A: The RS-232-C interface standard was introduced early, and as a result, it has several limitations. These include the following four main issues:
Firstly, the signal level of the interface is high, which can easily damage the chips in the interface circuit. It is not compatible with TTL levels, so a level conversion circuit is needed to connect it with TTL circuits.
Secondly, the transmission rate is low, with a baud rate of 20 Kbps in asynchronous transmission.
Thirdly, the interface uses a signal line and a signal return line to form a common ground transmission. This common ground transmission is prone to common mode interference, resulting in weak noise immunity.
Fourthly, the transmission distance is limited, with a maximum of 50 feet or about 15 meters. To address these shortcomings, new interface standards like RS-485 have emerged.
RS-485 has several advantages:
1) Electrical characteristics: Logic "1" is represented by a voltage difference between two lines of +2 to +6 V, while logic "0" is represented by -2 to -6 V. The signal level is lower than RS-232-C, making it less likely to damage the interface chip. It is also compatible with TTL levels, allowing easy connection to TTL circuits.
2) Maximum data transmission rate is 10 Mbps.
3) RS-485 uses balanced drivers and differential receivers, providing enhanced resistance to common mode interference and better noise immunity.
4) The maximum transmission distance is 4000 feet (about 1219 meters). Additionally, RS-485 supports up to 128 transceivers on the bus, enabling multi-station capability, which allows users to easily set up a device network using a single RS-485 interface.
5) Due to its good noise immunity, long transmission distance, and multi-station capability, RS-485 has become the preferred serial interface.
RS-485 interfaces typically use shielded twisted pair cables for transmission. The interface connector usually adopts a DB-9 9-pin connector. The RS-485 interface with intelligent terminals uses a DB-9 (hole), while the keyboard interface RS-485 uses a DB-9 (pin).
Second, the RS-422 and RS-485 serial interface standards differ from RS-232 in that they use differential transmission, also known as balanced transmission. They use a pair of twisted wires, with one line defined as A and the other as B.
Normally, the voltage between the transmit drivers A and B is +2 to +6 V for a positive logic state and -2 to -6 V for a negative logic state. There is also a signal ground C. In RS-485, there is an "enable" terminal used to control the disconnection and connection of the transmit driver and the transmission line. When active, the transmit driver enters a high-impedance state, called the "third state." The receiver also has a corresponding configuration. The receiving and transmitting ends are connected through a balanced twisted pair. When the voltage between the receiving ends AB is greater than +200 mV, a positive logic level is output; if it is less than -200 mV, a negative logic level is output. The receiver's balance line voltage is usually between 200 mV and 6 V.
The RS-422 standard defines the electrical characteristics of balanced voltage digital interface circuits. It allows multiple receiving nodes on the same transmission line, up to 10 nodes. RS-422 supports point-to-multipoint two-way communication. It has a maximum transmission distance of 4000 feet (about 1219 meters) and a maximum transmission rate of 10 Mb/s. The length of the balanced twisted pair is inversely proportional to the transmission rate. At lower rates, longer distances are possible.
RS-422 requires a terminating resistor equal to the characteristic impedance of the transmission cable. No termination is needed at short distances. The terminating resistor is connected to the far end of the transmission cable.
RS-485, developed from RS-422, shares many electrical regulations. Balanced transmission requires termination resistors on the transmission line. RS-485 can operate in two-wire and four-wire modes, supporting true multi-point two-way communication. When using a four-wire connection, only point-to-multipoint communication is possible, but it is better than RS-422. RS-485 differs from RS-422 in its common mode output voltage and receiver input impedance. RS-485 can be used in RS-422 networks. It has a maximum transmission distance of about 1219 meters and a maximum transmission rate of 10 Mb/s. Terminating resistors are required at both ends of the transmission bus.
Third, when installing RS-422 and RS-485 networks, the bus topology generally adopts a terminal-matched bus structure. Multiple nodes can be connected, but ring or star topologies are not supported. When building a network, it is important to use a twisted pair cable and connect each node in series. The lead line from the bus to each node should be as short as possible to minimize reflected signals. Proper installation is crucial to avoid signal degradation.
Fourth, when using RS-422 and RS-485 bus networks, termination resistors are often used to match the bus. However, at short distances and low speeds, matching may not be necessary. Matching is essential when the signal conversion time exceeds three times the time it takes for the electrical signal to travel unidirectionally along the bus. For example, with a typical twisted pair and a data rate of 250 kb/s, matching is not required within 16 meters.
Termination resistors are typically 100Ω for RS-422 and 120Ω for RS-485. Another method is RC matching, which saves power but requires careful selection of the capacitor value. Diode matching is another option, which helps reduce reflected signals without true matching.
Grounding is critical for RS-422 and RS-485 networks. Improper grounding can lead to instability and interface damage. A low-resistance signal ground is necessary to prevent common-mode interference. Ground loops can cause large currents, affecting communication. Measures such as adding current-limiting resistors, using floating technology, or adopting isolated interfaces can help mitigate these issues.
Network failure protection is important to prevent the receiver from entering an indeterminate state. Bias resistors can be used to ensure the bus remains in a known state. Transient protection methods like isolation and bypass components can protect against high-frequency interference.
When using the RS-485 interface, the maximum cable length depends on the data signal rate. Longer cables require lower data rates to maintain signal integrity. Different wire diameters affect the maximum cable length. For example, a 24AWG cable allows a maximum length of 200m at 600Kbit/s, while a 28AWG cable limits it to less than 200m.
Multi-point communication on RS-485/422 buses requires careful management of transmitter and receiver states. Only one transmitter can send at a time, and in full-duplex mode, the master station can always send while only one slave sends at a time.
Terminal matching is necessary to avoid signal reflection and echo, especially in long-distance transmissions. The termination resistor value depends on the cable's characteristic impedance, typically 100–120Ω. If the farthest site is unknown, proper wiring ensures correct termination.
RS-485/422 interfaces may still have data output when communication stops due to passive driving states. Solutions include clamping the bus, using fail-safe interface products, or software-based synchronization.
Factors affecting communication speed and reliability include signal reflection, cable attenuation, and distributed capacitance. Signal reflection occurs due to impedance discontinuities and mismatches, leading to errors. Cable attenuation reduces signal strength, and distributed capacitance can cause timing issues.
Understanding simplex, half-duplex, and full-duplex communication is essential. Simplex allows one-way communication, half-duplex allows bidirectional communication but not simultaneously, and full-duplex allows simultaneous two-way communication. Telephone lines are an example of full-duplex channels.
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