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I am working on a reader device that I designed in slave mode to instantly read the information of the buttons pressed on an automation packaging machine and a master device that reads the data live from this device via RS485. The system was working without any problems at first, but after 2-3 days, the RS485 communication stopped working. The MAX3485 integrated circuit in the slave device breaks down. When I replace this integrated circuit, the device continues to work. There is no heating in the integrated circuit. After a while, this chip breaks down again chronically. I have been researching for a long time what could be causing this, but unfortunately I could not reach a conclusion. I would be very happy if someone with experience in this regard could give me an idea.

I am using a 2m long cable coded "643621120305" between the master and slave devices. My power supply comes from the machine. Different components in the machine are also powered from this source. I was previously powering it with a separate adapter, the result was the same.

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Figure 1: System general design

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Figure 2: Master Device RS485 Circuit Design

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Figure 3: Slave Device RS485 Circuit Design

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  • \$\begingroup\$ Are you connecting GND (Slave) to GND_ISO (MSTR) via the cable? If not, give it a try. If not possible per system design bridge GND and GND_ISO in the MSTR Device with 1Meg||1pF 1kV to avoid high voltage differentials. \$\endgroup\$
    – ElectronicsStudent
    Commented Sep 4 at 14:29
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    \$\begingroup\$ Umm... The master is driving the RS485 with 5V, but the slave is powered at 3.3V. That is not a good idea. \$\endgroup\$
    – Steve Mathwig
    Commented Sep 4 at 14:31
  • \$\begingroup\$ Have you read this ... fig 10 st.com/resource/en/application_note/… \$\endgroup\$
    – Antonio51
    Commented Sep 4 at 14:39
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    \$\begingroup\$ @SteveMathwig MAX3485 is powered by 3.3V, but is capable to have -7V to +12V Common-Mode Input Voltage Range. Why do you think this is a problem? \$\endgroup\$
    – Chupacabras
    Commented Sep 4 at 14:47
  • \$\begingroup\$ That cable has two pairs of wires. If one pair is unused, connect both wires to ground at least at one end, and at both if the devices have a common ground, to avoid picking up EMI that might be damaging it. \$\endgroup\$
    – DrMoishe Pippik
    Commented Sep 4 at 15:00

2 Answers 2

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Of course it does not work, the slave and master are properly designed themselves, but as a system, you have two RS-485 transceivers that are fully isolated from each other trying to communicate without a common reference.

This is not supposed to happen.

RS-485 is not a two-wire bus, it is a bus with two data wires and a need for common reference between all parties involved.

Which means, in case of devices being isolated from each other, there must be a common ground reference wire between the bus sides of the RS-485 ICs:

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  • \$\begingroup\$ Thanks for your answer, If GND and GND2 are connected in this way, the GND of the circuit and the GND of RS485 communication will be the same, then isolation will have no meaning. But according to your description, if the slave device was isolated and I connected the isolation GNDs of these two circuits to each other, I think there would be a general ground integrity. This idea makes more sense. \$\endgroup\$
    – Electronn
    Commented Sep 5 at 15:32
  • \$\begingroup\$ @Electronn Exactly. Imagine if you had two isolated transceivers, you would again need to connect the two grounds that relate to the isolated bus interface together. Same thing basically. The two RS-485 ports must be within 7V within each other or they cannot communicate or even damage. \$\endgroup\$
    – Justme
    Commented Sep 5 at 16:11
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Note that the isolation in the master is unnecessary. I'm not sure if the master's design is in your control or not. But, since both the master and the slave use the same power supply, there's no reason to isolate the master.

In fact, without isolation you probably could use a single pair cable to connect the master and the slave, depending on the shared ground via the 24V supply. This would likely work OK given the fairly wide input common-mode range of the transceivers.

When the master is isolated, this common ground reference is broken, and an explicit ground wire must connect the slave and the master, as per this answer.

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