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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?

ocrdu
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Sairext Irkaris
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  • Have a look at 74S vs 74LS ICs UND EE201L, especially the bit under "Ability of FANOUT". – ocrdu Nov 08 '21 at 02:32
  • 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. – jay Nov 08 '21 at 02:58
  • 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? – Syed Nov 08 '21 at 06:00
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    @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. – Sairext Irkaris Nov 09 '21 at 10:11
  • @Syed I notice some input have 0.2uA but some 0.4uA of current flow back so I'm not really sure. I need like 5 to 15 inputs for my computer instruction logic unit because it's have 8 processing unit, those run with many and, or, nor and xor gate and I want to avoid pull down resistor as much as I can to reduce the size of the board and reduce power consumption. – Sairext Irkaris Nov 09 '21 at 10:14
  • Fan-in and Fan-out of the technology will be listed in the literature from the manufacturers. It should be sufficient information for your project at this point. Design the computer at this high-level. If you are exploring logic families, it is a separate project. For using chips from different families, you have to know much more. Don't confuse your task with details of "board" design. One thing at a time. Try to split on different boards according to the task at hand and use a ribbon (rainbow) cable connector, so that changes can be made when required. e.g. memory board, IO board etc. – Syed Nov 09 '21 at 10:16
  • I'll check it out, thanks @Syed – Sairext Irkaris Nov 09 '21 at 10:18
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    Thank you @ocrdu – Sairext Irkaris Nov 09 '21 at 10:19
  • @SairextIrkaris look at hc125, 126, 244, 245, etc, called "buffer" if you need large number of fanout. AMD had ALU-s and bit-slice building blocks,,,,, long long ago when we were building mainframes. – jay Nov 09 '21 at 15:34

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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.

Neil_UK
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  • 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... –  Nov 08 '21 at 13:14