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Amiga 500, 74LSxxx vs. 74Fxxx

giobbi

Veteran Member
Joined
Dec 23, 2012
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987
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São Paulo country, Brazil
Hi,

I've noticed my Amiga uses both 74LSxxx and 74Fxxx ICs. Do you think I could use 74LS ICs instead of 74F for replacement?
Will 74LS ICs instead of the faster 74F ones prevent the computer to boot?

--Giovi
 
... Do you think I could use 74LS ICs instead of 74F for replacement?
Will 74LS ICs instead of the faster 74F ones prevent the computer to boot?
...

It is usually best to replace with the same type. There are more differences between the LS and the F types than speed (propagation delay from inputs to output); fanout and fanin are two of them. Typically the F series parts will be faster and will have greater fanout than LS series. If you replace the data bus buffers, for instance, with a part that is too slow or doesn't have enough fanout then you can get flaky boot or complete boot failure. So, yes, if the engineers designing the system put an F series part in a particular location they probably did it for a reason, since F series parts were more expensive than LS series parts.

What you want to really watch out for are TI AS and ALS series parts, as there was a pretty serious issue with those chips made in 1984 or earlier. It was bad enough that Tandy recommended shotgun replacement of all AS and ALS series TTL in their Model 12-based line (Tandy Tech bulletins 12/16B:039, dated 3/11/85, and 6000:005, also dated 3/11/85). Tandy's information includes the statement about affected parts being 1984 date codes and earlier.
 
Another thing to consider is slope time. A very fast slope contains more HF-components, which in turn might cause ringing on signals if the design/PCB is not laid out to prevent such.
 
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