1
\$\begingroup\$

enter image description here

Hi, I have RS485 communication circuit as shown in figure. But it has no isolation, only SM712 is used for ESD protection. Now, i think it will be better to isolate the it.So, how can i isolate it? so, should i replace the RS485 (SN65HVD20D) by a internally isolated RS 485 or have to use some isolators in the circuit to isolate it?

If I use a bi-directional digital isolator (say ISO1540), can it possible to isolate the bus side A and B? Can I exactlty want to know what would be the problem if I do so? Please go through the block diagram below:

enter image description here

\$\endgroup\$
11
  • 1
    \$\begingroup\$ That's a bit broad. Obviously, if you have an isolated replacement of your transceiver, that'll come with a datasheet or example board with a schematic of how to use it. \$\endgroup\$ Jan 8, 2019 at 12:23
  • 3
    \$\begingroup\$ "I think it will be better": Why do you think it is? "I think" doesn't sound like proper engineering; you will have a reasoning, and that reasoning is probably critical for how to design your circuit! \$\endgroup\$ Jan 8, 2019 at 12:24
  • \$\begingroup\$ @Marcus Müller, But if i want isolate my circuit, how can i do that without replacing the RS485 IC? \$\endgroup\$
    – Aakash Dey
    Jan 8, 2019 at 12:31
  • \$\begingroup\$ In general, getting an isolated transceiver means you get stuck with that supplier and they tend to be expensive. When there's an industry standard for SOIC8 RS485 transceivers with plenty of second source. So in general I would consider using a separate isolator. These are more reliable than optocouplers: analog.com/en/analog-dialogue/articles/… \$\endgroup\$
    – Lundin
    Jan 8, 2019 at 12:33
  • \$\begingroup\$ OK. If i use isolator (say digital isolator), where should i place this, on Input side of RS485 (Pin A and B) or on output side( RO and DI pin)? \$\endgroup\$
    – Aakash Dey
    Jan 8, 2019 at 12:49

1 Answer 1

0
\$\begingroup\$

You can isolate the lines going to the transceiver, or you can isolate the RS-485.

Isolated transceiver: enter image description here

Isolator + Transceiver: enter image description here

Typically there is less choice in bus-to-bus isolators or combined transceiver/isolator chips, thus I commonly isolate CAN bus with abundant generic logic isolators and a separate transceiver, bonus point is BOM re-use. There seems to be more available for RS-485 though.

However, if you're not looking for low maintenance longevity, the isolated transceiver should be fine.

Image sources:
https://forums.parallax.com/discussion/162277/isolated-rs-485 https://www.radiolocman.com/review/article.html?di=336193

This is how the isolator work when you do not use old school optocouplers.

\$\endgroup\$
4
  • \$\begingroup\$ I have understood the above to methods. But what will be the problem if I isolate in the bus side with a bidirectional isolator? Isn't possible? I have post the image below. \$\endgroup\$
    – Aakash Dey
    Jan 12, 2019 at 14:54
  • \$\begingroup\$ @AakashDey you cannot isolate the RS-485 side with any random digital isolator. RS-485 is a differential standard with particular voltage levels while a normal digital isolator deals with single-ended signals. \$\endgroup\$
    – nanofarad
    Jan 12, 2019 at 15:34
  • \$\begingroup\$ @AakashDey Research the signal levels used by RS-485. \$\endgroup\$
    – Jeroen3
    Jan 12, 2019 at 16:29
  • \$\begingroup\$ OK. I will study on this. Thanks for the guidance and help. \$\endgroup\$
    – Aakash Dey
    Jan 16, 2019 at 12:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.