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SN65LBC176A - failures on a node when the RS485 Master is dropped/removed from the line

Rob Mitrunen
Expert 1685 points
Other Parts Discussed in Thread: SN65LBC176A-EP, SN65HVD82, SN65LBC184, SN65HVD11-HT

We have an application using the SN65LBC176A-EP for RS485 communication with 2 nodes. We are seeing failures on a node when the RS485 Master is dropped/removed from the line. The failure presents itself as power ground short on the PCB with the node. Removing one of the nodes does not damage the Master however. USB voltage powers the node transceiver as well as the support circuitry on the board. The master and Slave are connected by 4 lines (A, B, GND, and node detect signal).

 

Is it possible that a voltage transient on the communication lines somehow makes its way to the VCC?

 

If I am reading the circuit diagram from page 2 of the datasheet correctly, the diodes will clamp the voltage to 16 volts but a transient less than 16V will pass through?

 

  • 0
    TI__Genius 14317 points

    Hi Rob,

    Can you describe physically how the master is "dropped/removed from the line"? Is the master transceiver on one board and the node transceivers on two other boards? Is the master transceiver board being removed from a card slot (power removed too) while the two node transceiver (boards?) remain powered?

  • 0
    TI__Mastermind 28265 points

    Rob, I'm assuming that the master node is unplugged manually. In this case they are various mechanisms that can destroy the transceiver. Either through human ESD  (several kV damaging the internal ESD structure) or simply by an ESD transient caused by the friction of board pins and connector pins during unplugging. Since the device can transmit up to 30 Mbps, awhat data rate do you require?

    regards, Thomas

  • 0 in reply to Thomas Kugelstadt
    Expert 1685 points

    Currently, we have the system set up in setup in the following way:

    The master and the two nodes are separated by 10 feet of cabling. The master and each node are on a separate circuit board and have their own dedicated power supply. The only thing common between the boards are the communication lines and ground. The master is removed from the system by disconnecting the 10 foot cable from it. At this point, both nodes are still powered.

     

    The master node is unplugged manually. The node is connected to the rest of the system using a micro-d connector.

     

    As for the transmit speeds we don’t require anything higher than 10kbps. We are definitely not using the device to its fullest extent.

  • 0 in reply to Rob Mitrunen
    TI__Mastermind 28265 points

    Rob, at 10kbps you should try the new SN65HVD82. It can go up to 250kbps but it has siginficantly stronger transient protection on-chip.

    Also at these short distances if your ground potential differences are below 7V you can remove the rgound wires too.

    regards, Thomas

  • 0 in reply to Thomas Kugelstadt
    TI__Genius 14317 points

    Another device to look at for this application is the SN65LBC184 since it also has Input TVS and "Circuit Damage Protection".

    * Integrated Transient Voltage Suppression

    * ESD Protection for Bus Terminals Exceeds:

    ±30 kV IEC 61000-4-2, Contact Discharge

    ±15 kV IEC 61000-4-2, Air-Gap Discharge

    ±15 kV EIA/JEDEC Human Body Model

    * Circuit Damage Protection of 400-W Peak

    (Typical) Per IEC 61000-4-5

    Since we don't know the failure mechanism of the slave node transceiver device, this device protects against most of the possible causes.

  • 0 in reply to Jeffery Pfarr
    Expert 1685 points

    Currently, the customer is having issues establishing a robust system on surface using the SN65LBC176A-EP. Once they are confident that the system is robust and working the way they want, they will be implementing it in a high temperature application using SN65HVD11-HT or DS16F95A.

  • 0 in reply to Rob Mitrunen
    TI__Mastermind 28265 points

    The LBC176A is a 5V device, the HVD11 is a 3V device.

    Is your customer aware of this?

  • 0 in reply to Thomas Kugelstadt
    Expert 1685 points

    Yes they are and will most likely stay with the 5V part. Any suggestion of what they should try with their SN65LBC176A-EP circuit or should this be a question for the Hi Rel Forum?

  • 0 in reply to Jeffery Pfarr
    TI__Mastermind 28265 points

    yes, it is also one of the oldest and most expensive devices. TK