Almost Works

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

• LOAD 0
• STORE 1
• ...
• READ 1
• LOAD M

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:

• the encapsulation is different
• the label is different
• TI does manufacture logic ICs in Malaysia
• there are similar chips with different manufacture dates on ebay, and
• they are from Hong Kong supplier rather than mainland China.

Hoping for the best.

AlanX