[Techtoolslist] Fwd: 7408 vs 74LS08

[John Robertson]

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 :-#)#