Just the title, more specifically, I’m trying to figure out the issue with pin 19 (CO) on the program counter/register when I want the program counter to put its value onto the bus.
Despite the high or low status of pin 19, nothing is outputting on the bus.
What should the other relevant control pins be set to as well for the PC to successfully output onto the bus?
I’m confused about the 74LS139 wiring in Ben Eater’s 8-bit computer output module.
In the schematic, it looks like pins 4–7 of the 74LS139 are connected to the common cathodes of the four 7-segment displays. But in the actual video, Ben seems to connect them to pins 9–12 instead.
Am I misunderstanding the schematic, or is there a difference between the chip/pin numbering being shown? I’m using a 74LS139 and want to make sure I wire it correctly.
Thanks!
I’m building kit 4 on the 8 bit CPU, and I’ve noticed that in the videos, he is using a 74ls76, whereas in the kit that was shipped to me I got a 74ls107 (another jk dual flip-flop), but on the schematics and in the videos, they both use the ‘76 which is sort of annoying so I was just wondering if there is any updated schematic or video with the newer chip, anything helps, thanks!
Hey everyone,
First off, sorry for the messy wiring.
Secondly, I'm currently building the RAM module from Ben Eater's 8-bit computer series and I've run into a strange issue.
My 16-pin DIP switch doesn't fit properly (I ordered a new one) in the breadboard, so for now I'm manually simulating the switch positions using jumper wires. The current switch pattern is:
ON ON OFF OFF ON ON OFF OFF
OFF OFF OFF OFF OFF OFF OFF OFF
(At least that's the intended state.)
The problem is that when I set the MAR to 0000 and try to write data to RAM using the red data LEDs, the LEDs end up showing:
00110111
instead of the value I'm expecting.
I've checked the obvious things (power rails, chip orientation, loose wires) but I haven't found anything yet. Since I'm not using the actual DIP switch right now, I'm wondering if my temporary wiring method could be causing floating inputs or some other issue.
Questions:
00110111 point to any common wiring mistakes on the RAM module?Any troubleshooting ideas would be greatly appreciated. Thanks!
I’m building and troubleshooting an ALU, and I’ve run into a weird issue.
When the B register is all 0s and the A register is set to 00000001, the SUM register is showing 01000001. This suggests the second LED is stuck high.
I traced the wiring and found that A_6 was incorrectly connected to pin 3 (which is tied to the second LED). I moved it to pin 14 of U20 (where it’s supposed to go), but then the first LED lights up instead of the second.
At this point I’m confused because:
• With B = 00000000 and A = 00000001, the SUM should be 00000001, but I get 01000001.
• When A_6 is floating, I get 10000001.
• When A_6 is connected to pin 14 (correct location), I get 01000001.
Even more strange:
• If I manually set only the second LED on the A register, the output shifts and the first LED lights instead.
• So instead of getting 01000000, I get 10000000.
It seems like the bit positions are shifting or getting swapped somewhere in the SUM output.
Final summary of behavior:
• Rightmost LED ON on A register → SUM looks mostly correct but with an extra second bit high.
• Second LED ON on A register → output shifts left, and the first LED becomes high instead.
What should I check next to debug this?
Just the title. Clock module took me about a week and I just referenced the schematic for 90% of it. The registers and ALU seem like tougher modules so just asking for any feedback, thanks 🙏.