• 목록
• 답변
• 글쓰기
• 게시판 리스트 옵션
  • 수정
  • 삭제

Normally, this connection is made at one end of the main cable. The cable shield must be earthed only in one point. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together - it is wise to add some current limiting to the SC connection. Rs485 is low impedance balanced , so twisting the pair helps with noise immunity as well as reducing the attenuation effect of higher baud rates due to the parallel capacitance of the wires in the cable jacket over the long lengths. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. RS-485 does not define a communication protocol; merely an electrical interface. The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system. The maximum number of devices that can be connected to a main cable is 32, including the Master. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. 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).

In order to increase the extent of the Modbus network, repeaters can be used; and signal amplifying and regenerating devices provided with two communication ports that transfer to each what they receive from the other. In some countries, inserting two cables into the same screw terminal is permitted. In this case it is possible to connect the main inlet and outlet terminal directly to the terminals of an instrument without creating a branch. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. Now there was certain instances I used STP wire for rs485 because I had a lot of devices connected in a long run chain (DMX512 lighting fixtures). 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. These are used in programmable logic controllers and on factory floors. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. The truth tables of most popular devices, starting with the SN75176, show the output signals inverted.

RS-485 signals are used in a wide range of computer and automation systems. 12 V, i.e. ±7 V on top of the 0-5 V signal range. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance.

For example, Belden 3105A cable can be specified, but different types of cable with equivalent characteristics can be used. The shielding may be braided (be formed by a mesh of thin conducting wires) or be a foil (consisting of a sheet of metal wound around the conductors): the two types are equivalent. 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. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal. 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, rs485 cable A may be considered the non-inverting signal and B as inverting. 5 should work in your situation because of the voltage you will be running (5-10V) and your signal is not high frequency (above 500Mhz) in the first place.