Cant figure out data bus register problem

[Simon_m74]

Hi,
I`m following Ben Eaters videos to make an 8bit CPU/Computer..
https://eater.net/bbcpu8-registers/
(look at video part 5)

I have not got far..:
The clock was first and thats working fine.
The registers are next - and thats the problem.

I can get the register to read data from the bus - tested by grounding some pins out here and there.
No matter what I try, I cannot get it to send data back to the bus.

I am VERY sure I have built it correctly, as I checked his schematic, board layout, and checked the datasheets for the IC`s.

The data is read fine into the 74LS173 -which has its output turned on, so that I can see the data on LED`s.
So that works fine.
But when I try to send the data back to the Bus, nothing happens.

I am not an expert, but I do understand logic like this - hence I`m stuck - as logically - is should work!

I have tried changing the 74LS245 (that releases the data to the bus, when `enable` goes low)
Also, I built two - both the same problem.

Its been rebuilt about 4 times now from scratch - still no luck

I guess its simple - but I cant see the problem!

Please help.

Thanks

 

[kellys_eye]

Please post a schematic of the area of concern

 

[Simon_m74]

I have just drawn this up quick -as I need to head out for the day.,

Problem I have is on the right - where the 74LS245 outputs to the bus.

The DIR is set high, so data should go from `left to right`
Enable on 74LS173 is high (no read)
Enable on 74LS245 is Low (output on)

Reading to the 173`s is fine
but I cant get the `245` to output back to the bus.

 

[hexreader]

Each LED should have a series resistor (I would choose 1k Ohm) to limit current draw. Without resistor, each LED will drag voltage down to around 1.6 Volts, assuming typical red LED. 1.6 Volts may not be high enough to be seen as a '1' by the 74LS245 inputs.

 

[Harald Kapp]

Each LED should have a series resistor (I would choose 1k Ohm) to limit current draw. Without resistor, each LED will drag voltage down to around 1.6 Volts, assuming typical red LED. 1.6 Volts may not be high enough to be seen as a '1' by the 74LS245 inputs.

+1 for that.

You're not alone, this is a rather common mistake. Read more on driving LEDs.

 

[Simon_m74]

Hi all
Resistors sorted it! Thanks!!

Reason I didnt put them on was the video tutorial didnt have them on, and it was stated that they were not needed as the 74LS173s have a current limit system - there dont require resistors on LEDs.

Thanks again guys..

Now onto building the ALU!