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THVD1452: RS485 Full Duplex

Helder Cunha
Prodigy 10 points
Part Number: THVD1452
Other Parts Discussed in Thread: THVD1451

Hello, 

We are using the following chip THVD1452DGSR to build an RS485 full duplex network,

when we connect 3 units in the same bus following your technical information, we detect that the line to transmit the information from Slaves to the MASTER don't work as expected...the other line that the Master uses to send the information to the Slaves works fine... we are testing the following system: 


We only get the system working if we disable all the Slaves transmission line (using the DE) and put one Slave at a time sending the information to the Master. 

if we able all the Slaves transmisstion line, the system don't work... it looks like that teh voltage levels on transmission line decrease for low values, that are not recognized... 

waiting for your help! 

regards,

Hélder Cunha 

  • 0
    TI__Guru 72242 points

    Helder,

    Do all nodes have termination resistance populated on the bus pins?

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    Prodigy 10 points

    HI Eric, 

    yes, we apply the 120ohm resistance (RT) on bus pins of the of the first unit and last unit of the bus, as well the other 10ohm resistances and TVS as following example shows:

    let me know your comments. 

    regards,

    Hélder Cunha 

  • 0 in reply to Helder Cunha
    Guru 286290 points

    On a RS-485 bus, only one device at a time is allows to transmit. (What should the bus state be when multiple devices try to transmit different values?)

  • 0 in reply to Clemens Ladisch
    TI__Guru 50835 points

    Hi Helder,

    Clemens makes a good point here. Devices that share a transmission line cannot be active at the same time. So the first case you describe where all non-active nodes have DE disabled when not actively communicating is the proper method for this configuration.

    Let us know if you have any more questions.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    Prodigy 10 points

    Hi Eric and Clemens,

    We know that only one slave should be in transmission mode at a time, and we are respecting that.

    But we think that all slaves could be active on bus line (without using the DE signal) if we respect the condition that we have only one slave communicating at a time correct? 

    From what we read in datasheet, we don't need to use the DE signal unless we need to make some power managment and disable the slave, correct? Even because texas have other chips (THVD1451) for full duplex that don't have this DE signal to control it! 

    Our understanding is that both lines (transmission and receiving) should be always active. but in this case it only works for the line that master uses to transmit information for the slaves. 

    Waiting for your comments. Thanks!

    regards,

    Hélder Cunha 

  • +1 in reply to Helder Cunha
    TI__Guru 50835 points

    Hi Helder,

    The RS-485 drivers will always be driving an output signal (based on D state) while they are active (DE enabled). So as long as DE is enabled, the node is transmitting, weather there is data on the D pin or the input is held in a constant state. Once DE is disabled, the driver outputs will be in a high impedance state and allow other nodes to transmit to the same signal line. 

    RS-485 devices that lack DE inputs are meant to be used in point-to-point systems where only one driver exists on a signal line. Alternatively, the transceiver could be unpowered entirely to force the outputs into a high impedance state, but this is non-ideal as it would also disable the receiver. 

    Note that RS-485 signal states are both actively driven. This is in contrast to interfaces such as CAN or I2C which have an active driven state (dominant or open drain pull-down) and a passive state (recessive or passive pull-up resistor). These protocols allow for other devices to drive a signal while other devices are in a passive state. RS-485 needs to deactivate any non-transmitting drivers in order to avoid contention. 

    Let me know if this is clear and if you have any more questions. 

    Regards,
    Eric Schott