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SN75C188 Unintentional output

masahiko shimizu
Guru 11235 points
Other Parts Discussed in Thread: SN75C188, DS14C88

Hi,

My customer is using SN75C188 as below picture.
He has a issue of unintention output at startup power supply.
This output wave is red circle at below picture.

I think that SN75C188 should output 0 voltage level until starting -12 supply.
(2A=High, 2B=High -> 2Y=Low)

Could you tell me about mechanism at this situation, please?

Best regards,
Shimizu

  • 0
    TI__Guru**** 165666 points
    Shimizu-san,

    Bipolar RS232 drivers expect VCC- to be the most negative voltage on the device. Does VCC- go positive at all? Even +500mV would be too much.
    Even if VCC- stays at zero, the output may still go high. Some negative voltage may be needed to bias the internal circuits.
  • 0 in reply to Ron Michallick
    Guru 11235 points

    Hi Ron,

    Thank you for your support of last.

    I would like to advice in case of DS14C88M at this time.
    My customer has needed this information.
    Customer accepts your comment as reference, too.
    Could you advice our, please?

    Please see the below picture.
    This waveform is DS14C88M.
    The circuite is same as SN75C188M.
    It doesn't go to High logic level at not supplying -12V.
    Why doesn't DS14C88M output high level at this situation unlike SN75C188M?
    Could you tell me the difference of structure between DS14C88M and SN75C188M, please?

    Best regards,
    Shimizu

  • 0 in reply to masahiko shimizu
    TI__Guru**** 165666 points
    Shimizu,

    The internal schematics for DS14C88M and SN75C188 are different.
    However, they perform the same function when all supplies are powered.
    I do prefer DS14C88M response.
  • 0 in reply to Ron Michallick
    Guru 11235 points
    Hi Ron,

    Thanks for your answering.

    But unfortunately answer that you have to prepare is not to satisfy our customers.
    What our customer want to know is
    the reason that DS14C88M does not output abnormal signal during no -12V supply.

    The following is the background of this inquiry.
    Our customer found that SN75C188 is not suitable for their use.
    So they tried different part, DS14C88M and found that can be used.
    They need reason in order to obtain the approval in the design review

    We expect an appropriate response from you.

    Best regards,
    Shimizu
  • 0 in reply to masahiko shimizu
    TI__Guru**** 165666 points
    Shimizu,

    I contacted group responsible for DS14C88M to give an answer.
    They should respond soon.
  • 0 in reply to Ron Michallick
    TI__Guru* 93670 points
    Greetings,

    DS14C88 input voltage range is from V+ +0.3 to GND -0.3. On the other hand SN75C188 input voltage range is a function of the negative supply(from VCC- +2 to VCC+).

    DS14C88 minimum input high voltage is 2.0V(relative to GND). Therefore when both inputs are 5V the output will be at 0V(independent of the negative supply).

    Regards,,nasser