1Introduction to Serial Ports
RS-232, RS-422, and RS-485 are all standards for serial data communication interfaces, initially formulated and published by the Electronic Industries Association (EIA). RS-232 was released in 1962, named EIA-232-E, as an industrial standard to ensure compatibility between products from different manufacturers. RS-422 was developed from RS-232 to address its shortcomings. To improve the short communication distance and low speed of RS-232, RS-422 defines a balanced communication interface, increasing the transmission rate to 10 Mbps and extending the transmission distance to 4000 feet (when the rate is below 100 Kbps), allowing up to 10 receivers to be connected on one balanced bus. RS-422 is a unidirectional, balanced transmission specification for single transmitter and multiple receivers, named TIA/EIA-422-A standard. To expand its application scope, the EIA also established the RS-485 standard in 1983 based on RS-422, adding multipoint, bidirectional communication capabilities, allowing multiple transmitters to connect to the same bus while enhancing the transmitter’s driving capability and collision protection features, and expanding the common mode range of the bus, later named TIA/EIA-485-A standard. Since the EIA’s proposed standards all use “RS” as a prefix, it is still common in the communications industry to refer to the aforementioned standards with the “RS” prefix.
RS-232, RS-422, and RS-485 standards only specify the electrical characteristics of the interfaces and do not involve connectors, cables, or protocols. Based on this, users can establish their own high-level communication protocols. Therefore, in the video industry, many manufacturers have established a set of high-level communication protocols, either publicly or for exclusive use by the manufacturer. For example, the RS-422 control protocol for video recorders from Sony and Panasonic differs, while the control protocols on video servers are more varied, such as the Louth and Odetis protocols being public, while ProLINK is based on Profile.
2RS-232 (EIA-232) Serial Communication Interface Standard
Currently, RS-232 is the most widely used serial interface in PCs and the communications industry. RS-232 is defined as a single-ended standard that increases communication distance in low-speed serial communication. RS-232 adopts an unbalanced transmission method, known as single-ended communication.
The data signal at both the transmitting and receiving ends is relative to the signal ground. For example, when data is sent from a DTE device using a DB25 connector, the signal level is at pin 2 relative to pin 7 (signal ground). Typical RS-232 signals fluctuate between positive and negative levels. When sending data, the transmitter driver outputs a positive level between +5 and +15V and a negative level between -5 and -15V. When no data is being transmitted, the line is at TTL level. From the start of data transmission to the end, the line level transitions from TTL level to RS-232 level and back to TTL level. The typical operating levels for receivers are between +3 and +12V and -3 and -12V. Since the difference between the sending level and receiving level is only about 2V to 3V, its common mode rejection capability is poor. Combined with the distributed capacitance on twisted pairs, the maximum transmission distance is about 15 meters, with a maximum speed of 20 Kbps. RS-232 is designed for point-to-point communication (i.e., only one pair of transmitting and receiving devices), with a driver load of 3 to 7 kΩ. Therefore, RS-232 is suitable for communication between local devices. Relevant electrical parameters are shown in the table below.
| Communication Standard | RS-232 | RS-422 | RS-485 |
| Operating Mode | Single-ended | Differential | Differential |
| Number of Nodes | 1 Transmit 1 Receive | 1 Transmit 10 Receive | 1 Transmit 32 Receive |
| Maximum Cable Length | About 15 meters (50 feet) | About 1219 meters (4000 feet) | About 1219 meters (4000 feet) |
| Maximum Transmission Rate | 20 Kbps | 10 Mbps | 10 Mbps |
| Maximum Output Voltage | ±25V | -0.25V to +6V | -7V to +12V |
| Minimum Driver Output Signal Level Load | ±5V to ±15V | ±2.0V | ±1.5V |
| Maximum Driver Output Signal Level No Load | ±25V | ±6V | ±6V |
| Driver Load Impedance | 3KΩ to 7KΩ | 100Ω | 54Ω |
| Maximum Slew Rate | 30V/μs | N/A | N/A |
| Receiver Input Voltage Range | ±15V | ±10V | -7V to +12V |
| Receiver Input Threshold | ±3V | ±200mV | ±200mV |
| Receiver Input Resistance | 3KΩ to 7KΩ | ≥4KΩ | ≥12KΩ |
| Driver Common Mode Voltage | N/A | ±3V | -1V to +3V |
| Receiver Common Mode Voltage | N/A | ±7V | -7V to +12V |
3RS-422 (EIA-422) and RS-485 (EIA-485) Serial Communication Interface Standards
1. Balanced Transmission
RS-422 and RS-485 differ from RS-232 in that they use differential transmission for data signals, also known as balanced transmission. It uses a pair of twisted wires, defining one wire as A and the other as B.
Typically, the positive level between the sending drivers A and B is between +2 and +6V, representing one logic state, while the negative level is between -2 and +6V, representing another logic state. There is also a signal ground C, and in RS-485, there is an “enable” pin, which is optional in RS-422. The “enable” pin is used to control the connection and disconnection of the sending driver and the transmission line. When the “enable” pin is active, the sending driver is in a high-impedance state, referred to as the “third state,” which is distinct from logical “1” and “0.”
The receiver is also defined in relation to the sender, with connections between AA and BB made through the balanced twisted pair. When there is a voltage greater than +200mV between the receiving terminals AB, it outputs a positive logic level; when less than -200mV, it outputs a negative logic level. The typical voltage range for the receiver is 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. In fact, there is also a signal ground wire, totaling five wires. Due to the high input impedance of the receiver and the stronger driving capability of the sending driver compared to RS-232, multiple receiving nodes can 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, RS-422 supports point-to-multipoint bidirectional communication. The input impedance of the receiver is 4k, so the maximum load capability for the sender is 10×4k+100Ω (termination resistance). The RS-422 four-wire interface uses separate sending and receiving channels, so there is no need to control data direction. Any necessary signal exchange between devices can be achieved either via software (XON/XOFF handshake) or hardware (a pair of separate twisted wires).
The maximum transmission distance for RS-422 is 4000 feet (about 1219 meters), with a maximum transmission rate of 10 Mbps. The length of the balanced twisted pair is inversely proportional to the transmission rate; the maximum transmission distance can only be achieved at speeds below 100 Kbps. The highest transmission rate can only be achieved over very short distances. Generally, the maximum transmission rate on a 100-meter twisted pair is only 1 Mbps.
RS-422 requires a termination resistor, which should be approximately equal to the characteristic impedance of the transmission cable. Termination resistors are not necessary for transmission over short distances, generally below 300 meters. The termination resistor is connected at the farthest end of the transmission cable.
For the electrical parameters of RS-422, see the table above.
3. RS-485 Electrical Specifications
Since RS-485 was developed from RS-422, many of its electrical specifications are similar to those of RS-422. Both use balanced transmission and require termination resistors on the transmission line. RS-485 can operate in both two-wire and four-wire configurations, with the two-wire system enabling true multipoint bidirectional communication.
When using a 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, whether in four-wire or two-wire configurations.
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 for RS-485 receivers is 12k, whereas for RS-422, it is 4k. RS-485 meets all RS-422 specifications, so RS-485 drivers can be used in RS-422 networks.
For the electrical specifications of RS-485, refer to the table above.
Like RS-422, RS-485 has a maximum transmission distance of about 1219 meters and a maximum transmission rate of 10 Mbps. The length of the balanced twisted pair is inversely proportional to the transmission rate; the maximum cable length can only be used at speeds below 100 Kbps. The highest transmission rate can only be achieved over very short distances. Generally, the maximum transmission rate on a 100-meter twisted pair is only 1 Mbps.
RS-485 requires two termination resistors, with values equal to the characteristic impedance of the transmission cable. Termination resistors are not necessary for transmission over short distances, generally below 300 meters. Termination resistors are connected at both ends of the transmission bus.
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