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I'm studying Intel 8086 processor.

I can't understand why the "Fan-out" column is like that in the table below

recommended fan-out from 8086

I understand that only one "74 family" chips or five "74LS family" gates can be connected to 8086 output pin.

8086 can sink up to 2.0mA. So it can handle up to five(=2.0mA/0.4mA) 74LS family chips or one(=2.0mA/1.6mA) 74 family chip.

But why does 8086 handle up to ten 74ALS family chips? I thought that it can sink up to 20(=2.0mA/0.1mA) gates and source up to 20(=400uA/20uA) gates. So it should drive up to 20 74ALS chips.

And why 10 74AS family chips? It should be four because 0.5mA current would flow from one 74AS chip to 8086 output pin and 8086 can sink up to 2.0mA.

It is very tricky to me, so please help me understand it.

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  • \$\begingroup\$ @hskim don't try to calculate at your own reason. I don't understand why you calculate 20 source up if the table describes only 10. LS, HC, AS is the technology to handle different speeds and low rates. \$\endgroup\$ – Fernando Baltazar Jun 6 '18 at 5:51
  • \$\begingroup\$ @jsotola You're right. I editted it \$\endgroup\$ – hskim Jun 6 '18 at 6:03
  • \$\begingroup\$ @FernandoBaltazar Because there were no other explanations for limitation of fan-out in Textbook except the sink and source current. If there are other technological reasons, my question is solved! Thank you \$\endgroup\$ – hskim Jun 6 '18 at 6:03
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I actually wire wrapped an 8088 (which is similar) shown below and I did have to pay attention to fanout (my max fanout is 5, which you may be able to tell on some of the processor pins)

Two words, noise margin, the current supports 20 chips: true but the noise margin does not. I really wish they would say something about capacitance at this point but they didn't, which also plays into fanout (if you have to much capacitance it increase your rise and fall time although this is not as much of a problem for standard logic families because they reduce capacitance)

If you read the rest of the chapter:

enter image description here
Source: The Intel Microprocessors by Barry B Brey

enter image description here

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  • 3
    \$\begingroup\$ If you ever do this, zif socket (the one with the lever) for a ROM is indispensable \$\endgroup\$ – Voltage Spike Jun 6 '18 at 6:13

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