Fake chip verification? - Page 1
Forum section: electronics / general

author: bitman
author_rank: Unknown
timestamp: March 01, 2020, 11:09:53 pm
content: I think I've got my first "real" set of fake chips from Ebay. Trying to build a plugin debugger for a 7400 IC CPU I needed something to quickly show/decode binary to hex - and the old DM9368N seemed to be it albeit nobody really had it at a reasonable price. I did find it on Ebay - not cheap but not at $20 a piece either.https://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/DM9368.pdfjust for reference.The chips are labeled with HP1901Y in addition to the model number. I cannot find any public datasheets with HP as a manufacturer of these chips. I guess there's a chance they used the DM9368N for something different?However, now that I'm trying the chips I received on the prototype board things aren't working. First, I've tested 3 chips and they all seem to behave slightly differently. One doesn't draw any current regardless of what I do to it. Others will look like a short (or close to it) but simply pull pin 3 (LE) high or low. Strangely when you remove the connection it stays high current - the chip gets very hot and well at that point I disconnect it.Since I'm getting different results and I can rub off the printing with a bit of Isopropyl Alcohol, it also doesn't seem to do any kind of current limiting on the outputs I am concluding this is a fake.EDIT: Above paragraph rephrased to make sense.First, I'm pretty sure whom-ever sold me this will deny and won't care. I'm not looking to convince him/her but more to to learn if I can trash these ICs or if I should keep on trying. From what I can see the pins aren't doing what the datasheet says they should. A "Latch Enable" should definitely not short anything regardless of setting. In particular since only gnd and vcc are connected when it happens. Of course when a chip doesn't seem to have any functionality that also indicates "bad chip" to me. I found on youtube a hint to use the Isopropyl Alcohol and that at least looks like it could be a good indicator.Sad part is I bought other chips from the same vendor I haven't tested yet so now I think the whole batch may be DOA.Now, this is definitely old technology - so if there's a better way to drive a dual 7 segment display using BCD that actually displays HEX and not those weird characters that most 7400 chips use, I'll gladly consider changing my design - in particular if those chips are cheaper and more available. It would really be great if it was current regulating too.

author: wraper
author_rank: Unknown
timestamp: March 01, 2020, 11:29:22 pm
content: Quote from: bitman on March 01, 2020, 11:09:53 pmNo photo provided and I don't think this marking has anything to do with HP.

author: bitman
author_rank: Unknown
timestamp: March 01, 2020, 11:40:34 pm
content: Quote from: wraper on March 01, 2020, 11:29:22 pmWell, I can fix that. Doesn't look like an HP logo at this angle so you may be on the right track.

author: syau
author_rank: Frequent Contributor
timestamp: March 01, 2020, 11:48:29 pm
content: You may consider to try 74C925.

author: oPossum
author_rank: Super Contributor
timestamp: March 01, 2020, 11:49:43 pm
content: That is the National Semiconductor logo.For some alternatives see this thread:https://www.eevblog.com/forum/projects/16-bit-to-4-digit-7-segment-decoder/

author: oPossum
author_rank: Super Contributor
timestamp: March 01, 2020, 11:51:06 pm
content: Looks like laser marking on that chip. I don't think that was in use when those parts where in production. Also looks the the original markings have been sanded off.

author: bitman
author_rank: Unknown
timestamp: March 02, 2020, 12:43:12 am
content: Quote from: oPossum on March 01, 2020, 11:49:43 pmThat's just a digital - not hex output. The 7400 series chips typically use some odd looking characters for A-E and doesn't typically include F as it's used for "special" things like a full blanking of the output.

author: oPossum
author_rank: Super Contributor
timestamp: March 02, 2020, 01:00:24 am
content: That whole thread is about hexadecimal digits on 7 segment displays using a PLD or microcontroller.

author: oPossum
author_rank: Super Contributor
timestamp: March 02, 2020, 01:12:12 am
content: This is what the DM9368 should look like.

author: bitman
author_rank: Unknown
timestamp: March 02, 2020, 01:12:35 am
content: Quote from: oPossum on March 01, 2020, 11:49:43 pmThanks - it looked like a bad HP logo under a magnifying glassAnd it's one of the threads I've seen (a while ago) but it seems very inconclusive. Yes I can make a microcontroller do this - that seems to be the only option - and I loved the "guy" who posted a Z80 with the display signalling the overkill here. I've used the EPROM trick in the past too, but it too becomes much bigger than the display.I realize that old tech like TIL311 is no longer applicable for modern designs and as such they aren't really an option today ( at $30+ per piece on Jameco, nope not even going to consider going there). I'm trying to keep my design within the 1970ies computer design using mainly 7400 series ICs and I was really hoping I could get mightly close with the DM* which seemed to use TTL logic at the very least (and having been "decommissioned" a long time ago too). My EPROMS are "huge" compared to the 7400 chips and if think about using ATTINYs I need at least 3 more chips as a buffer and 2 shift registers. So that's more than a Nano. I'm sure I could just do a surface mount but that would really kill the feeling of "like in the old days" - at least for me.Then there's sites like this:http://www.paleotechnologist.net/?p=1386which uses a PIC - and while that's a standard chip it still becomes much larger than two side-by-side 7 segment will, which wouldn't fit on my build. But at least it's simple enough compared to 3-4 chips. Thanks for the reminder though. I'll comb through it again to see if there's anything I missed.

author: oPossum
author_rank: Super Contributor
timestamp: March 02, 2020, 01:20:41 am
content: Buy from someone other than chipsgate and you may get genuine DM9368 chips.

author: jmelson
author_rank: Super Contributor
timestamp: March 02, 2020, 02:09:30 am
content: Quote from: bitman on March 01, 2020, 11:40:34 pmAhh, very funny! Looks like a date code of 2019, week 1. But, of course, National Semiconductor did NOT EXIST in 2019. (Maybe TI still makes some parts with the National logo.)So, I'd be plenty suspicious of these parts.Jon

author: wraper
author_rank: Unknown
timestamp: March 02, 2020, 07:55:38 pm
content: That's some ugly sanded surface. One of the ugliest I've ever seen.