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RS-644 Datasheet(PDF) 1 Page - Fairchild Semiconductor |
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RS-644 Datasheet(HTML) 1 Page - Fairchild Semiconductor |
1 / 3 page © 2002 Fairchild Semiconductor Corporation AN500526 www.fairchildsemi.com Fairchild Semiconductor Application Note July 2002 Revised July 2002 AN-5023 LVDS Compatibility with RS422 and RS485 Interface Standards Abstract Low Voltage Differential Signaling (LVDS) technology offers one of the best serial data transmission profiles available today for physical layer interfaces. With many existing interface standards to choose from, this applica- tions note provides guidelines on the inter-operation of LVDS devices with other differential interface standards such as RS-422/485. An example is included of an LVDS receiver configured to be inter-operable with an RS-422 driver. Interface Standards With so many electrical interface standards having sepa- rately evolved within particular industries, many serial data interface standards now exist. To simplify designers’ choices, the following table summarizes some of the key electrical specifications for the different serial data interface standards listed. TABLE 1. Interface Standards Specifications Note 1: The transmission distance is shorter with faster signaling rates. The three standards in Table 1 are hardware specifications that define the driver and receiver electrical characteristics. Software protocols are not discussed in any of the stan- dards, and it is up to the system designer to define a proto- col suitable for their system. The RS-422 differential standard was established to provide a balanced intercon- nect system (in preference to the single-ended interface) for use at higher signaling rates. The RS-422 standard is suited for point-to-point and multi-drop interconnects and not so well suited for multipoint systems. The RS-485 dif- ferential standard was issued to provide the driver/receiver electrical characteristics for balanced multipoint systems. One of the principle differences between RS-422 and RS-485 standards is that the RS-485 driver can be placed into a high impedance tristate mode, allowing drivers to transmit over the same pair of wires. Compatibility with RS-422 and RS-485 Standards The LVDS interface circuit is not intended for direct inter- operation with other interface technologies, like RS-422, RS-485, or even Positive Emitter Coupled Logic (PECL). Under certain conditions, inter-operation of LVDS with other interface circuits mentioned above may be possible but may require modification in the interface or within the equipment. Limitations on certain performance parameters, like common mode range, may be required; satisfactory operation is not assured and additional provisions may need to be employed. In determining whether direct inter-operation of LVDS TIA/ EIA-644 compliant devices is possible with other interface standards, it is necessary to compare the generator (driver) output and the receiver input electrical specifications. Spe- cifically the driver’s differential output voltage (VOD) and the driver offset voltage (VOS) must be within the bounds of the LVDS RS-644 receiver’s input ranges. Correspond- ingly, the receiver’s input thresholds and voltage range must be able to accept the LVDS RS-644 driver’s output levels. Drivers compliant to the TIA/EIA-644 standard feature a current source capable of delivering a loop current in the range of 2.5 to 4.5mA. As illustrated in Figure 1, the result- ing differential voltage (VOD) will be a minimum of 250 mV up to a maximum of 450mV across the 100 Ω termination resistor. The driver offset voltage (VOS), also referred to as the center point, is typically +1.2V referenced to circuit common ground. Any balanced receiver that guarantees the input voltage range of 0V to +2.4V and input thresholds of 200mV or less may be compatible and directly inter-operate with other balanced drivers. Compatibility is possible provided the balanced driver does not violate the maximum receiver input voltage range and develops a differential voltage (VOD) of at least 100mV and not greater than 600mV. Inter- operation with drivers with larger output differential volt- ages is possible by using an attenuation circuit with the interface points of the components. Refer to Figure 2 for the illustration. Parameter RS-422 RS-485 RS-644 (LVDS) Differential Voltage Swing (typ) 3.0V 3.0V 350mV Common Mode Voltage 1.8V 1.8V 1.2V Driver Output Rise Time 10%tui 30%tui 30%tui Receiver Common-Mode Voltage ± 7V −7V/+12V 0V to +2.4V Receiver Sensitivity ± 200mV ± 200mV ± 100mV Signaling Rate < 50Mbp/s < 50Mbp/s > 400Mbp/s Transmission Distance (Note 1) ≤1200m ≤1200m ≤100m |
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