I'm using 74lsXX ICs for my 4-bit computer. I noticed that 74XX ICs have reverse current at the input; I know that the output of the IC allow the reverse input current flow back to it.

But I'm a bit confused because I don't know how to find the reverse current of the input and output.

I want to connect multiple inputs to an output and I want to know if the output can handle that much current. I've done some research and tried to find it in the datasheet but I can't find it. Does anyone know where to find it?

  • \$\begingroup\$ Have a look at 74S vs 74LS ICs UND EE201L, especially the bit under "Ability of FANOUT". \$\endgroup\$
    – ocrdu
    Nov 8, 2021 at 2:32
  • \$\begingroup\$ You are looking for something like "input current / input leakage current / bias current/ Iil & Iih/ etc" vs. "output drive current / Iol & Ioh / etc.". Meantime, input capacitance become an important parameter for fanout count when the operating speed has to go faster. \$\endgroup\$
    – jay
    Nov 8, 2021 at 2:58
  • \$\begingroup\$ How much current have you noticed? How many inputs is "multiple" inputs? Can you add chips one by one and figure out where it fails? \$\endgroup\$
    – Syed
    Nov 8, 2021 at 6:00
  • 1
    \$\begingroup\$ @jay My computer only run at 1kHz so I don't think that I need to care about input capacitance. I just want to know what is the input current that flow back to pull it down to reduce amount of pull down resistor to keep my board size small and reduce power consumption. \$\endgroup\$ Nov 9, 2021 at 10:11
  • 1
    \$\begingroup\$ Thank you @ocrdu \$\endgroup\$ Nov 9, 2021 at 10:19

1 Answer 1


74LSxx chips are designed as a compatible family.

The concept you are looking for is Fanout.

The output current, both the source and the sink current, but mainly the sink current which is much larger, is designed to be large enough to drive the input current of 'several' inputs.

The input and output currents are thus standard values, and are sometimes not shown explicitly on the data sheets of individual parts. You have to go to the Family Characteristics data sheet for that.

74LSxx input is 0.4 mA, and output is 4 mA to 8 mA, depending on the noise margin you want to guarantee. This allows 10 inputs to be driven from one output, and if conditions are kind, 20 inputs. Some devices in that family are described as having 'buffer outputs', and the higher output current drive is detailed in the individual data sheets.

  • \$\begingroup\$ But note that mixing families, e.g. 74LS, 74H, 74S and plain 74 series, fanout numbers vary : 74LS can't drive very many 74S inputs, for example. Get the brange book (TTL Databook) if you need to know... \$\endgroup\$
    – user16324
    Nov 8, 2021 at 13:14

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