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enter image description hereCan anyone confirm the correctness of D27, D28 diode polarities or should they be in opposite direction? Please suggest an which diode is suitable for this purpose OR is it better to use a zener diode in place of simple diode? Also what is the purpose of C69 capacitor and what will be its appropriate value?

edit: picture added now.

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    \$\begingroup\$ Schematic? Without one, we don't know what you're asking about. \$\endgroup\$ – Peter Bennett Feb 8 at 21:21
  • \$\begingroup\$ SM712 is a good diode along with series resistors of about 10 ohms \$\endgroup\$ – uglyoldbob Feb 8 at 21:27
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    \$\begingroup\$ This circuit is confusing, what is it for? why a receive-only node? is this some piece of test equipment? If so why terminate? \$\endgroup\$ – Jasen Feb 9 at 2:46
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D28 and D27 are clamping diodes; whenever either of the data lines sinks below ground, the diode will source current into it to clamp it back to ground. The polarity of the diodes here is correct. Note that because RS-485 is designed to operate with a wide common-mode range these diodes will wrongly clamp the signal when common mode dips below ground (as pointed out by Dan Mills in a comment). As a result, your system will not operate when this happens, even though RS-485 transceivers should (by spec) be able to operate in this case. Considering that the absolute maximum rating for the bus terminals allows voltage down to -10 V, this squanders a significant portion of the allowable common-mode range. You may want to consider back-to-back Zeners instead, in order to clamp on the range of (-10 V, 15 V).

C69 is used for terminating the transmission line. This guide by TI discusses AC termination in section 4.3; the arrangement typically consists of a resistor and capacitor in series, rather than simply a capacitor. Typically the resistor is equal in value to the characteristic impedance of the transmission line. During a transition, the capacitor initially looks like a short-circuit, and the two parts together terminate the line with the resistor's impedance. During the steady-state, the capacitor blocks DC to save power and limit current draw.

The arrangement in your schematic seems odd, since the capacitor alone will present the wrong termination impedance because of the missing resistor. While you might be able to get away with it like this, you might encounter significant reflections from the receiver, especially if the transmission line is long compared to the signal wavelength.

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  • \$\begingroup\$ So what i understand is that C69 is necessary for line termination.. the RS485 will not work correctly without it. But D27 and D28 are only for protection.. the circuit will work even without these diodes.. please correct me if i am wrong. \$\endgroup\$ – scico111 Feb 8 at 21:55
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    \$\begingroup\$ @scico111 The two diodes are for protection. C69 is for termination but it doesn't seem correct; I suspect that it will work better than unterminated but worse than properly terminated. \$\endgroup\$ – Andrey Akhmetov Feb 8 at 22:35
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    \$\begingroup\$ RS485 has a common mode range that extends from +12 to -7V with respect to ground so those diodes are overly conservative, and while they will protect the chip, they will also stop the receiver working in the presence of common mode. C69 is also just weird, for it to be an AC termination it needs a resistor of about 120R in series (but only if this thing is at one end of the line), I suspect it is some kind of half assed RFI measure possibly put in to solve a particular problem in whatever kit that is. \$\endgroup\$ – Dan Mills Feb 8 at 23:52
  • \$\begingroup\$ @DanMills Thank you for the reminder as I forgot that common-mode could swing that far negatively. I've restated your remarks with attribution and adjusted the diode recommendations to clamp using Zeners instead. \$\endgroup\$ – Andrey Akhmetov Feb 9 at 0:08
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    \$\begingroup\$ @scico111 Is there any particular reason why the resistor or AC termination in the link I provided doesn't work for you? And as for the Zener, you just need a pair that clamps to the range I specified, so a 15 V and a 10 V zener back to back. \$\endgroup\$ – Andrey Akhmetov Feb 9 at 1:22
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R122 and R121 look like part of a termination and bias network, but the third resistor is missing. replace C69 with a 120 ohm resistor.

D27 and D28 should be bi-directional TVSs - the line should be allowed to go a few volts negative, ans also be limited in the positive direction.

but then the RS485 transceiver is strapped to only receive... what is the purpose of the bias network?

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