tailieunhanh - RS-485 INTERFACE

STANDARD FOR ELECTRICAL CHARACTERISTICS OF GENERATORS AND RECEIVERS FOR USE IN BALANCED DIGITAL MULTIPOINT SYSTEMS | RS-485 INTERFACE STANDARD FOR ELECTRICAL CHARACTERISTICS OF GENERATORS AND RECEIVERS FOR USE IN BALANCED DIGITAL MULTIPOINT SYSTEMS Introduction The RS-485 is the recommend standard by the Electronic Industries Association EIA that specifies the electrical characteristics of generators and receivers that may be employed for the interchange of binary signals in multipoint interconnection of digital equipments. When implemented within the guidelines multiple generators and receivers may be attached to a common interconnecting cable. An interchange system includes one or more generators connected by a balanced interconnecting cable to one or more receivers and terminating resistors. Electrical Characteristics The electrical characteristics that are specified are measured at an interconnect point supplied by the devices manufacturer. Figure 1 shows an interconnection application of generators and receivers having the electrical parameters specified. The elements in the application are generators receivers transmission cables and termination resistances Rt . The loads on the system caused by each receiver and passive generator shall be defined in terms of unit loads. Each generator can drive up to 32 unit loads consisting of both receivers and generators in the passive state. Described in LEGEND This Standard G Generator R Receiver G R Combination Generator I Receiver L Length of stub the guideline assumes length of stub to be effectively zero R t Termination resistance location and value are not specified in this standard but a generator can drive 32 unit loads plus two termination resistances of l2o ohms each. Figure 1. Multipoint Interconnect Application The loading caused by receivers and passive generators on the interconnect must be considered in defining the device electrical characteristics. Two areas are of concern the DC load and the AC load characteristics. The DC load is defined as a number or fractions of unit loads . The AC loading is not standardized but