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CD4536B: LED indicating data traffic

Part Number: CD4536B


We are an OEM firm and have been building an industrial controller for our customer for about 10 years.  The controller has a timing circuit that controls blinking of a LED when there is data traffic between the controller and peripherals.  Just like those on modems and routers.  Recently we notice that the LED blinks one time when power up.  It happens on chips procured recently.  It does not happen on chips procured before 2011.  I am wondering if chip design has been changed during this period.

Regards and Thanks,


  • Hi Edwin, and welcome to the forums.

    As far as I know, the CD4000 series of logic hasn't been changed in 40 years. We do test every device that leaves TI, and they continue to meet the datasheet standards. It's possible that the manufacturing plant may have had to buy a new piece of equipment (for example, a photo-lithography machine), and the process may have shifted slightly due to the change. Some system designs are more sensitive to minor changes than others.

    Can you provide a schematic of the circuit being used? We might be able to narrow down why the system is sensitive to this type of slight change in process.
  • Hi Emrys,

    Thank for response.  Please find schematic of the LED blinking circuit in below:


  • The reset doesn't affect the monostable multivibrator's latches, which inherently start in an unknown state. The older devices must have had a slight imbalance in the process to give a default "low" output to the latches, and now they have a slight imbalance to give a default "high". Unfortunately, this is just part of working with latched devices.

    I assume that the device operates normally once the system is stable and running?
  • Hi Emrys,

    Yes, the unexpected blinking just happen at the moment power up and it operates normally after that.

    Thank you for the analysis. I am expecting a new batch in this week. I will test them and see what happen. If necessary I will approach you again.

    Thank you again for your help.