8-bit Computer Register

[tggzzz]

Okay, so is there any way to troubleshoot the registers using multi-meters I am not able to get the correct values from the bus to the register and vice versa.

Make sure you understand V_OH V_OL V_IH V_IL listed in the data sheet. You can measure the output voltage with a multimeter.

But a multimeter won't show any transients due to poor construction technique.

[Balakumaran Selvaraji]

So does the 74ls245 chip is pulled high or low? and also I referred to the 74ls245 datasheet and it tells me that the V_IH Min = 2V and V_IL Max = 0.8... and can anybody tell me what is the exact reason behind this problem... I am so confused...
Also, the datasheet is attached below.

[paulca]

I'm confused.  In the original video the yellow LEDs showing the input to the bus are all 0, OFF, nada.  When you set the load enable and pulse the clock the register shows all 0, off, nada.

What's the problem?  More importantly, why do you think the first two LEDs should change and the others remain the same?  Because you pulled them to ground?  What on earth would that do when there is no power on them anyway?  The only thing I can think is that pulled to ground gives a clear logical low, rather than the pretty much disconnected state the others seem to be in.

If those yellow LEDs are showing the state of the bus, then the register is correctly capturing the bus.

Remove all the yellow LEDs.  Wire the odd wires to Vcc and the even wires to GND.  Set the load enable and pulse the clock.  What happens?

EDIT:
Further.  In the second video you posted about the clock problem.  The bus is all 0.  When you enable the load and pulse the clock the register then shows all 0.  That's what's meant to happen!

I currently have the flu and maybe I'm away with the fairies but why would you expect something else in the register?  Where would it get that data from?

Maybe you are trying to load the bus from the register, in which case the direct link to ground without resistor for the first two bits is surely just going to short the bus transceiver output to ground when it sets those outputs high, dropping Vcc into the floor and resetting the logic chips.

[Balakumaran Selvaraji]

Okay, so my registers(A,B and Instruction) and ALU seems to be working perfectly fine. But I faced this very problem and would like to know the possible solutions to it :

Scenario:
   So I tried to connect all the led's to ground via some jumper wires and the only power which was to these led's was a common ground from some register. Now, I haven't connected the power as I burnt up one of my led's  as passed in too much voltage through it. So, now when I try to connect some of these jumper wires to ground and as well as have my load active in one of the registers, I noticed that the value was not taken by the registers and it defaulted to zero. So, I watched his video(Ben Eater) on "TroubleShooting the ALU" and he seems to get it working perfectly alright. So, I am wondering what might be wrong... 
   

[danadak]

Your leds have a series current limiting R in their connection, right ?


Regards, Dana.

[Balakumaran Selvaraji]

Your leds have a series current limiting R in their connection, right ?


Regards, Dana.

The LED's in my register have them but not the LED's on the bus.

[ebastler]

Scenario:
   So I tried to connect all the led's to ground via some jumper wires and the only power which was to these led's was a common ground from some register. Now, I haven't connected the power as I burnt up one of my led's  as passed in too much voltage through it. So, now when I try to connect some of these jumper wires to ground and as well as have my load active in one of the registers, I noticed that the value was not taken by the registers and it defaulted to zero. 

Sorry, I find your description hard to follow. Can you please try again, and be more specific? What is connected to one side of the LED, what is connected to the other?

Also, *all* your LEDs should be connected via series resistors! Without resistors, you can damage your chips or cause the circuit to work unreliably due to nasty current peaks. You should read up on how to calculate the right resistor value based on the drive voltage, the LED's forward voltage, and the desired LED current. But for the time being, simply use 470 Ohm or so -- much better than connecting them without a resistor!

[Balakumaran Selvaraji]

I too had that doubt in my mind but when I watched the youtuber's model then I was like, it was working for him but why not me?
I will try to add resistor's and see if the problem resolves.

[cracked_machine]

Seriously just buy this https://www.amazon.co.uk/Hobby-Components-24MHz-Logic-Analyser/dp/B00DAYAREW/ref=sr_1_1?ie=UTF8&qid=1514187041&sr=8-1&keywords=logic+analyser

It's less than £15 and you would have resolved the problem by now already...  Is there a reason you are resisting this?

[Nusa]

I too had that doubt in my mind but when I watched the youtuber's model then I was like, it was working for him but why not me?
I will try to add resistor's and see if the problem resolves.

5V LED's can be had with integrated resistors that are not visually obvious, which might explain the confusion: https://www.mouser.com/new/kingbright/kingbright-resistor-LEDs/
It's more expensive than adding the resistor yourself, but having them can make breadboarding easier.

[Balakumaran Selvaraji]

Okay, So he was not using the LED's with built in resistors as you said. He once mentioned that some of the TTL chips have current limiting resistors on the output so that no resistors are needed

[Old Printer]

 I have watched Ben Eater's logic gate videos to death, but I am still not knlowadgble enough to troubleshoot your problem. I will say that the devil is in the details, and judging by the system he has built from wires and breadboards he has discovered a lot of Devils. Have you watched his video on choosing good breadboards from bad ones?

[Balakumaran Selvaraji]

The problem is resolved few days back. I have no idea what it was....

[Old Printer]

The problem is resolved few days back. I have no idea what it was....

That's too bad, you had a lot of input on that situation, it would have been nice to come away from it with a conclusion, though by reading between the lines it seems possible there was nothing actually wrong, just a different expectation. Rule #1 in troubleshooting - never change more than one thing at a time if you really want to come to a solid conclusion.

[Balakumaran Selvaraji]

Yup, I learnt that when I was troubleshooting the ALU module  

[Nusa]

Okay, So he was not using the LED's with built in resistors as you said. He once mentioned that some of the TTL chips have current limiting resistors on the output so that no resistors are needed

That may be adequate if all you're doing is displaying the output on the LED. However, if you're also trying to use that output as an input to something else, remember that the diode (that's what an LED is!) tied to ground on that input may pull the voltage down enough to change it's digital value.