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In one of my PCB design, for a RS 485 signal termination resistor of 120 ohm is there. But the differential characteristics impedance of the signal is taken as 100 ohm instead of 120 and the signal travelling down the trace is of low frequency. Does this impedance mismatch/change affects the low frequency signal particularly at the destination with reflections or some other SI issues?

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    \$\begingroup\$ ti.com/lit/an/slla272c/slla272c.pdf (see section 6) \$\endgroup\$ – kva Jun 27 '18 at 11:04
  • \$\begingroup\$ A small mismatch is often not an issue; in particular, even if there is a reflection, having an approximately right resistive termination means there's a lot of dissipation and reflections will likely die out within few round trips. However, to formally analyze this you'd need to consider the length of the line and the applicable frequency. \$\endgroup\$ – Chris Stratton Jun 27 '18 at 13:55
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If you can afford to have a delay between sending messages on the line I would do that. As it will ensure that previous messages will have enough time to dissipate. The time needed for the signal to dissipate is depending on the length of the line so experimentation might be needed, but a bit of code can be written that tests reliability of the line with an increasing delay and you stop increasing when the message transmission is reliable.

Most lines typically range from 100Ω to 150Ω for twisted-pair cables. As long as you have resistors on both ends of the line you should be fine.

For additional reading look at this page as it explains how to deal with the reflection coefficient optimally.

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    \$\begingroup\$ A delay between messages wouldn't really help, if there is an issue, it would be overlap of the individual transitions encoding bits or symbols with un-expended reflections of preceding ones. \$\endgroup\$ – Chris Stratton Jun 27 '18 at 13:53

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