msignor
Member
Hi,
Quick question.. I am trying to wrap my head around testing TTL chips as faulty. I am working with the 74LS138 which looks pretty common as a component.
I am following a datasheet per: http://www.datasheetcatalog.com/datasheets_pdf/7/4/L/S/74LS138.shtml
From what I can tell, pins 4,5,6 should always be Low - Low - High regarding inputs
** Is this a correct assumption that all 74LS138 Chips should follow this? Did I read this thing completely wrong?
When I compare two chips side by side on my PCB Pins 456 are -- LOW - X - High and the other is LOW - HIGH - HIGH
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Can I easily be fooled here? Is it possible that one of these 74LS138 could be getting "broken" inputs from other defective chips when the 138 is really good?
-----
Any tutorials out there or maybe a video explaining this a little more?
Quick question.. I am trying to wrap my head around testing TTL chips as faulty. I am working with the 74LS138 which looks pretty common as a component.
I am following a datasheet per: http://www.datasheetcatalog.com/datasheets_pdf/7/4/L/S/74LS138.shtml
From what I can tell, pins 4,5,6 should always be Low - Low - High regarding inputs
** Is this a correct assumption that all 74LS138 Chips should follow this? Did I read this thing completely wrong?
When I compare two chips side by side on my PCB Pins 456 are -- LOW - X - High and the other is LOW - HIGH - HIGH
-----
Can I easily be fooled here? Is it possible that one of these 74LS138 could be getting "broken" inputs from other defective chips when the 138 is really good?
-----
Any tutorials out there or maybe a video explaining this a little more?

