RS-485 INTERFACE

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STANDARD FOR ELECTRICAL CHARACTERISTICS OF GENERATORS AND RECEIVERS FOR USE IN BALANCED DIGITAL MULTIPOINT SYSTEMS

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Nội dung Text: RS-485 INTERFACE

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 Rt Rt specified are measured at an interconnect point supplied by the devices manufacturer. Figure 1 A/A1 A1 A shows an interconnection R G/R Balanced G application of generators and Interconnecting B B1 B/B1 Cable receivers having the electrical C1 C L C/C1 parameters specified. The Interface elements in the application are: Described in LEGEND: This Standard generators, receivers, transmission G = Generator cables, and termination resistances R = Receiver (Rt). The loads on the system G/R = Combination Generator / Receiver caused by each receiver and 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 passive generator shall be defined standard, but a generator can drive 32 unit loads plus two termination in terms of unit loads. Each resistances of 120 ohms each. generator can drive up to 32 unit Figure 1. Multipoint Interconnect Application loads consisting of both receivers and generators in the passive state. 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 must be considered in the design of a system using the devices meeting this standard. General System Configuration: The generators and receivers conforming to the RS-485 standard can operate with a common mode voltage between -7 volts and +7 volts ( instantaneous ). The common mode voltage is defined to be any uncompensated combination of generator-receiver ground potential difference and longitudinally coupled peak noise voltage measured between the receiver circuit ground and cable with the generator ends of the cable short circuited to ground, plus the generator offset voltage (Vos). Grounding Arrangements: Proper operation of the generator and receiver circuits requires the presence of a signal return path between the circuit grounds of the equipment at each end of the interconnection. The grounding arrangements are shown in Figure 2. Where the circuit reference is provided by a third conductor, the connection between circuit common and the third conductor must contain some resistance ( e.g., 100 ohms ) to limit circulating currents when other ground connections are provided for safety. Some applications may require the use of shielded interconnecting cable for EMI or other purposes. The shield shall be connected to frame ground at either or both ends, depending on the application. 9-4.1
Balanced Interconnecting Rt Rt Cable Pair Third Conductor No Connection No Connection A/A1 A1 A R G/R G B B1 B/B1 C/C1 C C1 100 S 100 S 100 S Logic reference 100 S * 100 S * 100 S * Optional 1/2 W 1/2 W 1/2 W Strap Chassis reference Earth (safety) reference NOTE: Third conductor not required if earth * Resistor must become an open reference provided in each using equipment. circuit when overloaded Figure 2. Grounding Arrangements Similarity with RS-422-A: In certain instances, it may be possible to produce generators and receivers that meet the requirements of both RS-422-A and of RS-485. Table 1 depicts the differences in parameter specifications which exist between the two documents. Table 1. Comparison of RS-422-A and RS-485 Characteristics Characteristic RS-422-A RS-485 Min. output voltage 2V into 100 ohm 1.5 V into 54 ohms > 1/2 open circuit V Ishort to ground 150 mA maximum Ishort to -7, +12 volts 250 mA peak trise time < 0.1 tb , 100 ohm load < 0.3 tb , 54 ohm, 50 pF load Where tb = time duration of the unit interval at the applicable data signalling rate (pulse width). 9-4.2