable to remain active after data transmission is complete, the bus driver enters a high-impedance state, and the receiver can monitor for new communication data on the bus. However, since the bus is in a passive drive state at this time (if the bus has a terminating resistor, the differential level between lines A and B is 0, the receiver output is uncertain, and it is very sensitive to changes in the differential signal on lines A and B; if there is no terminating resistor, the bus is in a high-impedance state, and the receiver output is uncertain), it is easily affected by external noise. When the noise voltage exceeds the input signal threshold (typically ±200mV), the receiver will output data, causing the corresponding UART to receive invalid data, resulting in subsequent normal communication errors; another situation may occur at the moment of turning the transmit enable control on/off, causing the receiver to output a signal, which will also cause the UART to receive incorrectly.
Solution:
1) The non-inverting input terminal is pulled up (A line) and the inverting input terminal is pulled down (B line) to clamp the bus, ensuring that the receiver output is a fixed "1" level;
2) Replace this interface circuit with an interface product from the MAX308x series that has a built-in fault-prevention mode;
3) Eliminate this through software, that is, add 2-5 start synchronization bytes to the communication data packet, and the actual data communication will only begin after the synchronization header is satisfied.
Thirteen, Three Factors Affecting RS-485 Bus Communication Speed and Reliability
1. Signal reflection in communication cables
During communication, two types of signals cause signal reflection: impedance discontinuity and impedance mismatch. Impedance discontinuity occurs when a signal encounters a very low or even nonexistent cable impedance at the end of the transmission line, causing reflection, as shown in Figure 1. This signal reflection principle is similar to the reflection of light when it travels from one medium to another. To eliminate this reflection, a terminating resistor of the same magnitude as the cable's characteristic impedance must be connected across the cable end to ensure impedance continuity. Since signal transmission on a cable is bidirectional, theoretically, connecting a terminating resistor of the same magnitude at the other end of the communication cable should eliminate signal reflection altogether. However, in practical applications, the characteristic impedance of the transmission cable is related to the communication baud rate and other application environments, and it is impossible for the characteristic impedance to be perfectly equal to the terminating resistor. Therefore, some degree of signal reflection will still exist.
Another cause of signal reflection is impedance mismatch between the data transceiver and the transmission cable. Reflections caused by this are mainly manifested as data corruption across the entire network when the communication line is idle.
The impact of signal reflection on data transmission ultimately stems from the fact that the reflected signal triggers the comparator at the receiver's input, causing the receiver to receive an incorrect signal, resulting in CRC check errors or errors in the entire data frame.
In signal analysis, the parameter used to measure the intensity of reflected signals is the RAF (Reflection Attenuation Factor). Its calculation formula is shown in equation (1).
RAF=20lg（Vref/Vinc）（1）
In the formula: Vref - the voltage magnitude of the reflected signal; Vinc - the voltage magnitude of the incident signal at the connection point between the cable and the transceiver or terminating resistor.
The specific measurement method is shown in Figure 3. For example, if the peak-to-peak value of the incident sine wave at 2.5MHz is measured to be +5V and the peak-to-peak value of the reflected signal is +0.297V, then the reflection attenuation factor of the communication cable at a communication rate of 2.5MHz is: RAF = 20lg(0.297/2.5) = -24.52dB
To reduce the impact of reflected signals on communication lines, noise suppression and the addition of bias resistors are commonly used. In practical applications, for relatively small reflected signals, the addition of bias resistors is often preferred for its simplicity and convenience. The principle behind improving communication reliability by adding bias resistors in communication lines will be explained in detail later.
XIV. Signal Attenuation in Communication Cables
The second factor affecting signal transmission is signal attenuation during cable transmission. A transmission cable can be viewed as an equivalent circuit composed of distributed capacitance, distributed inductance, and resistance.
The distributed capacitance C of the cable is mainly generated by the two parallel conductors of the twisted pair. The resistance of the conductors has a negligible impact on the signal. Signal loss is mainly due to the LC low-pass filter composed of the cable's distributed capacitance and distributed inductance. The attenuation coefficient of the PROFIBUS standard LAN two-core inductor (the standard cable chosen by Siemens for the DP bus) at different baud rates is shown.
15. Purely resistive loads in communication cables
The third factor affecting communication performance is the size of the purely resistive load (also called a DC load). The purely resistive load referred to here mainly consists of the terminating resistor, the bias resistor, and the RS-485 transceiver.
The EIA RS-485 specification mentions that an RS-485 driver with 32 nodes and 150Ω terminating resistors can output at least 1.5V differential voltage. The input resistance of one receiver is 12kΩ, and the equivalent circuit of the entire network is shown in Figure 5. Based on this calculation, the load capacity of the RS-485 driver is: RL = 32 input resistors in parallel || 2 terminating resistors = ((12000/32) × (150/2)) / (12000/32) + (150/2)) ≈ 51.7Ω
Currently, commonly used RS-485 drivers can handle loads up to 20Ω. Without considering other factors, based on the relationship between drive capability and load, the maximum number of nodes a single driver can support is far greater than 32.
When the communication baud rate is relatively high, a bias resistor on the line is essential. The bias resistor's connection method is as follows: Its function is to pull the voltage level away from 0 when there is no data on the bus (idle mode) after the line enters an idle state. This way, even if small reflected signals or interference appear on the line, the data receiver connected to the bus will not malfunction due to these signals. The value of the bias resistor can be calculated using the following example: terminating resistor Rt1 = Rr2 = 120Ω;
Assuming the maximum peak-to-peak value of the reflected signal, Vref, is ≤0.3Vp-p, then the voltage of the negative half-cycle, Vref, is ≤0.15V; the reflected current across the terminal resistor caused by the reflected signal, Iref, is ≤0.15/(120||120) = 2.5mA. The hysteresis value of a typical RS-485 transceiver (including SN75176) is 50mV, i.e.:
(Ibias-Iref)×(Rt1||Rt2)≥50mV
Therefore, the bias current Ibias generated by the bias resistor can be calculated to be ≥ 3.33mA.
+5V=Ibias（R pull-up+R pull-down+（Rt1||Rt2））（2）
Equation 2 shows that R<sub>pull-up</sub> = R<sub>pull-down</sub> = 720Ω.
In practical applications, there are two methods for adding bias resistors to the RS-485 bus:
(1) The bias resistors are evenly distributed to each transceiver on the bus. This method adds a bias resistor to each transceiver connected to the RS-485 bus and adds a bias voltage to each transceiver.
(2) Use only one pair of bias resistors on a section of the bus. This method is effective for buses with large reflected or interference signals. It is worth noting that the addition of the bias resistors increases the load on the bus.
XVI. The relationship between the load capacity of the RS-485 bus and the length of the communication cable
When designing a network configuration consisting of an RS-485 bus (bus length and number of loads), three parameters should be considered: purely resistive load, signal attenuation, and noise margin. The purely resistive load and signal attenuation parameters have already been discussed; now we will discuss noise margin. The noise margin of an RS-485 bus receiver should be at least greater than 200mV. The previous discussions were based on the assumption that the noise margin would be zero.
In practical applications, to improve the bus's anti-interference capability, it is generally desirable for the system's noise margin to be better than that specified in the EIARS-485 standard. The following formula shows the relationship between the bus load and the communication cable length: Vend = 0.8 (Vdriver - Vloss - Vnoise - Vbias) (3)
Where: Vend is the signal voltage at the end of the bus, specified as 0.2V in standard measurements; Vdriver is the output voltage of the driver (related to the number of loads. When the number of loads is between 5 and 35, Vdriver = 2.4V; when the number of loads is less than 5, Vdriver = 2.5V; when the number of loads is greater than 35, Vdriver ≤ 2.3V); Vloss is the signal loss during transmission in the bus (related to the specifications and length of the communication cable). Using the attenuation coefficient of the standard cable provided in Table 1, and according to the formula attenuation coefficient b = 20lg(Vout/Vin), Vloss = Vin - Vout = 0.6V can be calculated (Note: The communication baud rate is 9.6kbps, and the cable length is 1km. If the baud rate increases, Vloss will increase accordingly); Vnoise is the noise margin, specified as 0.1V in standard measurements; Vbias is the bias voltage provided by the bias resistor (typically 0.4V).
Multiplying by 0.8 in equation (3) is to prevent the communication cable from entering a fully loaded state. From equation (3), it can be seen that the value of Vdriver is inversely proportional to the number of loads on the bus, the value of Vloss is inversely proportional to the bus length, and the other parameters are only related to the type of driver used. Therefore, on an RS-495 bus with a selected driver, under a fixed communication baud rate, the number of loads is directly related to the maximum distance the signal can be transmitted. The specific relationship is:
Within the limits allowed by the bus, the more loads a signal carries, the shorter the transmission distance; conversely, the fewer loads a signal carries, the longer the transmission distance.
17. The Impact of Distributed Capacitance on RS-485 Bus Transmission Performance
The distributed capacitance of a cable is mainly generated by the two parallel conductors of the twisted pair. Additionally, there is also distributed capacitance between the conductors and ground, which, although small, cannot be ignored in the analysis. The impact of distributed capacitance on bus transmission performance is primarily because the bus transmits a fundamental frequency signal, represented only by "1" and "0". In special bytes, such as 0x01, the signal "0" allows sufficient charging time for the distributed capacitance. However, when the signal "1" arrives, the charge in the distributed capacitance does not have time to discharge, and (Vin+) - (Vin-) is still greater than 200mV. As a result, the receiver mistakenly interprets it as "0", ultimately leading to a CRC check error and an error in the transmission of the entire data frame.
Due to the distribution of data across the bus, data transmission errors occur, thus degrading the overall network performance. There are two ways to solve this problem:
(1) Reduce the data transmission baud rate;
(2) Use cables with low distributed capacitance to improve the quality of transmission lines.
18. Definitions of simplex, half-duplex, and full-duplex
1. If at any given moment in a communication process information can only be transmitted from one party A to another party B, it is called simplex.
2. If at any given moment, information can be transmitted from A to B, and also from B to A, but only one direction of transmission is possible, this is called half-duplex transmission.
3. If at any given time there is bidirectional signal transmission from A to B and from B to A on the line, it is called full-duplex.
Telephone lines are two-wire full-duplex channels. Thanks to echo cancellation technology, bidirectional signals are not confused. Sometimes, full-duplex channels separate the transmitting and receiving channels, using separate lines or frequency bands to transmit signals in opposite directions, such as loopback transmission.
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