Struggling with use of 74ls189an in 8 Bit Computer Project - Page 1
Forum section: electronics / beginners

author: Geor322
author_rank: Newbie
timestamp: August 12, 2017, 07:20:52 pm
content: Edit: I have also one much simpler question: Is there any reason at all that the 74ls189an should ouput on address input 0? As it most definitely is for me and I'm suspecting that may have received some dodgy IC's. Thanks.Hi,I am currently following Ben Eater's youtube tutorial to building an 8 bit breadboard computer and have made it up to building the RAM module however, I have now become stuck with a problem I cannot figure out for the life of me. Link to the relevant tutorial video:https://youtu.be/KNve2LCcSRc?list=PLowKtXNTBypGqImE405J2565dvjafglHU.In the pictures below you can see that the most significant yellow LED(furthest left) is lighting when it shouldn't be. This LED is connected to a0 on the 74ls189an (These are the chips circled in red in the first picture) (datasheet link:http://www.datasheets360.com/pdf/3120074256527141813). This LED lights only when either one of the two 74ls189's are in the breadboard as can be seen below. The address lines of the 74ls189's are connected together (in order to give the computer 8 bit rather than 4 bit functionality); however, I have tested this and no matter whether both of the IC's are in the breadboard or either one, there is still an output. Only when neither of the 74ls189's are in the breadboard does the LED turn off. It seems like the address input 0 of both IC's are acting as outputs !?!? I also tried only inserting the vcc and a0 pins only of the IC's and this causes the LED to light also. This is very strange as obviously the IC shouldn't be outputting anything when there is no ground. It almost seems like there is a direct link between the vcc pin and a0 pin??Maybe I'm missing something obvious as I am a complete electronics novice but any help or suggestions would be greatly appreciated.Thanks,George

author: rstofer
author_rank: Super Contributor
timestamp: August 12, 2017, 09:45:59 pm
content: Schematic?

author: Ian.M
author_rank: Super Contributor
timestamp: August 12, 2017, 11:01:31 pm
content: I would be extremely suspicious of the authenticity of the 74LS189 chips. While there are still a few stockists of the original 74189, the LS variant doesn't seem to be in stock at any reputable distributor and is widely counterfeited (on EBAY and from Chinese sources).e.ghttps://www.eevblog.com/forum/projects/d-flip-flops-(ttl)-driving-me-crazy-(going-haywire-instead-of-1-or-0)/

author: Geor322
author_rank: Newbie
timestamp: August 13, 2017, 02:51:27 pm
content: Sorry as this is not my project just a tutorial I am following I have no schematic

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 13, 2017, 03:35:01 pm
content: Is anything driving the A0 line or is it just floating? It is not uncommon for a TTL input to show high when it is floating, see for example this NAND input, 4K + a BE diode to VCC:

author: Geor322
author_rank: Newbie
timestamp: August 13, 2017, 04:31:46 pm
content: Yes the A0 line is definitely connected to ground however it is still ouputting high.

author: Ian.M
author_rank: Super Contributor
timestamp: August 13, 2017, 05:18:08 pm
content: Connect the 74S189 to +5V Vcc and Gnd on its own with all other pins open circuit. It should draw less than 60mA*. If its more the chip is most probably bad or fake. Measure the current it takes to pull each input down to Gnd. All should be under 0.4mA. Again if any take more than that the chip is bad or fake.See page 2 of the datasheet for details of the individual pins input and output circuits.* The max supply current is specifed with /S and R//W grounded and all other inputs pulled high. As the chip is inactive with /S high, and TTL inputs float high, it should be less with all inputs floating.Quote from: Geor322 on August 13, 2017, 02:51:27 pmThat's the trouble with video tutorials for anything except manual skills (e.g. drag soldering). If the author can't be bothered to create supplementary material like schematics etc. one should question the quality and usefulness of the tutorial. You'd probably benefit considerably from drawing up your own schematic . . . .

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 13, 2017, 06:04:36 pm
content: Quote from: Geor322 on August 13, 2017, 04:31:46 pmThat is impossible...Edit: And I don't see a ground connection on your breadboard. I still think that your A0 is open and because of that it is high.

author: Geor322
author_rank: Newbie
timestamp: August 13, 2017, 06:08:31 pm
content: I mean that in the breadboard the a0 line is connected to ground but if I take an output directly from the pin on the IC itself, rather than from a hole on the breadboard, it is high. Therefore even though the IC is getting a low input on a0 it doesn't seem to be affecting the output of the a0 pin. Does that make more sense?

author: Geor322
author_rank: Newbie
timestamp: August 13, 2017, 06:11:10 pm
content: The connection to ground is through the brown wire and onto the breadboard above where it is connected to a 74ls157 and through there it is connected to ground. In other words, the brown wire connects a0 to ground.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 13, 2017, 06:11:55 pm
content: Quote from: Geor322 on August 13, 2017, 06:08:31 pmNo, sorry. So either your A0 from the IC is not connected to the breadboard or the breadboard connection is not connected to ground. Can you point to the ground connection on your first picture?Edit: I see the brown wire going to an IC that looks to be there to drive the LED but I still don't see the ground connection.

author: Geor322
author_rank: Newbie
timestamp: August 13, 2017, 06:15:50 pm
content: I will attempt to draw up my own schematic and also try a few more suggestions to rectify this problem. If I manage to draw up a schematic I will post it here if my problem still persists then.Thank you for all your help

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 13, 2017, 06:18:03 pm
content: I'm also interested to see where the led is connected, because as far as I can tell from the picture A0 goes to pin 1 of the ls157 which is the select pin.