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SN65HVD3082E: Communication problem with 256 nodes

Part Number: SN65HVD3082E

Hi Experts,

I am posting this on behalf of the customer. Here it is below.

We have currently developed a product for one of our clients' requirements where we required  RS485 communication for up to 256 Nodes, for which we are using SN65HVD3082 IC.

We are using Nuvoton M0516LDN Controller for Master and Nuvoton MS51FB9AE Controller for slaves.

Currently, we have succeeded in communicating up to 100 slaves with their masters. but the time taken for communication to finish from 1 to 100 goes around 6 to 7 seconds which is quite far from our expectations. we are expecting the communication to get completed from 1 to 256 within maximum of 2 seconds. baud rate that we are using is 9600. 19200 and greater baud rate does not work.

 

Communication Pattern

  1. Master sends one Byte of data to slave 1 (DATA = "1")
  2. Slave 1 Acknowledges it and sends one byte of data back to the master (NO DELAY IS PROVIDED) (DATA = "K")
  3. NO DELAY IS PROVIDED
  4. Master sends one byte of data to slave 2 (DATA = "2")
  5. Slave 2 Acknowledges it and sends one byte of data back to the master (NO DELAY IS PROVIDED) (DATA = "K")
  6. and so on upto 256 slaves

Inside a loop of 256, the master sends one byte of data to each slave one by one. and the same is acknowledged by each slave. If the acknowledgment is not received by the master, the Master registers the same as COMMUNICATION FAULT. 

As mentioned, we have succeeded in Communicating 100 Slaves to date, but takes a lot of time. but when we add a 120 Ohms resistor at the end of line, we are not able to communicate even with a single slave. Currently, the distance between each slave and master is not more than 24 to 30 Inchs. 

I am attaching a circuit diagram we are using for RS485 Communication. Request you to please suggest any changes required to meet our requirements. 

 

Immediate response will be highly appreciated.

Best regards,

Jonathan

 

  • The circuit diagram is incomplete. Please show (or describe) everything connected to the bus lines. What bus topology are you using?

  • Hi Jonathan,

    The device can support 256 nodes - but not with the biasing resistors - they add to to the loading of the driver - if these biasing resistors exist on every node there is most likely a loading problem. 

    Is the speed that you are measuring only include the time for the RS-485 devices or is there processing time included in that as well? What is the bus capacitance and how long is the total cabling length?

    Please let me know!

    Best,

    Parker Dodson

  • Hi Parker,

    Thank you for waiting. Here is the customer's feedback.

    Where has to be the biasing resistor? do we need to place the biasing resistor only on the master? if so, what value has to be the resistor or how can we calibrate what the value of the resistor has to be?
     
    The speed mentioned here is the speed of RS485 and the controller processing time. But as we increase the nodes, the speed/rate also decreases. it takes longer to communicate the nodes connected to the end. 

    For Eg.  if 80 Nodes are Connected. 
    1 to 40 Nodes it takes around 4 mSec
    41 to 60 Nodes it takes around 8 mSec
    61 to 80 Nodes it takes around 12 mSec
    and so. 
    The Time mentioned here is just a rough calculation, just for making things understandable. 
    The distance between each node and the master is approx 24 Inches.
    Best regards,
    Jonathan
  • Hi Jonathan,

    In general to calibrate the passive failsafe resistors we use this note: https://www.ti.com/lit/an/slyt324/slyt324.pdf which goes into how to size the passive failsafe resistors. I'd say it would be about 1.2K for the passive fail-safe - it is only on one node. The caveat is that with a fully loaded bus technically  you shouldn't have any passive fail-safe because the common mode loading will be < 375 (~287) which will be overloading the bus. 

    For the time increase there are a few things to consider:

    Based on the quick math they did every 40 node adds about 4ms. So based on how I think they are measuring the time - the transceiver is most likely not the issue.

    At most you are looking at a a couple us for the signal to go from bus to controller and controller to bus through the transceiver. So even being generous maybe 200us delay for 40 nodes. But there is still ms of time that is missing and I think its the time for teh controller to process the command from the host device.  Unless the bus is very capacitive that the bus charge time is taking longer. 

    What is the bus topology - how are the nodes connected together? 

    Do they know the capacitance per foot for the cabling?

    Please let me know!

    Best,

    Parker Dodson