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RS-232 and RS-485 PCB Layout Application Note MaxLinear Confidential TBD/18 MaxLinear Confidential • www.maxlinear.com • 293ANR00293ANR00 i
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RS-232 and RS-485 PCB Layout Application Note Revision History Revision History Document No. Release Date Change Description 293ANR00 December 1, 2022 Initial release. MaxLinear Confidential December 1, 2022 293ANR00 ii
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RS-232 and RS-485 PCB Layout Application Note Table of Contents Table of Contents Introduction...........................................................................................................................................................................1 General Recommendation ...................................................................................................................................................1 MaxLinear Confidential December 1, 2022 293ANR00 iii
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RS-232 and RS-485 PCB Layout Application Note List of Figures List of Figures Figure 1: Optimal Power Supply Decoupling Capacitor Location...........................................................................................1 Figure 2: Optimal Use of Power Supply Traces, Ground Planes, and Power Planes ............................................................2 Figure 3: Input Traces and Unused Input Pin Strapping Recommendation ...........................................................................3 Figure 4: Trace Routing Recommendation.............................................................................................................................3 Figure 5: Example of a Guard Ring around Critical Traces....................................................................................................3 Figure 6: Component with Long Loop Traces Bad vs Good...................................................................................................4 MaxLinear Confidential December 1, 2022 293ANR00 iv
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RS-232 and RS-485 PCB Layout Application Note Introduction Introduction This document describes recommended printed circuit board (PCB) design and layout practices for RS-232 and RS-485/RS-422 serial transceiver products. The PCB is one of the most important factors that affect device performance, ESD and EMI. The expected PCB design minimizes the system noise and provide shielding between the PCB’s internal circuitry and the external environment. MaxLinear recommends that the PCB layout practices described in this application note apply to the serial products and the entire system PCB design. General Recommendation This section list the tips you can follow for your PCB design. Tip #1: Use power supply decoupling capacitors and place them close to the device supply pin Voltage sources in electronic systems can generate a lot of noise that affects both digital and analog circuits. Analog systems are extremely prone to supply voltage noise. This unwanted noise can distort the analog signal resulting in false values detected by devices further down the signal path. In most systems, the direct current (DC) supply is a constant voltage with ripple less than 50mV. Although small, it is good practice to minimize these supply voltage noises by adding decoupling capacitors to the device supply pin. ■ Connect a low-ESR, 0.1F (or 0.01F) ceramic bypass capacitors between each supply pin and ground. ■ Always keep the filtering bypass capacitors as close to the device supply pin as possible. Figure 1: Optimal Power Supply Decoupling Capacitor Location ■ Keep traces as short as possible to reduce the trace inductance. ■ Use a solid ground plane and solid power plane instead of ground traces and power traces. This lowers the current-path inductance and ensures the expected supply to each component in the system. MaxLinear Confidential December 1, 2022 293ANR00 1
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RS-232 and RS-485 PCB Layout Application Note General Recommendation ■ Place solid or filled vias into solid ground planes or power planes. Vias provide excellent connections and lower thermal dissipation. Figure 2: Optimal Use of Power Supply Traces, Ground Planes, and Power Planes MaxLinear Confidential December 1, 2022 293ANR00 2
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RS-232 and RS-485 PCB Layout Application Note General Recommendation Tip #2: Keep Input and output traces short and tie unused pins to ground ■ To reduce the noise coupling to input traces, run the input traces as far away from output traces and power supply sources. ■ Always keep the length of the input traces as short as possible because the input traces are the most sensitive part of the device. ■ Unused input pins must be tied to ground via 50 or directly to ground. Figure 3: Input Traces and Unused Input Pin Strapping Recommendation ■ Avoid routing traces at right angles to prevent from generating radiations and reflections. Figure 4: Trace Routing Recommendation Tip #3: Digital switching signals and clock lines Digital signals are binary with a defined voltage level. These signals transition between specific high and low voltage levels. They are used to convey logic data of the implemented protocols. Due to their sharp edge transitions and switching characteristics digital signals and clock lines can be a source of noise in the system layout. For critical applications, consider a low-impedance guard ring around critical traces. A guard ring can significantly reduce switching noise from coupling to nearby traces. Figure 5: Example of a Guard Ring around Critical Traces MaxLinear Confidential December 1, 2022 293ANR00 3
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RS-232 and RS-485 PCB Layout Application Note Disclaimer Tip #4: General recommendations ■ Place components on the PCB in such a way as to avoid long loop traces. Figure 6: Component with Long Loop Traces Bad vs Good ■ Avoid placing vias and pads in the path of critical signals. Vias and pads add additional unwanted capacitance and inductance which can cause signal reflection and distortion. ■ Use a power supply with adequate voltage and current for the application and with ground noise less than 50mV peak-to-peak. ■ Terminate all high-speed digital signal traces and clock lines to prevent reflections and signal ringing. For additional support, contact your MaxLinear Contact Support team or visit www.maxlinear.com. Corporate Headquarters: 5966 La Place Court, Suite 100 Carlsbad, CA 92008 Tel.: +1 (760) 692-0711 Fax: +1 (760) 444-8598 www.maxlinear.com The content of this document is furnished for informational use only, is subject to change without notice, and should not be construed as a commitment by MaxLinear, Inc. MaxLinear, Inc. assumes no responsibility or liability for any errors or inaccuracies that may appear in the informational content contained in this guide. Complying with all applicable copyright laws is the responsibility of the user. Without limiting the rights under copyright, no part of this document may be reproduced into, stored in, or introduced into a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise), or for any purpose, without the express written permission of MaxLinear, Inc. Maxlinear, Inc. does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the life support system or to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless MaxLinear, Inc. receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has been minimized; (b) the user assumes all such risks; (c) potential liability of MaxLinear, Inc. is adequately protected under the circumstances. MaxLinear, Inc. may have patents, patent applications, trademarks, copyrights, or other intellectual property rights covering subject matter in this document. Except as expressly provided in any written license agreement from MaxLinear, Inc., the furnishing of this document does not give you any license to these patents, trademarks, copyrights, or other intellectual property. MaxLinear, the MaxLinear logo, any MaxLinear trademarks (MxL, Full-Spectrum Capture, FSC, G.now, AirPHY, Puma, and AnyWAN), and the MaxLinear logo on the products sold are all property of MaxLinear, Inc. or one of MaxLinear’s subsidiaries in the U.S.A. and other countries. All rights reserved. Other company trademarks and product names appearing herein are the property of their respective owners © 2022 MaxLinear, Inc. All rights reserved.
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