This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74S140: input pulldown problem

Part Number: SN74S140

we need urgent help rsp. an answer to this question:

problem with a charge of SN74S140D: normally a pulldown resistor of 4k7 is used to tie open inputs to gnd. all ics of our last charge (50 pcs, marked '53C59KK G4') needed a 300 ohm resistor for the same effect (pulldown). So what's wrong with these ics ?

  • Hello,
    Can you please provide scope shots of the input signals with the old setup and the new, both with 4k7 and 300 ohm resistors?

    Also, a schematic showing all connections to that line would be very helpful.
  • No input signals at this ic, 3 inputs tied to Vcc, 1 inputs connected directly to a 68pin-connector. Vcc is 5V.

    For testing proposes a resistor of 4k7 ohms to gnd is connected to this single input, no other connection is made. During test Vcc is switched to this input and output is measured by multimeter. Only the change of the output under 50 ohms load is of interest during test. We build already about 50 devices with this ic (five ics in one devices), and up to now no problem occured.

    We measured a voltage of about 1.6V at the input without any connections made to the input, output is about 200 mV. With a resistor 4k7 to gnd connected to the input we measured to voltage of about 1.3V, output stays at about 200mV. With a resistor 300R to gnd instead of the (4k7-resistor) the measured voltage at the input goes down to 0.75V and the output goes high (about 3.7V).

  • My first guess would be damage due to ESD.  These are old devices and are very simple internally, as shown by the schematic:

    Have you tried replacing the devices that are showing problems with new ones?