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SN65HVD485E: SN65HVD485ED

Dattatray Salunke
Prodigy 80 points
Part Number: SN65HVD485E

Hi,

I am using "SN65HVD485ED"  RS-485 chip. The termination resistor value is 120E. Pull to pin A is 1.2K & pull down to pin B is 1.2K. I am using Modbus poll software to generate the query. I have connected three device on the bus. I am sending query to only one device after every 3 sec. The response received from end device having 20% error from the total query sent. Means if Modbus poll software sent 100 query, i am getting response for all with 20 having "Checksum error" , "Insufficient byte received" such a error.

Can anybody help me out that what is going wrong?

Thanks

  • 0
    TI__Guru 74252 points

    Dattatray,

    I'm sorry you are having trouble with this device. I'd like to ask a few questions just to get a bit more information.

    • What is the status of the other pins in a typical condition? Because 80% of your messages are going through correctly, I assume you have everything correct, but I'd like to have all the possible information I can.
    • What bit rate is your message being sent?
    • Are you monitoring the signals on an oscilloscope (if possible)? And if you are, could share some scope shots of your waveforms? I realize this may not be easy to capture any errors, but it'd be interesting and helpful to see what the bus looks like during transmission.
    • Is it a consistent 20% error rate?
    • From the responses you are getting, are they consistent with what you'd expect? That is, is the correct device on the bus responding with something that makes sense given what you are sending to it.

    Can you try removing the 1.2k resistors and see if it helps? This device doesn't need pull up and pull down on the A and B lines, and while it shouldn't affect the signaling, it may be creating a mismatch on your termination impedance to what the transceiver expects, which would affect your signal integrity.

    Regards,

  • 0 in reply to Eric Hackett
    Prodigy 80 points
    Hi Eric,

    Thanks.
    Message bit rate is 19200.
    Error rate is increasing as we connected more than 3 devices. 20% error is for 3 devices.
    We tried with 1.2K removed. But no improvement observed.
    With one device connected it works fine.
    As in the datasheet suggested on bus 64 devices can be connected. In that case what is the terminating resistor & how it will be calculated?

    Regards,

    Dattatray
  • 0 in reply to Dattatray Salunke
    TI__Guru 62920 points
    Hi Dattatray,

    Typically you would terminate the network via a 120-Ohm resistor between the A and B lines at the two furthest ends of the bus. For such a low data rate, though, I would not expect this termination to be all that critical. (You should be careful not to have more than two termination resistances present, though, since each resistor connects in parallel and reduces the overall resistive loading of the bus.)

    Do you think you could measure the signal waveforms at the D input to the transceiver, at the A and B bus outputs/inputs, and at the eventual R output of the transceiver when this issue occurs? That could help us better isolate what exactly is going wrong to result in bit errors.

    Regards,
    Max