Is RS-422 TTL Compatible?

Discussion in 'Embedded Systems and Microcontrollers' started by eb123, Jun 13, 2018.

  1. eb123

    Thread Starter New Member

    Jul 3, 2017
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    Does the RS-422 standard expect TTL logic levels? Is a TTL encoder with differential signalling (what would be the closest protocol to this) compatible with the standard?
     
  2. nsaspook

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  3. eb123

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    Not sure how much of the document I understand, but it suggests that RS-422 doesn't have specified input logic levels.

    http://www.ti.com/lit/ds/symlink/am26ls32ac.pdf

    This RS-422 Receiver seems to have TTL input levels for the differential signals. Can you confirm that I am reading this correctly?
     
  4. MrChips

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    If you mean is RS-422 voltage levels the same as TTL voltage levels, i.e. 0 to 5V, then the answer is yes.
     
  5. eb123

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    I have come across RS-422 receivers which don't accept TTL logic level inputs.
     
  6. MrChips

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    If that were the case it would be indicated in the data sheet.
     
  7. ebp

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    RS-422 levels, when they arrive at a receiver, can be considerably less than TTL levels and there is allowance for the voltages to be less than zero volts or more than 5 volts. The receiver datasheet covers this. TTL levels are always within the allowable range.

    RS-422 signals are differential, and the receiver responds to the difference while, within reason, rejecting the common-mode voltage. For example if the levels were +7 V and +6 V for the [+] and [-] inputs, respectively, the output would be HIGH. -3V and -2V would also cause a HIGH output, as would +5V and 0V or +4V and +1V. In the first case, the signal could be considered to be ±0.5 V on 6.5 volts of common mode voltage. In the second example the signal would be ±0.5 V on -2.5 volts of common mode.

    If you want to use TTL level signals as inputs effectively, the logic levels of the two inputs should always be opposite, as presumably they are from your encoder.
    If you have only a single TTL level signal, then the other receiver input should be resistively or otherwise biased to (typically, but not necessarily ideally) 2.5 V with respect to ground.There is generally little merit to using a 422 receiver in this way - an ordinary schmitt trigger device would do just as well or better.

    Note that there MUST be a common or "ground" between the signal source and the receiver circuit. It doesn't necessarily need to be dedicated only to signal use and could be a power supply common (e.g. if your encoder is powered from the same supply as the receiver circuit, the supply common ("ground") will do the job.

    Again, there is a ton of information on RS-422 on the web.
     
  8. eb123

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    Jul 3, 2017
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    "TTL levels are always within the allowable range."

    Do you mean TTL levels will always be accepted within RS-422 receivers? What is the allowable range?

    For example, http://www.ti.com/lit/ds/symlink/ua9637a.pdf, -0.2 to 0.2v for input.
     
  9. John P

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    If the question is whether TTL levels (one high and one low, with minimal common mode voltage) will be decoded correctly by an RS-422 receiver, then the answer is yes. The RS-422 specification covers those voltage levels, and a considerably wider range too.

    "Common mode voltage" means that the sending and receiving devices may not agree on what the level of the ground voltage is. RS-422 allows several volts of common mode (I think it's +/- 7) but for a system that operates over short range indoors, it's never likely to be an issue. If you do get a mismatch of that kind, it's time to look for a serious wiring problem.
     
  10. eb123

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    Do you know what the maximum Low voltage, and minimum High voltage is for RS-422 inputs?
     
  11. John P

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    WIth RS-422, it's not a question of maximum or minimum voltage, but which line is higher than the other one. The difference must be at least 0.2V or the standard isn't being met, which means the output will be unpredictable. The good news is that TTL levels should easily meet that requirement.
     
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  12. AnalogKid

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    According to the datasheet you linked, +/-7 V.
    That is the sensitivity of the input, the minimum required voltage difference between the + and - inputs that will cause the output to change state, *not* the input voltage range.

    Again, a RS422 receives is a *differential* analog comparator with a digital-compatible output, not a digital logic circuit. If you bias one input to approx. +1.5 V, then the other input will be TTL compatible. If you bias it to +6 V, the other input will not be TTL compatible.

    ak
     
    Last edited: Jun 14, 2018
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