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