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Original question:

AM6421: ARM64 MCU for industry motion control application

SN65LBC174: RS422 wire break with the SN65LBC174 transceiver

Umesh Swamy
Prodigy 90 points
Part Number: SN65LBC174
Other Parts Discussed in Thread: AM6442,

Tool/software:

AM6421: ARM64 MCU for industry motion control application.

Is it feasible to detect RS422 wire break with the SN65LBC174 transceiver circuit, recommended inorder to achieve interface with EQEPx_ block of AM6442  

  • 0
    TI__Guru 59680 points

    RS485/422 transceivers don't have a circuit to detect if there's been a wire break. I'm not aware of a way to detect if a wire break occurred.

    If one of the wires broke in a RS422/485 system the bus would probably treat the bus similar to a short between A/B where one of the nodes would probably lag the other due to a slight RC constant difference from the termination resistor. If the receiver device had an internal fail safe protection then it may be possible for it to atleast be able to avoid any glitches on the bus. (The Routput would likely remain high if it had fail safe protection).

    -Bobby

  • 0
    Prodigy 90 points

    Hi Bobby,

    In your answer below can I understand this phrase - "...short between A/B where one of the nodes would probably lag the other due to a slight RC constant difference from the termination resistor.." as mentioned in https://infosys.beckhoff.com/english.php?content=../content/1033/el500x/2211033099.html&id= 

    "If no data transmission takes place the SSI input of the terminal is on low level."

  • 0 in reply to Umesh Swamy
    TI__Guru 59680 points

    Umesh,

    The RS422 driver would be transmitting data but the RS422 (if it has an internal fail safe) would likely not see the data in the wire break case. Essentially you communication through RS422 would be blocked by the cable break (kind of like if the cable wasn't connected).

    This isn't guaranteed though. In this hypothetical situation, I think it could be possible for communication to still get through if the RC constant on the broken side of the cable were large (ViD seen by the receiver exceeds +/-200mV), the communication would probably not be stable and there could be bit errors. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 90 points

    Hi Bobby,

    I want to share my thoughts of implementation of SSI sensor with EQEPx block of AM6442x in a block diagram, how do I share the images with you

  • 0 in reply to Umesh Swamy
    Prodigy 90 points

     how should encircled signals be handled in HW?

  • +1 in reply to Umesh Swamy
    TI__Guru 59680 points

    Hi Umesh,

    Umesh Swamy said:
    I want to share my thoughts of implementation of SSI sensor with EQEPx block of AM6442x in a block diagram

    I'm not very familiar with this kind of specific application. From the block diagram I can see, it looks like you circled two nets named INC_A and INC_B. Generally from the cable connector you would go through a termination resistor (which for RS422 is usually 100 ohms but depends on the cable's characteristic impedance), which would then go into the RS422 differential inputs. 

    Umesh Swamy said:
    how should encircled signals be handled in HW?

    I don't think there is an easy way to detect a break in the wire on the circled lines. Maybe if there was a circuit to detect if the differential input were seeing an output difference between +/-200mV for several milliseconds, you could potentially detect a break? Though you would probably also get false positives when there is no cable connected. Any kind of circuit added to those lines could also interfere with the termination seen by the device and potentially load the bus as well. 

    -Bobby