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I would like to confirm my understanding.

Suppose I want to have two battery-powered MCUs inside an (unmanned) aircraft communicate with each other via RS-485. If those MCUs are powered from the same battery, or if their grounds are connected, both MCUs should be able to communicate since they are both on (nearly?) the same ground potential. This should especially hold true, when the aircraft is small and interconnecting ground-lines are short and low-impedance. Correct?

Now, suppose one MCU is powered by a separate battery with no ground connection to the others. In my understanding the different between the two battery grounds is arbitrary - could be some mV (in which case communication will be possible) - or much more (in which case the RS-485 transceivers break). Correct?

Provided above points are correct, I figure that I either need an:

  • isolated RS-485 transceiver on the separate node with the 'outward' ground tied to the ground of the remaining MCUs. However, I then also need to power the outward interface either with an isolated DC/DC converter or from the other nodes.
  • isolated RS-485 transceivers on all nodes, powered by local isolated DC/DC converters
  • RS-485 transceiver with high common-mode range like LTC2863

I'm more inclined towards the last option due to the lower part count and complexity. Yet, I'm also interested in other suggestions.

Curiously, while there are many non-isolated DC/DC modules available in any form and power, I've yet to find a small, low-profile isolated DC/DC regulator of ~100mA output current. Can someone suggest a suitable part?

Clarification I'm looking for a small footprint (< 10mm x 10mm), low-profile (<2.5mm) part. All those 5W bricks are way too large for my purpose, considering that the transceivers require <100mA of current. Note that I've no regulatory requirements since we are not dealing with high voltages. Isolation is only needed to protect some components from harm. Therefore we really don't need too large packages that - as I understand - are mostly there to achieve enough distance between isolated pins.

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  • \$\begingroup\$ The issue is not so much a difference in ground potential, moreover a signal needs a return path. An isolated RS-485 driver simple provides a return path that is isolated at DC. This means the DC ground voltage at each side can be different. \$\endgroup\$
    – Oliver
    Commented Oct 16, 2014 at 9:09
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    \$\begingroup\$ Is there any reason why you can't connect a common ground between your nodes? Just because they'r all running off their own batteries doesn't mean you can't common the grounds. It'll make your RS485 comms so much simpler ... \$\endgroup\$
    – brhans
    Commented Oct 16, 2014 at 15:34

2 Answers 2

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You can connect the two grounds if they're truly floating wrt each other. Then the CM range is controlled and you're back to the original situation. Several volts of difference is okay (-7 to +12) so any reasonable connection over a short distance should meet this requirement. A high CM range transceiver would still require some kind of control of the common mode voltage.

Isolated DC-DC converters are very common, just do a parametric search at some distributor such as Digikey. I see 176 pages (25 per page) of different isolated DC-DC board-mount modules before filtering for required power and for input and output voltages.

Edit: One of my favorites is the TI (nee Burr-Brown) DCR01 series, which has an LDO (no problems out-of-spec voltages with light loads) and fits in an SOP that is 2.5mm high. It can supply 1W, so 200mA at 5V.

There are also RS-485 hybrid transceiver modules with the DC-DC converter built-in, but they're a bit expensive, and the DC-DC converter is probably a bit inefficient compared to one with a less constrained transformer.

enter image description here

You should connect the isolated ground to the ground on the remote system to control the common mode range.

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  • \$\begingroup\$ @Arne See my edit above. It's more like 10mm x 18mm x 2.5mm tall.. I don't think you'll find much much smaller than that, and low profile and capable of 0.5W to 1W. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Oct 16, 2014 at 9:01
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I guess the question is a bit old now, but I think the LTM2881 (http://www.linear.com/product/LTM2881) fulfills your requirements.

Edit: In response to the comment, I should have pointed out that this part is a complete isolated RS-485 transceiver including DC-DC converter with footprint 15x11.25mm and height 2.8mm. So actually it is almost the same size as the DCR01 part mentioned in the reply above, but includes the RS-485 transceiver as well. Price of the LTM2881 is somewhat higher than the DCR01, but total solution cost would probably be comparable.

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  • \$\begingroup\$ Link-only answers are discouraged so it's worth adding a few more details on why it'd be suitable so if in the future the link dies or the part becomes EOL it might give a hint on things to look for in other parts. \$\endgroup\$
    – PeterJ
    Commented Jan 20, 2015 at 12:10

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