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Original question:

SN74LS123: Glitch upon power-up

SN74LS123: SN74LS123 design

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Intellectual 2375 points
Part Number: SN74LS123
Other Parts Discussed in Thread: SN74AHCT123A

HI Team

I am using a chip,SN74LS123.But I have a problem now.The following is my schematic.

In this schematic,U16B this part is normal,but U16A something is wrong.

U29 is a optocoupler,it has no input.Pull up the voltage of the No.2 Pin and No.3 Pin,make them 5V.Now,No.1 Pin is stable 1.1V,no falling edge.But the No.13 Pin is high all the time.After this,I measured the voltage of No.7 Pin and No.15 Pin,they are not the same.No.7 Pin is 1.1V,but No.15 Pin is 0.6V.

In response to this phenomenon, I have some questions to ask.

First,The output of No.13 Pin is high, is it caused by the voltage of No.15 Pin?

Second,what could cause the voltage of No.15 Pin to be incorrect?

Best Regards,
William

  • 0
    Guru 270740 points

    1.1 V at pin 1 is not a valid logic voltage.

    LS family devices require quite a lot of input current (see IIL in the electrical characteristics). I strongly suggest to use a CMOS device like SN74AHCT123A.

    If you must use the LS device, make R167 and R169 small enough so that they can supply enough current with a total voltage drop of less than 0.8 V.