Program counter build

Building the program counter module for the 8-bit computer.

Complete parts list (everything in the program counter module):
- 1x 74LS161 (4-bit synchronous binary counter)
- 1x 74LS245 (Octal bus transceiver)
- 4x LEDs

[pavelhegler]

Lets take a moment and appreciate how there is no nerv wracking intro on this videos and you can just go from one to another in your flow state learning so much without getting interrupted.

[joverstreet]

If your program counter appears to be resetting or erratic, make sure you tie pin 1 of the 74LS161 to 5V! Ben’s seemed to work fine without doing so. Mine was obviously resetting as the pin was floating.

[TarousDT]

I really appreciate your editing to smooth the moments where you need to do something off camera and not just leaving silence in there.

[aminuteforchrist]

For those with problems, I had two and was able to solve them.

1. I had only 1 LED flashing. After tying pin 1 of the 74LS161 to HIGH, other LEDs began to flash.
2. But, the LEDs seemed to flash in an unpredictable manner. It wasn't counting 0-15. I then tied pin 9 on the 74LS161 to HIGH. This fixed it.

After those 2 changes, the program counter was working as Ben's did.

[jamessowin]

Who would down vote this? This whole series is amazing. I've been looking for a series like this for a very long time. I think this is the first video series to ever do this. Thank you for making this!

[polymetric]

"You can never have too many LEDs"
Truer words have never been spoken

[evanparshall]

Make sure to tie the clear pin to high on the counter. Had some issues with that affecting my counter.

[ninjajoe]

Abnormal counting behavior additional solution:

TL;DR: Ben is using his clock circuit by itself, but my clock circuit is still hooked to the RAM module. The capacitor on the NAND gate for clock edge detection on the RAM manual input causes this to screw up the voltage in the circuit. I still don't have a solution other than just pulling the capacitor for now.


Many comments talk about the abnormal behavior of the circuit and its inability to count properly. I had the same problem. Many solutions mention tying pin 1 of the 161 to 5v and one mentions a putting a 470 ohm resistor in line with the clock circuit at pin2. Both changed the behavior for me but didn't solve the problem. While troubleshooting I accidentally tied the first bit of the 161 to high with a loose wire and all of a sudden the other three bits were counting just fine (albeit only with three digits). This made me realize it was a power problem. After many mods with various resistor and breadboard layouts, and measuring voltages all over, I realized the issue wasn't at the 161, but at the clock input. The only other thing hooked up is the capacitor from the "RAM module testing and troubleshooting" video he inserts at 13:10 for edge detection. since Ben only has the clock-module hooked up in this video he doesn't have the same problem. For now I'm just removing the capacitor and simple NAND-ing my RAM. I havn't watched further yet, so hopefully he encounters and discusses this issue in later videos. I would love to hear solutions to this problem if others have it! Thanks.

[ryannichols]

Ben is single-handedly keeping the LED companies in business.

[bkzzzzz]

and the counting starts for the controller/sequencer circuit! cant wait to see how that magic works. thanks a lot Ben for your efforts and continuing the series.

[noxim_]

New video? oh man.. Ive known how computers work on the logical level for a while, but thanks to you I now know how to actually implement them with electrical components. Someday I hope to build my own pc with a subleq instruction set.

[jeffnay]

I have to voice a small complaint here. Keep in mind that I do love Ben's videos, but he has spent the last 6 videos
building the simplest board of all the boards on the SAP-1. I actually built my program counter with 2 74LS107's and a 74LS126
a long time ago.
We have been working on this project for almost a year now. and still not near completion yet. When we should be moving on to more advanced projects like programming and burning micro controllers and ROMs for our boards and or building boards with
full 8-bit processors like the 6502.
Ben has gotten us all hooked on learning, and we are all grateful for him sharing his knowledge with us. He has a great talent for
teaching electronics in a clear and precise manner.

Thank you Ben

[smiley]

Awesome new video! I have bought a lot of things online since I started watching this series.

[randomnickname]

Thank you Ben for your great videos! I'm going to start building copy of your computer in few month. Already ordered bunch of TTL/breadboards/jumpers from ebay. Can't wait to start :)

[mjsymes]

Thoroughly enjoying the series. Am building this slowly although not always with the same 74 series chips - if I can find a cheaper alternate on E-bay / Farnell I'm going for those and hoping I can work out the difference. Big step up from my only previous custom electronics which was a simple reaction timer game.

[Contango]

And it was! Now for the final, please with sugar on top, show us how the controller works!

[Contango]

😆😆😆😆 Awesome! Been waiting for this, and couldn't wait to comment before I watch it. No preassure, it will be great.

[cnstantin]

it's so nice how you explain. makes me want to start doing it my self <3 i have zero experience in this.

[fifaham]

Building computer circuits is a lot of fun.

[Fruzan-vv]

Ben eater in his last video: instead of these components that we used to built an computer, i am gonna use an 8 bit computer.