Currently, most building intercom systems on the market use signal control cables (such as PVC sheathed flexible cord RVV and shielded wire RVVP) and coaxial cable SYV-75 series cables. The system requires cables to transmit voice, data, and video signals, and the requirements also extend to the quality of voice transmission, data transmission rate, and video signal transmission quality and rate. In building video intercom systems, the quality requirements for connecting cables are quite high. Cables transmitting voice and alarm signals mainly use 4-core or higher PVC sheathed flexible cord (RVV), while coaxial cable SYV-75-5 is primarily used for video transmission. Nowadays, some new technologies using twisted-pair network cables for signal transmission, including video, have emerged, eliminating the need for coaxial cable. If the system needs to prevent external interference or cannot be grounded, the connecting cables used in the system must be shielded wire (RVVP) type cables.
Cables for connecting video intercom systems
The wiring standard for direct-button building video intercom systems is as follows: the video, two-way audio, and remote unlocking terminals of each indoor unit are connected to the door unit via a bus, but each call line is directly connected to the door unit separately. Therefore, this type of multi-resident video intercom system uses a relatively large amount of cable: SYV-75-3 or SYV-75-5 coaxial video cable; a 4-core unshielded PVC sheathed flexible wire (RVV) or shielded wire (RVVP) for the microphone/speaker/unlocking line; a 2-core PVC sheathed flexible wire (RVV) for the power connection; and a 2-core shielded wire (RVVP) for the call line.
Digital coded push-button video intercom systems are generally used in multi-residential high-rise buildings. The standard wiring configuration varies depending on the manufacturer, but generally, the basic wiring includes: main cables (video coaxial cable, SYV-75-3, SYV-75-5, etc.), power cables (2-core PVC sheathed flexible wire RVV), audio/data control cables (4-core shielded wire RVVP), and individual unit signal cables (6-core shielded wire RVVP), etc.
Most residential buildings equipped with video intercom systems also have a management center unit and a perimeter unit at the entrance of the community, enabling residents, the management center, and visitors to have a so-called "three-way call" for security purposes. The wiring for such a networked system adds networking cables between the unit entrance units, the community entrance units, and the management center unit. These cables typically include video coaxial cables (SYV-75-3, SYV-75-5, SYV-75-7, etc.) for transmitting video image signals, and 4-core shielded cable (RVVP) for transmitting audio and control signals.
The above describes the selection of different cables for different signal transmissions. With the increasing R&D capabilities of cable manufacturers, some comprehensive cables suitable for access control and video intercom systems have also appeared on the market. These cables use specific process structures, so that the cable core assembly includes coaxial cables for transmitting video signals, signal lines for transmitting audio signals, signal lines for transmitting control signals, and power lines for transmitting power signals. This allows the required effect to be achieved with a single wiring, saving a lot of engineering work. However, the cables need to be manufactured according to the specific requirements of various video intercom system devices.
Main cable for access control and video intercom systems
1. RS-232-C is a serial physical interface standard developed by the Electronic Industries Association (EIA) of the United States. RS is an abbreviation for "Recommended Standard," 232 is the identifier, and C indicates the revision number. The RS-232-C bus standard has 25 signal lines, including one main channel and one auxiliary channel. In most cases, the main channel is used. For general full-duplex communication, only a few signal lines are needed, such as one transmit line, one receive line, and one ground line. The RS-232-C standard specifies data transmission rates of 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, and 19200 baud per second. The RS-232-C standard specifies that the driver can have a capacitive load of 2500pF. The communication distance will be limited by this capacitance. For example, when using a communication cable with 150pF/m, the maximum communication distance is 15m. If the capacitance per meter of cable is reduced, the communication distance can be increased. Another reason for the short transmission distance is that RS-232 is a single-ended signal transmission, which has problems such as ground noise and inability to suppress common-mode interference. Therefore, it is generally used for communication within 20m.
2. RS-485 bus: The RS-485 serial bus standard is widely used when communication distances range from tens of meters to thousands of meters. RS-485 employs balanced transmission and differential reception, thus possessing the ability to suppress common-mode interference. Furthermore, the bus transceiver has high sensitivity, capable of detecting voltages as low as 200mV, allowing signal recovery even at distances exceeding one kilometer. RS-485 operates in half-duplex mode, meaning only one point can be in transmission mode at any given time; therefore, the transmitting circuit must be controlled by an enable signal. RS-485 is very convenient for multi-point interconnection, eliminating the need for many signal lines. RS-485 can be networked to form distributed systems, allowing up to 32 drivers and 32 receivers to be connected in parallel.
Previously, communication between PCs and smart devices primarily relied on RS232, RS485, and Ethernet, depending mainly on the device's interface specifications. However, RS232 and RS485 only represent the physical media layer and link layer of communication. To achieve bidirectional data access, custom communication applications had to be written, but most of these programs do not conform to ISO/OSI standards, offering only limited functionality and applicability to a single device type, lacking versatility. In device networks connected by RS232 or RS485 devices, if the number of devices exceeds two, RS485 must be used as the communication medium. Intercommunication between devices in an RS485 network can only be achieved through a "master" device, typically a PC. Only one master device is allowed in such a network, with all others being "slave" devices. Fieldbus technology, based on the ISO/OSI model, has a complete software support system and can solve problems related to bus control, collision detection, and link maintenance.
Cable manufacturers have also developed a series of twisted-pair shielded cables for RS-485 transmission (the wire cores use a twisted-pair structure to enhance their anti-interference performance) to meet these requirements. If the above system specifies that the wiring should be suitable for RS-485 transmission, it is recommended to use such cables for connection to ensure that the system performance meets the standards.
