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예식일 : Brief Story: The truth About Rs485 Cable
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Circuits may be terminated on screw terminals, D-subminiature connectors, or other types of connectors. In some countries, inserting two cables into the same screw terminal is permitted. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel. The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation. The issue also exists in programmable logic controller applications. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals.

Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. In order to avoid errors when many devices are connected, cables of the same colour should be used for all the connections to the terminals A and cables of the same colour should be used for all the connections to the terminals B of the various devices (e.g. white for A and blue for B). If on the other hand each terminal can accept only a single cable, a proper branch must be created using three auxiliary terminals for each instrument to be connected. Control and monitoring systems: A single pair of rs485 signal cable can connect up to 32 devices, with a signal transmission distance of up to 1200m. Therefore, using rs485 cables is an ideal choice for industrial control and monitoring systems, especially those with large-scale and high dispersion. What made it such an advancement over RS232 was the fact that it could transmit not only a single device to device transmission, but also a communications bus to connect multiple devices at once.

The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. RS232 referenced ground to decipher the data it was transmitting. Access control systems: Rs485 cables are mainly used to connect and transmit data between access control devices such as security gates, RFID cards, or time attendance machines and the control center. Alarm systems: Rs485 cables in alarm systems help transfer alarm signals and information from alarm devices to the control center. With your RS485 network, you can remotely monitor and control that pump from a control center. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. By using repeaters very large RS-485 networks can be formed.

This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices. Another advantage over RS232 that I mentioned earlier is the number of connectable devices. The maximum number of devices that can be connected to a main cable is 32, including the Master. This ensures maximum protection for the inner core. This makes it easier to identify cabling errors. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. Industrial automation systems: In the industrial automation sector, rs485 signal cables play a vital role in connecting and remotely controlling motors.

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