ON Semiconductor

To learn more about onsemi™, please visit our website at www.onsemi.com

onsemi and and other names, marks, and brands are registered and/or common law trademarks of Semiconductor Components Industries, LLC dba “onsemi” or its affiliates and/or subsidiaries in the United States and/or other countries. onsemi owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of onsemi product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent-Marking.pdf. onsemi reserves the right to make changes at any time to any products or information herein, without notice. The information herein is provided “as-is” and onsemi makes no warranty, representation or guarantee regarding the accuracy of the information, product features, availability, functionality, or suitability of its products for any particular purpose, nor does onsemi assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. Buyer is responsible for its products and applications using onsemi products, including compliance with all laws, regulations and safety requirements or standards, regardless of any support or applications information provided by onsemi. “Typical” parameters which may be provided in onsemi data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. onsemi does not convey any license under any of its intellectual property rights nor the rights of others. onsemi products are not designed, intended, or authorized for use as a critical component in life support systems or any FDA Class 3 medical devices or medical devices with a same or similar classification in a foreign jurisdiction or any devices intended for implantation in the human body. Should Buyer purchase or use onsemi products for any such unintended or unauthorized application, Buyer shall indemnify and hold onsemi and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that onsemi was negligent regarding the design or manufacture of the part. onsemi is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.

AN-5023 LVDS Compatibility with RS422 and RS485 Interface Standards

| 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 applications note provides guidelines on the inter-operation of Interface Standards With so many electrical interface standards having separately 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 | Fairchild Semiconductor Application Note July 2002 Revised July 2002 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. | | --- | --- | --- | --- | --- | --- | | 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 | | | 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 standards, and it is up to the system designer to define a protocol suitable for their system. The RS-422 differential standard was established to provide a balanced interconnect system (in preference to the single-ended interface) for use at higher signaling rates. The RS-422 standard is 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. Specifically the driver’s differential output voltage (V the driver offset voltage (V the LVDS RS-644 receiver’s input ranges. Correspondingly, the receiver’s input thresholds and voltage range must be able to accept the LVDS RS-644 driver’s output levels. |

Compatibility with RS-422 and RS-485 Standards (Continued)

AN-5023

FIGURE 1. Driver Output Levels

FIGURE 2. Application of RS-422 Data to an LVDS Receiver

The R1, R2, and R3 resistor divider network collectively is an attenuation circuit. Refer to the equation below to calculate a total differential load of 100Ω to match the characteristic the voltage drops across the resistors. The majority (90%) of the RS-422 voltage is dropped across resistors R1 and R2, leaving the remaining voltage to be dropped across R3 which is compatible to the LVDS receiver.

(Voltage across R1)(Voltage across R2)(Voltage across R3)
V1 = ITOT * R1
V2 = ITOT * R2
V3 = ITOT * R3
= (33mA)(45 Ω) = 1.485V
= (33mA)(45 Ω) = 1.485V
= (33mA)(10 Ω) = 330mV