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Almost Works and Fake 74LS189

A project log for CHUMP

CHUMP stands for Cheap Homebrew Understandable Minimal Processor. It is the invention of Dave Feinberg.

agpcooperagp.cooper 09/14/2021 at 00:173 Comments

Almost Works

The Monitor program almost works, but I have a memory read fault:

Should read back 0 (reads back B).

Likely my optimism for write timing was unfounded?

Strange I checked the schematic and I don't see any errors.

I checked the voltages on the 74LS189 pins (while stepping through the program) and they are a little strange. While MD can be tri-state, it was unexpected on D2 (around 2 volts).

I checked the power supply current which was high (267 mA).

I removed the 74LS189 and tried another, and then another chip without success.

I removed the 74LS189 and check the socket voltages (while stepping through the program) and they were as expected. I really cannot fault the board.

I checked the power supply current without the 74LS189 and it was quite reasonable (35 mA). How can the 74LS189 draw 232 mA and not destroy itself?

I checked the pin outs for the 74LS189 from another source, it matches. I am puzzled!

I have designed a Nano powered 74LS189 checker on vero-board, but I have no spare Nanos. I have ordered 3 but it will take two weeks to arrive.

Note: the 100R resistor is to limit power supply current to the 74LS189.

Note: the other resistors (all 1k), limit current to the pins of the 74LS189.

Fake 74ls189 chips

Last night while I slept I realised the only conclusion was that the 74LS189 must be a fake chip. This morning I found this post:

https://hackaday.com/2017/08/22/fake-ram-identifying-a-counterfeit-chip/

and

https://www.righto.com/2017/08/inside-fake-ram-chip-i-found-something.html

Here is what the fake chip looks like (mine are identical):

And the 10 chips I have are identical to the one shown in the posts.

Rather than throw away the PCBs, I have order some more 74LS189. This time:

Hoping for the best.

AlanX

Discussions

agp.cooper wrote 09/14/2021 at 11:37 point

Hi Ken

I can repurpose the 74LS189 Nano tester I just built.

AlanX

  Are you sure? yes | no

agp.cooper wrote 09/14/2021 at 10:41 point

Hi Ken,

Yes, that would work. I did that when testing the TTA8 many years ago.

One thing to watch is that the Nano can source 40 mA, enough to damage other chips, if by accident two outputs try to drive each other. That is why I have 1k resistors on each Nano pin. It took months for me to work this out.

I have ordered some more 74LS189s so hopefully they are real this time.

In the interim I have design a version for the common data Input/Output of modern SRAM.

Still, I am pretty happy with the result. It is nice to program even for a 4 bit CPU.

Regards AlanX

  Are you sure? yes | no

Ken Yap wrote 09/14/2021 at 01:43 point

Solder two rows of pin headers on your Nano, and corresponding pin sockets on the board. Then you can reuse the Nano in many places. In addition you can also use it with breadboard.

  Are you sure? yes | no