Re: Fwd: 7408 vs 74LS08

On 05/03/2013 7:32 PM, Jose Luiz Martins wrote:
> Hi !
>
> Have you guys experience problems using 74LSXX instead of 74XX in address
> bus?
>
> I'm trying to troubleshoot one Taito Moon Cresta board for months, and
> could not find the problem.
>
> I solve the garbage on the screen changing one 7408 at Data Line A5,A6 and
> A7 but now I have the screen filled with Zeroes and Ones.
>
> I was noticed by a friend that the 2114 is very speed sensitive, but I cant
> believe the 74LS08 is the guilty.
>
> I'm asking you guys because I don't have any good 7408 IC, I'll have to
> order it from USA and wait about 30 days to arrive.
>
>
> Thanks
As I've found in repairing games the problem with using 74LSXX in place
of 74XX devices is the drive current of the 74LSXX series is 1/10th
(more or less) of the original 74XX. So if you find the LS device is
only driving one standard TTL gate that is usually OK, however if
driving 2 or 3 it can be marginal, and 4+ is normally pointless. So try
a 74HCT08 if available - High Speed CMOS TTL compatable outputs, I
believe this is closer to the 74XX family.

If you have some 74XX data books they do go into drive capabilities of
the various families (often called fan-out too). I'm also certain there
are searchable info on this topic.

John :-#)#
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Received on Sat May 4 00:35:39 2013