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SN74121: SN74121D input pin cannot be pulled low by a pull low resistor.

Tom Tsao
Prodigy 70 points
Part Number: SN74121
Other Parts Discussed in Thread: SN74LVC1G123

Hi expert,

I am attempting to use pin 5 (input B) of the SN74121D as a trigger source, and have connected a 2kohm resistor from pin 5 to ground to pull the pin low.

However, I am still measuring a voltage of 1.55V at pin 5.

 Since the SN74121D requires a voltage level below 0.8V to be considered a logic low, the voltage at pin 5 not be low enough to trigger the device.

 

Should I choose the lower pull low lower resistor?

If yes, what value is suitable? Or have any suggestions?

 

Thanks!

  • +1
    Guru 243120 points

    The IIL specification says that you must be able to sink up to 3.2 mA to pull the pin down to 0.4 V. This corresponds to at most 125 Ω. Just use 0 Ω.

    Please note that TTL devices are very much out of date and are sold only to support legacy designs. Can't you use a modern device like the SN74LVC1G123?