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below is my schematic for isolated RS485 section. FYI B0305XT-1WR2 is used for power isolation and ISO1500DBQ as transceiver.
As you can see I forgot to place GND & GND_RS485 connections at pins 2 & 15 respectively.
As per the TI datasheet's typical application circuit, they are connected to their respective GNDs.
I have checked my PCB also, tracks ended up just like the schematic.
So the question is -

  1. Are the GND1 pins (2,8) & GND2 (9,15) are internally shorted? (I didn't find this info in datasheet) If yes, does my circuit work?

As of now, I've tried sending data from MCU to the transceiver. I cannot see any data on Serial terminal (connected D+,D- to USB-RS485 converter). Any help is appreciated.

RS485-ISO1500DBQ

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  • \$\begingroup\$ Since you have the board in your hands, can you measure with a multimeter if the grounds are connected or not? Did you also connect the RS485-side ground to PC RS485 adapter ground? \$\endgroup\$
    – Justme
    Dec 3, 2020 at 18:26
  • \$\begingroup\$ There's continuity between respective GNDs. No I didn't connect the grounds. Never connected before but it used to work on other boards without common GND. \$\endgroup\$ Dec 3, 2020 at 18:34
  • \$\begingroup\$ Power IC B0305XT-1WR2 is outputting 5.4V instead of its supposed 5V. It is same in 3 boards I checked. Does that pose a problem? \$\endgroup\$ Dec 3, 2020 at 18:37
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    \$\begingroup\$ That's misuse of the part, since you have the capacitors there you have easy opportunity to solder a connection to a nearby ground. As for debugging, break out the test equipment. If you only have a meter, make a temporary firmware which commands transmit mode and a steady level from the MCU side and then measure the input and output of the RS485 chip, then try it with the other level. Then command receive mode and drive the RS485 side from something else, possibly even a few flashlight cells held one way or the other. \$\endgroup\$ Dec 3, 2020 at 18:58
  • \$\begingroup\$ How do you expect RS-485 to work without common reference between two devices, with simply two data wires? Mind you, that you specifically have GND_RS485 completely isolated and floating (unless they are connected to common ground with the PC via some other route). And the isolating might have a lot of common mode switching noise. This manufacturer does not seem to even care mentioning it. Also, the RS-485 chip consumes very little current, and the isolating supply minimum requirement is 20mA, so this explains why there is 5.4V, it is still within (but uncomfortably close) to the 5.5V limit. \$\endgroup\$
    – Justme
    Dec 3, 2020 at 20:00

1 Answer 1

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Are the GND1 pins (2,8) & GND2 (9,15) are internally shorted? (I didn't find this info in datasheet) If yes, does my circuit work?

Likely. Use a multimeter to verify. Best practice is to connect all gnd pins to their respective ground.

I cannot see any data on Serial terminal (connected D+,D- to USB-RS485 converter

Check the level on your RE pin. Looks like you signal controls both receive (RE) and transmit (DE).

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  • \$\begingroup\$ Yes. The GNDS are internally shorted. Checked with meter. So you say it works if yes? Yeah DE and RE are shorted and controlled with a GPIO pin. I don't have oscilloscope to check signals. \$\endgroup\$ Dec 4, 2020 at 4:19
  • \$\begingroup\$ Correct, the chip will work if the internal grounds are connected. Having multiple ground pins helps with noise. Your issue is somewhere else. \$\endgroup\$ Dec 4, 2020 at 13:19
  • \$\begingroup\$ Try swapping D+ and D- (or A and B). TI RS485 chips usually have polarity opposite to a standard, and A usually goes into D-. \$\endgroup\$ Dec 4, 2020 at 13:21
  • \$\begingroup\$ @AndrejsGasilovs - Checked reversing the d+ and d- lines. No luck. \$\endgroup\$ Dec 5, 2020 at 7:02
  • \$\begingroup\$ @Mr_Engineer - looks like caps are the problem. It's working now. Thanks. \$\endgroup\$ Dec 5, 2020 at 9:23

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