Take The Stress Out Of Rs485 Cable > 자유게시판

Be sure to use a wire pair such as the blue and blue/white pair for the data lines and a third wire or pair twisted together for the common connection. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. 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. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. 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.

Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices. 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. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. Line A voltage, implying A, the green wire, is indeed connected to the driver inverting signal, as seen in a whitepaper. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. 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.

Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. Shield on communication cable is to remain continuous and ground only at the controller. The issue also exists in programmable logic controller applications. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, rs485 cable but engineers and applications guides continue to use the RS-485 designation. Category 5 (Cat 5, Cat 5e, and Cat 6) cables are the most common Ethernet cables in use today. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds.

Ideally, the two ends of the cable will have a termination resistor connected across the two wires. End of line termination resistor (120 ohm 1/4 watt) to be installed at both ends of network. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. Splicing of communications cable is not permitted. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology. RS-422 and RS-485 Standards Overview and System Configurations, Application Report (pdf). It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. The EIA once labeled all its standards with the prefix "RS" (Recommended Standard), but the EIA-TIA officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards.