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Isolated RS 485 for Harsh Environment and long Distance

Atul Bhakay
Genius 3205 points
Other Parts Discussed in Thread: SN65HVD22

6683.RS485 Design guide_slla272b.pdf

Hi, With ref to the attachment:-

1) What is meant by Single Ground Reference (refer figure 10 of the attachment)

2) Can RS485 communication happen over a distance of more than 1Km (1000 meters) where there is electrical noise (Marine application)

3) How is earthing carried out in ships? (Marine applications) 

4) What should be the voltage difference between Transmitter lines (Tx + and Tx - ) and also betwen reciever lines (Rx + and Rx - ) of RS485 in full-duplex mode, considering this distance of more than 1 Km ?

5) How can we monitor Tx and Rx pulses of RS485 when communication is going on (with CRO or logic analyser or multimeter in freq mode )?

6) What should be the basic, primary, mandatory conditions while designing RS485 communication for any distance (in the form of hardware and software) ?

Kindly reply on these quiries

Regards

Atul Bhakay

  • 0
    TI__Genius 14015 points

    Hi Atul,

    1) We recommend that RS-485 buses have a single ground reference for best performance. All transceivers except for one are isolated, so that the bus side of each node can float to whatever common-mode level is on the bus. The last node is non-isolated, to provide ground reference for the entire bus. This eliminates ground loops cause by ground potential differences between nodes.

    2) It depends on the data rate, bus structure, type of cable used, and how much electrical noise there is. With a slow data rate, good quality cable (Belden 3109A, or similar) and a point to point bus (you mentioned full-duplex), it is possible that communication would work at that distance. 

    3) I would guess there is a ground reference determined by the ships power source / generator. I don't have much experience with marine applications, however.

    4) Normal RS-485 levels should be fine: at least 1.5V differential at the driver, and at least +/-200mV at the receiver. 

    5) We typically use an oscilloscope on each of the bus lines, and on the TX and RX pins. 

    6) I'm not sure what you are looking for. Can you elaborate?

    Thanks,
    Jason Blackman 

  • 0
    Prodigy 50 points

    my data rate =4800

    max is 16 node

    straight cable is used 

  • 0 in reply to GOPAL jadhav
    Prodigy 50 points

    Preparing RS 485  communication for longer distance is not bigger (new) thing so please guide me proper sollution

  • 0 in reply to GOPAL jadhav
    TI__Genius 14015 points

    Hello,

    The SN65HVD22 transceiver is probably your best bet. It is designed to go 1200m at 500kbps. 

    http://www.ti.com/lit/ds/symlink/sn65hvd22.pdf

    It also has slew rate limiting, which means that networks with longer stubs perform better. The maximum allowable stub length for good communication is based on the rise and fall time of the driver. You can read about that more here: http://e2e.ti.com/support/interface/industrial_interface/m/videos__files/264978.aspx

    We recommend using good quality twisted pair with shielding, especially when going long distances. Straight cable allows any noise to be coupled in as differential signal, and also radiates more than twisted pair. Belden 3109A is one of our favorite cables. 

    Please let me know if you need anything else. 

    Thanks,
    Jason Blackman 

  • 0 in reply to Jason Blackman
    Intellectual 480 points

    Jason,

    Do you have isolated transceivers suitable for SN65HVD22* (I'm looking for communication that can handle a network with ~250kbps for one 10km segment (very large scale network)?

    * see Figure 5 and Figure 6 for example, "Data-rate independent half-duplex repeater design for RS-485", Applications Engineer.

    Thanks,

    Idan

  • 0 in reply to Idan Ozana
    TI__Mastermind 21336 points

    Hi Idan,

    It looks like Thomas has already addressed this question in a separate thread. Please let me know if I am mistaken.

    Thanks,

    John