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LTC2862 fail safe operation

Hi Team,
I'm using LTC2862 in one of my designs.
The datasheet of LTC2862 (page no. 13) says, LTC2862 has an internal failsafe biasing and do not request any external resistance biasing..
But in the typical application on page number 17, there is resistance biasing.
So do we really need resistance biasing, if yes, what is the value of those bias resistors. Also, what is the terminal resistance value of RS485?.

  • Hi Shibinvarghese,

    This transceiver has internal fail safe biasing, so you do not need external fail safe resistors provided it is used in a network of transceivers with similar internal failsafe features.

    I think Page 17 just shows how you make an RS485 bus compatible with the PROFIBUS standard. This standard requires that pull up and pull down be provided at both ends of the bus in order to maintain a differential voltage in the event of an idle bus. It basically shows you PROFIBUS termination. PROFIBUS is a little different from a standard RS485 transceiver (differential output of the driver and the differential output capacitance etc). 

    Regards,

    Naveen

  • Hi Shibinvarghese, 

    Naveen is correct, the termination network shown on Page 17 is specific to Profibus DP networks. The termination and failsafe biasing resistors in this figure are required for a node in a Profibus DP network. 

    The LTC2862 receiver will work correctly and exhibit proper failsafe functionality, with or without a failsafe biasing network present on the bus. If your network will contain only LTC2862 devices, you do not need failsafe biasing resistors.

    If the network might include other RS-485 transceivers without failsafe functionality, you can choose to add failsafe biasing resistors to the network to support these legacy devices. The exact values for the biasing resistors will depend on the supply voltage of the LTC2862 device.  

    If you are using the LTC2862 in a Profibus network, to be compliant with the standard, you should include failsafe biasing resistors and the series resistors per the Figure on page 17 of the LTC2862 datasheet. 

    The input resistance to GND on the A or B terminals of the LTC2862 is greater than 84kohms, 

    Regards,

    Neil

  • Hi , 

    [ If your network will contain only LTC2862 devices, you do not need failsafe biasing resistors.]

    Could you please let me know internal biasing resistors used so we can decide on the external biasing .

    Thanking you 

    With best regard 

  • Hi Parziwal, 

    There are no internal biasing resistors in the LTC2862 receiver, 

    The failsafe works by sensing when the differential input voltage, VID, is 0V, which indicates when the input is idle, shorted or floating. When the VID is 0V for 3us or more, the receiver switches to failsafe mode and outputs a logic 1 on the RO pin. This is implemented with a parallel window comparator, 

    The "Full Failsafe Operation" section on page 13 of the LTC2862 datasheet includes some details on how this failsafe operation works, and comparison to other types of receiver failsafe implemented in other transceivers, 

    regards,

    Neil 

  • Hi Neil , 

    Thank you for the quick reply. 

    I asked this because for true failsafe it have to be from -30mV so in this case we will have to use external biasing circuit .

    Thanking you 

    With best regards 

  • The LTC2862 failsafe window comparator has thresholds of -200mV and 0V. 

    If the VID is within this window for 3 us or more, the output will failsafe.

    This should cover your -30mV case 

    Regards,

    Neil