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ISO1500: used as Replacement for iso3082

Part Number: ISO1500
Other Parts Discussed in Thread: ISO3082

In an existing product the formerly used ISO3082 was replaced by the ISO 1500 as recommended.

The application and the used custom protocol uses the fact, that in case of two busmembers sending messages at the same time (collision is possible and allowed by the protocol) on the receiver-output (PIN R) the confused signal of the collision is visible for collision detection. It shows undefined messages due to undetermined voltage levels on the bus. The sended message is bitwise compared to the received one. when differences occur, a collision is presumed.

The ISO 1500 doesnt show this behavior. During transmission on the receiver output only the exact sended message can be seen unaffected of the collision going on on the bus. Not till transmitting stops an the rest of the colliding message may be received. 

Looking at the datasheet i encountered differences in timing, voltage threshholds an driver-impedances. I assume, that due to the improved improved HW-drivers and high bus impedances due to quite long bus cabling the receiver doesn't see the foreign signal but only his own, when transmitting.

Looking at the standard there's no collision detection specified for the RS485 bus.

Looking at the datasheet a collision detection function is not mentioned in both transceiver-versions.

My questions:

  1. Is the behavior of the ISO3082 an explicit function of the driver? (Or has it been used for detecting collisions although it's not intended for that explicidly?)
  2. Are there isolated RS485-transceivers with distinct collision detection (maybe actually with a distinct Pin) available? (If yes pls name them since i didn't find some)
  3. Is collision detection with the 485-bus usual in the applications, since it is not defined in the standard?

  • Hi Mirko,

    Thank you for reaching out to us!

    The RS-485 transceivers are only the physical layer, and they are not responsible for collision detection. The RS-485 protocol layer should handle the collision detection.

    1. No! ISO3082 does not have collision detection circuitry. Your observation on the ISO3082 could have resulted from the electrical characteristics such as propagation-delay, and the bus input threshold. 
    2. We currently do not have an RS-485 transceiver with collision detection circuitry.
    3. Collision happens when more than two transceivers are driving a bus at the same time. The frequency is higher when the number of transceivers is higher on the bus.