The RS-485 interface forms a half-duplex network, typically using a two-wire system with shielded twisted pair transmission. This wiring method allows for a bus topology structure, enabling up to 32 nodes to be connected on the same bus. Initially, data was output as analog signals for simple process quantities, and later, the RS-232 interface was used for instrument connections. This interface allows for point-to-point communication but does not support networking capabilities. The RS-485 interface emerged to address this limitation. This article provides a detailed introduction to the RS-485 interface in a question-and-answer format.
1. What is the RS-485 interface? How does it compare to the RS-232-C interface?
Answer: The RS-232-C interface standard was established quite early, and it inevitably has some shortcomings, mainly in the following four aspects:
- (1) The signal voltage levels of the interface are relatively high, which can damage the interface circuit chips. Additionally, because it is not compatible with TTL levels, a level conversion circuit is required to connect with TTL circuits.
- (2) The transmission rate is relatively low, with a baud rate of 20Kbps during asynchronous transmission.
- (3) The interface uses one signal line and one signal return line, forming a common ground transmission method. This common ground transmission is susceptible to common-mode interference, resulting in weak noise immunity.
- (4) The transmission distance is limited, with a maximum standard transmission distance of 50 feet, which practically 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) The electrical characteristics of RS-485: Logic “1” is represented by a voltage difference of + (2-6) V between the two wires; Logic “0” is represented by a voltage difference of – (2-6) V. The signal voltage levels of the interface are lower than those of RS-232-C, making it less likely to damage the interface circuit chips, and this level is compatible with TTL levels, facilitating connections with TTL circuits.
- 2) The maximum data transmission rate of RS-485 is 10Mbps.
- 3) The RS-485 interface uses a combination of balanced drivers and differential receivers, enhancing common-mode rejection capability, thus providing good noise immunity.
- 4) The maximum standard transmission distance of the RS-485 interface is 4000 feet, practically reaching up to 3000 meters. Additionally, while the RS-232-C interface allows only one transceiver to be connected on the bus, the RS-485 interface allows for up to 128 transceivers to be connected, providing multi-station capability, enabling users to easily establish a device network using a single RS-485 interface.
- 5) Due to the excellent noise immunity, long transmission distance, and multi-station capability of the RS-485 interface, it has become the preferred serial interface. The half-duplex network formed by the RS-485 interface typically requires only two wires, so RS-485 interfaces use shielded twisted pair transmission. The RS-485 connector uses a DB-9 9-pin plug, with the intelligent terminal RS-485 interface using a DB-9 (socket) and the keyboard connection using a DB-9 (pin).
2. RS-422 and RS-485 Serial Interface Standards
1. Balanced Transmission
Unlike RS-232, RS-422 and RS-485 use differential transmission methods, also known as balanced transmission. It employs a pair of twisted wires, defining one wire as A and the other as B. Typically, the positive level between the sending driver A and B is +2 to +6V, representing one logic state, while the negative level is -2 to +6V, representing another logic state. There is also a signal ground C; in RS-485, there is an “enable” terminal, which is optional in RS-422. The “enable” terminal is used to control the connection and disconnection of the sending driver and transmission line. When the “enable” terminal is active, the sending driver is in a high-impedance state, referred to as the “third state,” which is distinct from logic “1” and “0.”
The receiver is also defined relative to the sender, with the receiving and sending ends connected through the balanced twisted pair, where a voltage greater than +200mV between A and B outputs a positive logic level, and less than -200mV outputs a negative logic level. The input voltage range for 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 practice, there is also a signal ground wire, making a total of five wires. Figure 1 defines the pin configuration of its DB9 connector. Since the receiver uses high input impedance and the sending driver has stronger driving capability than RS-232, it allows multiple receiving nodes to be connected on the same transmission line, with a maximum of 10 nodes. This means one master device and the rest as slave devices, with no communication between slave devices, thus supporting point-to-multipoint bidirectional communication. The receiver input impedance is 4k, so the maximum load capacity of the sender is 10×4k+100Ω (termination resistance). The RS-422 four-wire interface, due to its separate sending and receiving channels, does not require control of data direction, and any necessary signal exchange between devices can be achieved either by software (XON/XOFF handshake) or hardware (a pair of separate twisted wires). The maximum transmission distance of RS-422 is 4000 feet (approximately 1219 meters), with a maximum transmission rate of 10Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate; the maximum transmission distance can only be achieved at rates below 100kb/s. The highest transmission rate can only be achieved over very short distances. Generally, the maximum transmission rate over a 100-meter twisted pair is only 1Mb/s.
RS-422 requires a termination resistor, which should be approximately equal to the characteristic impedance of the transmission cable. Termination resistors are generally not required for short-distance transmission, typically below 300 meters. The termination resistor is connected at the far end of the transmission cable.
3. RS-485 Electrical Specifications
Since RS-485 is developed based on RS-422, many of its electrical specifications are similar to those of RS-422. Both use balanced transmission methods and require termination resistors on the transmission line. RS-485 can be implemented in both two-wire and four-wire configurations; the two-wire system can achieve true multipoint bidirectional communication.
In the four-wire connection, like RS-422, it can only achieve point-to-multipoint communication, meaning there can only be one master device and the rest as slave devices. However, it has improved capabilities, allowing up to 32 devices to be connected on the bus, regardless of whether it is a four-wire or two-wire connection.
Another difference between RS-485 and RS-422 is their common-mode output voltage; RS-485 ranges from -7V to +12V, while RS-422 ranges from -7V to +7V. The minimum input impedance of the RS-485 receiver is 12k, while RS-422 is 4k. RS-485 meets all RS-422 specifications, so 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 10Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate; the maximum cable length can only be used at rates below 100kb/s. The highest transmission rate can only be achieved over very short distances. Generally, the maximum transmission rate over a 100-meter twisted pair is only 1Mb/s.
RS-485 requires two termination resistors, which should be equal to the characteristic impedance of the transmission cable. Termination resistors are generally not required for short-distance transmission, typically below 300 meters. The termination resistors are connected at both ends of the transmission bus.
4. Key Points for Installing RS-422 and RS-485 Networks
RS-422 supports 10 nodes, while RS-485 supports 32 nodes, allowing for multi-node network configurations. The network topology generally adopts a terminal-matched bus structure and does not support ring or star networks. When constructing the network, the following points should be noted:
- 1. Use a twisted pair cable as the bus, connecting each node in series. The lead length from the bus to each node should be kept as short as possible to minimize the impact of reflected signals in the leads on the bus signal. The figure shows some common incorrect connection methods (a, c, e) and correct connection methods (b, d, f). Although the three incorrect network connections (a, c, e) may work normally at short distances and low speeds, their adverse effects will become more severe as communication distance increases or communication speed rises. The main reason is that signals reflected at the ends of each branch will superimpose with the original signal, leading to a decline in signal quality.
- 2. Pay attention to the continuity of the bus characteristic impedance; signal reflections will occur at points of impedance discontinuity. The following situations are prone to such discontinuities: different cable types used in different sections of the bus, or too many transceivers installed closely together in one section of the bus, or excessively long branch lines extending to the bus.
In summary, a single, continuous signal channel should be provided as the bus.
5. Matching Considerations for RS-422 and RS-485 Transmission Lines
Termination resistors are generally required for matching RS-422 and RS-485 bus networks. However, for short distances and low speeds, termination matching may not be necessary. Under what circumstances can matching be disregarded? Theoretically, when sampling at the midpoint of each received data signal, as long as the reflected signal decays to a sufficiently low level before sampling begins, matching can be ignored. However, this is difficult to manage in practice. An article from MAXIM in the USA mentions an empirical rule that can be used to determine when matching is necessary based on data rates and cable lengths: when the signal transition time (rise or fall time) exceeds three times the time required for the signal to travel unidirectionally along the bus, matching can be omitted. For example, the RS-485 interface MAX483 has a minimum rise or fall time of 250ns, and the typical signal transmission rate on twisted pair cable is about 0.2m/ns (24AWG PVC cable). Therefore, as long as the data rate is below 250kb/s and the cable length does not exceed 16 meters, using MAX483 as the RS-485 interface does not require termination matching.
Generally, termination matching uses termination resistors, as previously mentioned. RS-422 requires termination resistors at the far end of the bus cable, while RS-485 requires termination resistors at both the beginning and end of the bus cable. The termination resistance is typically 100Ω for RS-422 networks and 120Ω for RS-485 networks, corresponding to the characteristic impedance of the cable, as most twisted pair cables have a characteristic impedance of about 100 to 120Ω. This matching method is simple and effective, but it has a drawback: the matching resistors consume considerable power, making it unsuitable for systems with strict power consumption limits.
Another more power-efficient matching method is RC matching, which uses a capacitor C to block the DC component, saving most of the power. However, selecting the value of capacitor C is challenging, requiring a compromise between power consumption and matching quality.
There is also a diode matching method; although this scheme does not achieve true “matching,” it uses the clamping effect of diodes to quickly attenuate reflected signals, improving signal quality. The energy-saving effect is significant.
6. Grounding Issues with RS-422 and RS-485
Grounding in electronic systems is crucial but often overlooked. Improper grounding can lead to unstable operation of electronic systems and even jeopardize system safety. Grounding for RS-422 and RS-485 transmission networks is equally important, as an unreasonable grounding system can affect the stability of the entire network, especially in harsh working environments and over long transmission distances, where grounding requirements are even stricter. Otherwise, the interface damage rate will be high. In many cases, when connecting RS-422 and RS-485 communication links, a simple twisted pair is used to connect the “A” and “B” ends of each interface, neglecting the connection of the signal ground. This connection method may work normally in many situations but poses significant risks for the following two reasons:
- 1. Common-mode interference: As previously mentioned, both RS-422 and RS-485 interfaces use differential signal transmission methods, which do not require detection of signals relative to a reference point; the system only needs to detect the potential difference between the two wires. However, people often overlook that transceivers have a certain common-mode voltage range, such as -7 to +7V for RS-422 and -7 to +12V for RS-485. Only by meeting these conditions can the entire network operate normally. When the common-mode voltage in the network line exceeds this range, it can affect the stability and reliability of communication, even damaging the interface. For example, when sending driver A sends data to receiver B, the output common-mode voltage of sending driver A is VOS. Since the two systems have their own independent grounding systems, there exists a ground potential difference VGPD. Therefore, the common-mode voltage at the receiver input can reach VCM=VOS+VGPD. Both RS-422 and RS-485 standards specify VOS≤3V, but VGPD can vary significantly (dozens of volts), possibly accompanied by strong interference signals, causing the receiver’s common-mode input VCM to exceed the normal range, generating interference currents on the transmission line, which can lead to communication disruptions or damage to the communication interface circuit.
- 2. EMI issues: The common-mode portion of the output signal from the sending driver requires a return path; without a low-resistance return channel (signal ground), it will return to the source end in the form of radiation, causing the entire bus to act like a large antenna radiating electromagnetic waves.
For the above reasons, despite using differential balanced transmission methods, the entire RS-422 or RS-485 network must have a low-resistance signal ground. A low-resistance 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 (non-shielded twisted pair) or the shielding layer of a shielded twisted pair. This is the most common grounding method.
It is worth noting that this approach is only effective against high-resistance common-mode interference; since the interference source has a high internal resistance, short-circuiting will not create significant ground loop currents, thus not greatly affecting communication. When the internal resistance of the common-mode interference source is low, large loop currents may form on the grounding line, affecting normal communication. The author believes that the following three measures can be taken:
- (1) If the internal resistance of the interference source is not very low, a current-limiting resistor can be added to the grounding line to limit the interference current. Increasing the grounding resistance may raise 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 isolate the ground loop. This is a commonly used and very effective method. When the internal resistance of the common-mode interference is very low, the above method may not work; at this point, consider floating the node introducing interference (for example, field devices in harsh working environments) (i.e., isolating the system’s circuit ground from the chassis or earth), thus breaking the ground loop and preventing large loop currents from forming.
- (3) Use isolation interfaces. In some cases, for safety or other reasons, the circuit ground must be connected to the chassis or earth and cannot be floated. In this case, isolation interfaces can be used to break the ground loop, but there should still be a ground wire connecting the common terminal on the isolated side to the working ground of other interfaces.
7. 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 level of receiver A is more than +200mV higher than that of receiver B, the output is positive logic; conversely, it outputs negative logic. However, due to the existence of the third state, when the host finishes sending a data message, it places the bus in the third state, meaning that when the bus is idle, there is no signal driving the bus, causing the voltage between A and B to drop to -200 to +200mV until it approaches 0V. This creates a problem: the output state of the receiver becomes uncertain. If the output of the receiver is 0V, the slave devices in the network will interpret this as a new start bit and attempt to read subsequent bytes, resulting in a frame error due to the absence of a stop bit, causing the network to become paralyzed. In addition to the aforementioned idle bus condition causing the voltage difference between the two wires to drop below 200mV, open or short circuits can also lead to this situation. Therefore, certain measures should be taken to prevent the receiver from being in an uncertain state.
Typically, a bias is added to the bus; when the bus is idle or open, bias resistors are used to bias the bus to a defined state (differential voltage ≥ -200mV). For example, pulling A to ground and pulling B to 5V, with typical resistor values around 1kΩ, though specific values may vary with cable capacitance.
This method is relatively classic, but it still cannot solve the problem of bus short circuits. Some manufacturers have moved the receiver threshold to -200mV/-50mV to address this issue.
8. Transient Protection for RS-422 and RS-485
The grounding measures mentioned earlier only protect against low-frequency common-mode interference and are ineffective against high-frequency transient interference. Since transmission lines act as inductors for high-frequency signals, the grounding line effectively becomes an open circuit for high-frequency transient interference. Although such transient interference lasts for a short time, it can have voltages in the hundreds or thousands of volts.
In practical application environments, there is still a possibility of high-frequency transient interference. Generally, switching large inductive loads such as motors, transformers, relays, or during lightning events can generate high-amplitude transient interference. If not adequately protected, it can damage RS-422 or RS-485 communication interfaces. To protect against such transient interference, isolation or bypass methods can be employed.
- 1. Isolation protection method. This scheme effectively transfers transient high voltage to the electrical isolation layer in the isolation interface. Due to the high insulation resistance of the isolation layer, no damaging surge current will occur, thus protecting the interface. High-frequency transformers, optocouplers, and other components are typically used to achieve electrical isolation, and some device manufacturers have integrated all these components into a single IC, making it very convenient to use. The advantage of this scheme is that it can withstand high voltages and prolonged transient interference, and it is relatively easy to implement, but the cost is higher.
- 2. Bypass protection method. This scheme uses transient suppression components (such as TVS, MOV, gas discharge tubes, etc.) to divert harmful transient energy to ground. The advantage is low cost, but the protection capability is limited, only protecting against transient interference within a certain energy range, and the duration cannot be long. Additionally, it requires a good connection to ground, making implementation more challenging. In practical applications, these two methods are often combined flexibly. In this method, the isolation interface isolates large transient interference, while bypass components protect the isolation interface from excessive transient voltage breakdown.
9. Considerations for Cable Length When Using RS-485 Interfaces
When using RS-485 interfaces, the maximum allowable cable length for data signal transmission from the generator to the load is a function of the data signal rate, depending mainly on signal distortion and noise. The relationship between maximum cable length and signal rate is derived using 24AWG copper twisted telephone cable (diameter 0.511mm), with inter-wire bypass capacitance of 52.5PF/M and terminal 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 practice, it is entirely possible to achieve longer cable lengths than this.
10. How to Achieve Multipoint Communication with RS-485/422
On the RS-485 bus, only one transmitter can send data at any given time. In half-duplex mode, only one master can send data. In full-duplex mode, the master station can always send, while the slave station can only send one at a time.
11. When is Terminal Matching Required for RS-485/RS-422 Communication? How is the Resistance Value Determined? How to Configure Terminal Matching Resistors?
In long-line signal transmission, terminal matching resistors are generally required at the receiving end to avoid signal reflections and echoes. The value of the terminal matching resistor depends on the impedance characteristics of the cable and is independent of the cable length.
RS-485/RS-422 generally uses twisted pair (shielded or unshielded) connections, with terminal resistors typically ranging from 100 to 140Ω, with a typical value of 120Ω. In practical configuration, a terminal resistor is connected at both ends of the cable, at the nearest and farthest ends, while intermediate nodes should not connect terminal resistors, as this would lead to communication errors.
12. What to Do if the Farthest Node in an RS-485 Network is Unknown?
This situation arises when users do not follow the principle of keeping the wiring from the node to the bus as short as possible when forming the RS-485 network. If the bus wiring adheres to this principle, there would be no issue of not knowing which node is the farthest. Additionally, it should be noted that such wiring will lead to poor system performance.
13. Why Does the Receiver Still Output Data When RS-485/RS-422 Communication Stops?
Since RS-485/RS-422 requires that all sending enable control signals be turned off and the receiving enable remains active after sending data, the bus driver enters a high-impedance state, allowing the receiver to monitor the bus for new communication data. However, since the bus is in a passive driving state (if there are terminal matching resistors, the differential level between A and B is 0, and the receiver’s output is uncertain; if there are no terminal matching resistors, the bus is in a high-impedance state, and the receiver’s output is uncertain), it is easily affected by external noise interference. When the noise voltage exceeds the input signal threshold (typically ±200mV), the receiver will output data, leading to invalid data being received by the corresponding UART, causing subsequent normal communication to fail. Another situation may occur during the moment of turning the sending enable control on or off, causing the receiver to output a signal, which can also lead to erroneous reception by the UART.
Solutions:
- 1) Use a pull-up on the non-inverting input (A line) and a pull-down on the inverting input (B line) on the communication bus to clamp the bus, ensuring the receiver outputs a fixed “1” level;
- 2) Replace the interface circuit with MAX308x series interface products that have built-in fault tolerance;
- 3) Eliminate the issue through software by adding 2-5 start synchronization bytes within the communication data packet, ensuring that actual data communication only begins after the synchronization header is met.
14. Three Factors Affecting RS-485 Bus Communication Speed and Reliability
During communication, two types of signals can cause signal reflections: impedance discontinuities and impedance mismatches. Impedance discontinuities occur when the signal encounters a significantly lower or nonexistent cable impedance at the end of the transmission line, causing reflections, similar to how light reflects when transitioning between different media. To eliminate such reflections, a termination resistor equal to the characteristic impedance of the cable must be connected at the end of the cable to ensure impedance continuity. Since signals on the cable are bidirectional, a termination resistor of the same size can also be connected at the other end of the communication cable. Theoretically, as long as a termination resistor matching the cable’s characteristic impedance is connected at the end of the transmission cable, signal reflections should not occur. However, in practical applications, due to the characteristic impedance of the transmission cable being related to the communication baud rate and other environmental factors, it is impossible for the characteristic impedance to perfectly match the termination resistor, so some signal reflections will still exist.
The other cause of signal reflections is the impedance mismatch between the transceiver and the transmission cable. This type of reflection mainly manifests when the communication line is idle, leading to data chaos across the entire network.
The impact of signal reflections on data transmission ultimately arises because reflected signals trigger the comparator at the receiver input, causing the receiver to receive erroneous signals, leading to CRC check errors or entire data frame errors.
In signal analysis, the parameter used to measure the strength of reflected signals is RAF (Reflection Attenuation Factor). Its calculation formula is as follows:
RAF=20lg(Vref/Vinc)
Where: Vref – the voltage magnitude of the reflected signal; Vinc – the voltage magnitude of the incident signal at the connection point between the transceiver and the termination resistor.
The specific measurement method is illustrated in the figure. For example, if the peak-to-peak value of a 2.5MHz incident signal sine wave is +5V and the peak-to-peak value of the reflected signal is +0.297V, then the reflection attenuation factor of this communication cable at a 2.5MHz communication rate is: RAF=20lg(0.297/2.5)=-24.52dB.
To mitigate the impact of reflected signals on communication lines, noise suppression and bias resistor methods are typically employed. In practical applications, for relatively small reflected signals, the bias resistor method is often used for simplicity and convenience. The principle of how to improve communication reliability by adding bias resistors in communication lines will be detailed later.
15. Signal Attenuation in Communication Cables
The second factor affecting signal transmission is the attenuation of signals during transmission through the cable. 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 minimal impact on the signal and can be ignored. Signal loss primarily occurs due to the LC low-pass filter formed by the distributed capacitance and distributed inductance. The attenuation coefficients of the LAN standard two-core inductance used in PROFIBUS (the standard cable selected by Siemens for the DP bus) vary at different baud rates.
16. Pure Resistive Load in Communication Cables
The third factor affecting communication performance is the size of the pure resistive load (also known as DC load). This refers to the combination of termination resistors, bias resistors, and RS-485 transceivers.
When discussing the EIARS-485 specification, it was mentioned that RS-485 drivers can output a differential voltage of at least 1.5V when connected to 32 nodes with 150Ω termination resistors. The input resistance of a receiver is 12kΩ, and the equivalent circuit of the entire network is illustrated in the figure. Based on this calculation, the load capacity of the RS-485 driver is: RL=32 input resistors in parallel || 2 termination resistors = ((12000/32)×(150/2))/((12000/32)+(150/2))≈51.7Ω.
Currently, commonly used RS-485 drivers include MAX485, DS3695, MAX1488/1489, and SN75176A/D used by Holley, among others, with some RS-485 drivers capable of achieving load capacities of up to 20Ω. Without considering many other factors, based on the relationship between driving capacity and load, the maximum number of nodes that a driver can support will far exceed 32.
When communication baud rates are relatively high, bias resistors are essential on the line. The connection method for bias resistors is to pull the bus level away from 0V when the line is in an idle state (no data). This way, even if small reflected signals or interference occur in the line, the data receivers connected to the bus will not malfunction due to these signals. The following example can be used to calculate the size of the bias resistor: termination resistors Rt1=Rr2=120Ω; assuming the maximum peak-to-peak value of the reflected signal Vref≤0.3Vp-p, then the negative half-cycle voltage Vref≤0.15V; the reflected current Iref due to the reflected signal on the termination resistor ≤0.15/(120||120)=2.5mA. The hysteresis voltage value of a typical RS-485 transceiver (including SN75176) is 50mV, thus:
(Ibias-Iref)×(Rt1||Rt2)≥50mV
From this, we can calculate that the bias current Ibias≥3.33mA.
+5V=Ibias(Rup+Rdown+(Rt1||Rt2))
From this equation, we can calculate Rup=Rdown=720Ω.
In practical applications, there are two methods for adding bias resistors to the RS-485 bus:
- (1) Distributing the bias resistors evenly among each transceiver on the bus. This method adds bias resistors to each transceiver connected to the RS-485 bus, providing each transceiver with a bias voltage.
- (2) Using a pair of bias resistors on a segment of the bus. This method is effective against large reflected signals or interference signals on the bus. It is important to note that adding bias resistors increases the load on the bus.
17. The Relationship Between Load Capacity of RS-485 Bus and Communication Cable Length
When designing the network configuration of the RS-485 bus (bus length and number of loads), three parameters should be considered: pure resistive load, signal attenuation, and noise margin. The pure resistive load and signal attenuation have been discussed earlier; now we will discuss noise margin. The noise margin of the RS-485 bus receiver should be at least greater than 200mV. The previous discussions assumed a noise margin of 0.
In practical applications, to enhance the bus’s anti-interference capability, it is desirable for the system’s noise margin to be better than that specified in the EIARS-485 standard. The relationship between the number of loads on the bus and the communication cable length can be seen from the following formula:
Vend=0.8(Vdriver-Vloss-Vnoise-Vbias)
Where: Vend is the signal voltage at the end of the bus, which is specified as 0.2V during standard measurement; Vdriver is the output voltage of the driver (which depends on the number of loads. For 5-35 loads, Vdriver=2.4V; for fewer than 5 loads, Vdriver=2.5V; for more than 35 loads, Vdriver≤2.3V); Vloss is the signal loss during transmission in the bus (which depends on the specifications and length of the communication cable), calculated using the attenuation coefficient provided in Table 1, with the formula for attenuation coefficient b=20lg(Vout/Vin), resulting in Vloss=Vin-Vout=0.6V (note: communication baud rate is 9.6kbps, cable length is 1km; if the baud rate increases, Vloss will also increase); Vnoise is the noise margin, which is specified as 0.1V during standard measurement; Vbias is the bias voltage provided by the bias resistors (typical value is 0.4V).
The multiplication by 0.8 in the formula is to prevent the communication cable from entering a fully loaded state. From this formula, it can be seen that the size of Vdriver is inversely proportional to the number of loads on the bus, and the size of Vloss is inversely proportional to the length of the bus; the other parameters depend only on the type of driver used. Therefore, once the RS-485 driver is selected, the number of loads on the communication baud rate is directly related to the maximum distance the signal can be transmitted.
Specifically, within the allowable range of the bus, the more loads connected, the shorter the distance the signal can be transmitted; the fewer loads connected, the farther the signal can be transmitted.
18. Definitions of Simplex, Half-Duplex, and Full-Duplex
- 1. If at any time during the communication process, information can only be transmitted from one party A to another party B, it is called simplex.
- 2. If at any time, information can be transmitted from A to B and also from B to A, but only one direction of transmission exists, it is called half-duplex transmission.
- 3. If at any time, there are bidirectional signal transmissions from A to B and from B to A on the line, it is called full-duplex.
The telephone line is an example of a two-wire full-duplex channel. By employing echo cancellation technology, bidirectional transmission signals do not become confused. Duplex channels sometimes also separate the receiving and transmitting channels, using separate lines or frequency bands to transmit signals in opposite directions, such as in loopback transmission.
（Source: Network）
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