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THVD1410: THVD1410 multiple unit interface issue

Yogesh Singhal
Prodigy 175 points
Part Number: THVD1410
Other Parts Discussed in Thread: THVD1452, THVD1424

Hi 

we are using the THVD1410 device in making a audio conference system

running the device at 3.3VDC.

Termination at the o/p (across  A B pins) is of 120 ohm

Conference system is like:

1 chairman unit ..and multiple (25 units ) as delegate units.

Output from Chairman unit (AB output)....is connected to Delegate unit (via RJ45 CONNECTOR, CAT 6 CABLE) 

When we are sending command  from Chairman unit to delegate unit ..it is going ..and we are receiving it back (delegate to chairman unit)

2 way communication is happening.

The problem:

Now if we are connecting ..second Delegate unit (2nd delegate unit is connected from first delegate unit.....via RJ45, CAT 6 cable)

than command from chairman unit t0 delegate unit 1  and 2 is going....but when receiving the command from 2nd delegate unit ..we are getting lot of garbage data .

one to one units are working OK...WHEN CONNECTING MANY IN PARALLEL....DATA IS NOT PROPER ..

Looking for suggestions.

rgds

yogesh singhal

  • 0
    Guru 268680 points

    An RS-485 bus should have exactly two termination resistors, at the two ends of the bus. More resistors will increase the load too much.

    On a half-duplex bus, it is not possible for multiple nodes to transmit at the same time. You need to use a protocol that prevents collisions.

  • 0 in reply to Clemens Ladisch
    Prodigy 175 points

    hI

    can u explain more..2 termination resistors ? we have placed 120 Ohm across A, B points..

    THVD 1410 data sheet, page 22, fig 31....we are having similar configuration.

    On a half-duplex bus, it is not possible for multiple nodes to transmit at the same time......does it means fig 31 is not correct?

    You need to use a protocol that prevents collisions.....can we get more details for this protocol

  • 0 in reply to Yogesh Singhal
    Guru 268680 points

    Figure 31 is correct. (Please note that there are only two RT.) "Half duplex" means that only one of the transmitters can be active at the same time.

    The THVD1410 is a transceiver that works on the electrical level. You can use whatever protocol you like. The firmware of your microcontrollers must be written so that only one of them pulls its DE high.

  • 0 in reply to Yogesh Singhal
    TI__Mastermind 46347 points

    Hi Yogesh,

    As Clemens has mentioned there should only  be 2 termination resistors on the bus (1 on each end) each equaling 120 Ohms - more than that and you could overload the bus and prevent proper communication. 

    In a half duplex system only 1 device can speak at a time. Preventing collisions really is dependent on what protocol you implement - since RS-485 is protocol independent there isn't a defined solution. Based on your overall system description - each node should have a defined address so only the delegate that is being communicated with responds and for commands that are supposed to go out to every delegate there needs to be firmware on each delegate that will help prioritize when to send a response signal - there will be a delay because each unit would have to respond individually.  So generally this is going to require a formatted data-packet with addresses, commands, and data (and for good measure some error checking may be useful) - how that is formatted doesn't matter to the transceiver as long as the timing is respected. I know that's very general - but protocol usage with RS-485 is very general due to its being protocol independent.

    You can probably increase throughput by using a full duplex device - this is a 4 wire system but it will allow the chairman node to send commands and receive responses at the same time. The closest to the THVD1410 is probably the THVD1452 - the biggest difference is duplex and max bus speed - so there could be some issues with the unterminated stubs if they are very long. But if the speed is necessary to keep at its current level than the THVD1424 could be interesting as it has a selectable data-rate (one of which is 500Kbps max) and it can be configured half or full duplex and the terminations are integrated (they can be switched on and off - off by default)  allowing for fewer passives on the bus.  

    Please let me know if you have any more questions and I will see what I can do!

    Best,

    Parker Dodson