ADM2485 parameters for RS485

Hi There,

We are using ADM2485 for use of RS485 Transceiver for MODBUS communication. I have questions on this chip about following parameters.

What is the maximum length it supports in RS485 network (Daisy chain)? What gauge? Must be shielded?

What should be baud rate in general for this chip? Is it possible to give table for baud rate vs distance?

What is proper Termination resistor values for Daisy Chain network?

What are the other factors or parameters need to be considered?

Here is what we are using: -

Baud Rate = 9600kbps

Cable Gauge = 24 (CAT-5)

Termination resistor = 150ohm

Cable type = Twisted Pair for A and B lines and one for Ground.

I would like to tell you that, when I hook-up three of our instruments, it works fine but when I go beyond three device in Daisy Chain Net-work, it stops communication. Any idea what could be the reason?

I was looking other manufacturer's ICs (similar to ADM485) and they have provided most of the information that I am asking this post. Where can I find this for ADM2485?

Any help or input will be greatly appreciated.

Thank you in advance,

pj

  • 0
    •  Analog Employees 
    on Apr 23, 2015 7:04 AM

    Hi PJ,

    Our AN-960 RS-485 Application Note provides you with answers to some of your questions:
    http://www.analog.com/media/en/technical-documentation/application-notes/AN-960.pdf

    1.
    The ADM2485 is capable of operating at data rates up to 16 Mbps.
    From a conservative estimate of cable length vs. data rate, AN-960 Figure 7:
    At a data rate of 9.6Mbps the cable length is limited to about 15 feet.

    If you operate the ADM2485 at a lower data rate, for example 2Mbps, then you can operate at up to 100 feet.

    2.
    RS-485 applications require termination at the master node and the slave node furthest from the master.
    The termination resistor value should equal the characteristic impedance of the cable.
    You are using 150 Ohm termination. Is this correct for your 24(CAT-5) cable?

    3.
    Using shielded cable minimizes EMI emissions and external noise coupling into the bus.
    If your application is in a noisy environment then you might consider using a shielded cable.
    The cable should be connected to Earth ground at one location only.

    4.
    If you are implementing the network in a daisy chain configuration, then you need to account for stub lengths.

    The electrical length of a stub, (the distance between a transceiver and cable trunk), should be much less than 1/4 of a wavelength of the frequency (equal to the inverse of the bit period).


    Can you please see how these guidelines compare to what you have implemented in your network?

    Best Regards,

    Richard

  • 0
    •  Analog Employees 
    on Apr 23, 2015 2:09 PM

    Hi Pj,

    At lower data rates, such as 10kbps or 20kbps it will be possible to communicate over much longer distances, up to 4000 feet.

    Yes, correct, the termination resistor value should equal the characteristic impedance of the cable.

    Yes, the 'inverse of the bit period' is the inverse of the baud rate 1/10kbps or 1/20kbps.

    Here's an example of what needs to be thought about when determining Data Rate vs. Cable Length:

    ---------------------------------------------------------------------------------------------------------------------------------------

    The AC effects of the cable limit the quality of the signal and limit the cable length to short distances when high data rates are used.

    Firstly, the approximate delay associated with standard RS-485 cabling is 1.5 ns/foot.
    Therefore, 330 feet (~100m) of cable will cause 495 ns of delay — which limits the data rate to 4 Mbps (2 MHz).

    Secondly, a number of other items must be considered when operating in a practical network, such as:
    A. transceiver device delays (propagation delays)
    B. encoding schemes/data pattern used (e.g. PRBS)
    C. protocol requirements (idle time)
    D. pulse width distortion of the signal
    E. system timing issues (e.g. jitter)

    As a result of these practical considerations, the 'conservative estimate graph' provides a data rate of < 1 Mbps corresponding to 330ft (100m).

    http://www.analog.com/media/en/technical-documentation/application-notes/AN-960.pdf

    ------------------------------------------------------------------------------------------------------------------------------------------

    Regards,

    Richard


  • Hi Richard,

    First of all Thank you for the response and pointing me write application note.

    I have mentioned you wrong baud rate. I said 10Mbps but it actually 10Kbps or 20Kbps. We are providing only this two options.

    Let me go through your points.

    1. I have already mentioned about baud rate.

    2. I will check impedance of CAT -5 wire. I think rule is "termination resistor value should be equal to impedance of wire". Correct??

    3. I will make sure about shielded cable.

    4. I need to calculate stub length. What do you mean by "inverse of bit period"? (Inverse of Baud rate of 10Kbps?).

    Now one general question,

    Is there any formula or calculation to calculate cable length or baud rate or other things related to RS-485?

    In other words, how one can say for specific this baud rate your length for cable must be this?? or vice-versa? I have found only one graph on most of the website between baud rate and cable length.

    I am wondering about calculation so if you can guide me in that direction would be great help.

    Thank you,

    pj......

  • Thank you Richard for the quick response.

    Let me do some calculations/exercises. I will get back if I have anything else.

    pj

  • Hi PJ,

    Some things to keep attention to with RS-485:

    1.) Besides the twisted wire for A/B, you also need a third wire to connect the ground. The Gnd2 of all the transceivers has to be connected to this Ground reference. In your post it sounds like you are doing this, but make sure.

    2) If you are using termination, there must be only two termination resistors, one on each end of your daisy chain. Nothing in between.

    Now, for 9.6kBd and a length of a few meters/feet, you can operate it just fine without any termination. Maybe try that.

    3) Make sure that you don't get a short from one of the additional devices that you hook up.

    4) Make sure that one of the additional devices that you hook up does not have its transmitter activated permanently. You could measure the voltage between the two A/B pins. When not activated, it should be close to Zero.

    I hope this helps a bit with your trouble shooting.

    Best Regards,

    Dirk