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RS422 and RS485 standards specify different common mode voltage limits. In some application notes, I see that ground connection between two node is somehow optional. Suppose that two nodes are isolated. How can we sure about common mode voltages? In optional case, is it assumed that scenario is known and potential of local grounds of two nodes are close enough to each other?

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  • \$\begingroup\$ 200mV difference is the same whether you're talking about 0V and 200mV or 10.8V and 11.0V. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 30 '14 at 16:54
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    \$\begingroup\$ But there is an upper limit for common mode voltages. For example, ADM2682E, an isolated transceiver need -7V < Vcm <12V for proper operation (see Table 1) which is actually defined by standard. \$\endgroup\$ – Alper Aug 30 '14 at 17:09
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In some applications, common-mode voltage bears zero (or nearly zero) significance. For example, and RS422 driver can drive (at the far end) an optoisolator. Since the only load on the twisted pair is the LED in the optoisolator, common-mode voltage need only be held lower than enough to arc across the PC board... one might get 240VAC common-mode induced voltage on the line, yet suffer no ill effects.

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  • \$\begingroup\$ Thank you. For other isolation cases which limits Vcm or non-isolated scenarios we should always consider Vcm, right? \$\endgroup\$ – Alper Sep 7 '14 at 8:16
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As always, it depends on application. I can't think of why won't you have another gnd line. But if i treat that question as riddle, i suggest taking instead a single 120R termination two 60R resistors. It will create a ccentral tap, which can then be connected to reference voltage on your side.

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  • \$\begingroup\$ In fact, you can also use serial caps and pull ups/downs. \$\endgroup\$ – Gregory Kornblum Jun 3 '15 at 9:00

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