Digging deeper into the Apollo Saturn V LVDC


[Fran] went all-out with her reverse engineering of the Apollo Saturn V LVDC board. Regular readers will remember that she was showing of the relic early this year when she took the board to her Dentist’s office to X-ray the circuit design. Since then she’s been hard at work trying to figure out how the thing functions using that look inside the board and components. When we say ‘hard at work’ we really mean it. Not only did she explore many different theories that resulted in dead ends, she also built her own version of the circuits to make sure they performed as she theorized. Above you can see her version of the NAND/AND gates used on the hardware.

We find her explanation of how the logic devices were originally fabricated to be very interesting. They started with a ceramic substrate and used additive processes to form the traces and add the gates. We’ve embedded her video explanation after the jump.

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How 6502 Overflow Works

6502 Overflow Circuit

The 6502 was a ubiquitous microprocessor back in the 80s, powering devices such as the Apple II and the Nintendo Entertainment System. [Ken] took an in depth look at a small part of the processor: the overflow circuit.

In order to do signed calculations, a microprocessor’s arithmetic logic unit (ALU) needs to be able to calculate when an overflow occurs. This happens when two numbers are added that cannot fit in a single byte. The result of a calculation will be incorrect, so the processor must inform the program that an overflow has occurred. This is done by setting an overflow flag.

[Ken] uses this example to first explain how the overflow circuit works in logic. He then looks at the gates and a transistor implementation of the logic. Finally, he walks us through the circuit on the actual 6502 die, showing how the circuit is constructed in silicon.

This is a great example to learn a bit more about how ALUs work and how integrated circuits get built.

Fabricating hardware from the original arcade Pong schematics


This heavily populated PCB is a recreation of the original arcade version of Pong. That is an important distinction because the home version of Pong used a specialized chip to do much of the work. This is basically all stock logic, which explains the high component count. We wonder how many quarters it took just to pay for all 66 chips at the time?

[Pong74ls] was the person who took on this project. There is an original schematic available, but it’s incredibly crowded and rather difficult to figure out. Fortunately [Dan Boris] has already done a lot of the heavy work. He took the one-page nightmare and turned it into a sixteen page plan for building the original board (look for the schematic link under technical details).

Before the board could be laid out some redesign work was necessary. It sounds like some of the original chips are out of production and suitable replacements needed to be found. The board was then laid out in Eagle before sending the design off to a fab house. There was just one error which didn’t allow the ball to bounce when hitting a paddle while travelling downward. A couple of jumper wires fixed that right up!

[via Reddit]

[Original Reddit Post]

Abstracting transistors into high-level design

Although it’s not the best way of understanding computers, most people tend to imagine electronic devices as black boxes filled with magic and blue smoke. Even microcontrollers, the most fundamental means of computation, are treated like little black plastic centipedes with metal legs. In a series of blog posts, [Andrew Gibiansky] is tearing down the walls of obfuscation and illuminating the world of transistors, gates, and FPGAs.

The first blog post goes over the idea of electronic circuits as a waterfall; a positive voltage is a reservoir on a mountain top and ground is sea level. This idea is extended to the lowly transistor acting as an electronic switch, able to turn a circuit on and off.

Continuing on to logic gates, [Andrew] covers the NOT, AND, and OR gates before moving on the flip-flops and SRAM. These can, of course, be modeled in Verilog and VHDL – programming languages that abstract the world of transistors and gates into a much more human-readable form.

[Andrew] is far from done with his series of blog posts, but judging from his work so far it seems to be a great resource for untangling the obtuse concepts of gates and memory into the coherent design of a computer.

7400 frequency counter

This frequency counter is [Miguel Pedroso's] entry in the 7400 Logic contest. After looking at the design we think this is a perfect project for those who have not worked with logic ICs before. The concept is simple and [Miguel] does a great job of explaining his implementation.

At its heart the device simply counts the oscillations of an input signal for one second, then latches the total to the 7-segment displays before zeroing the counter block and starting over. Six 4029 decade counters give the device a range of 1MHz. A set of 4511 BCD to 7-segment decoders translate the count to the display. A 4521 frequency divider chip uses an on-board 4.194304 MHz crystal oscillator to time both the display latching and the counter clearing. [Miguel] mentions that tuning the load capacitors is a bit tricky. Since breadboards have their own capacitance issues it may be necessary to change the load capacitor values when moved to protoboard or the crystal won’t start oscillating. You can see those caps are not the same value, but the tests in the video after the break show that this is pretty much spot-on.

If you’d rather give this a try in HDL here’s an FPGA-based frequency counter from which you can draw some inspiration.

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Two-player Tic-Tac-Toe from logic chips

Here’s another entry in the 7400 Logic contest. [Circuitchef] used gates and a few flip-flops to build a two-player electronic Tic-Tac-Toe game. The full details or shared in the PDF file he links to in his post. We’ve also linked to it after the break in case the Dropbox he is using becomes unavailable.

He provides a nice block diagram which helps to understand the game’s design. The board is arranged in a 3×3 matrix of momentary push buttons and bi-color LEDs. Each player takes turns pushing the button in the square they’d like to claim. The input circuitry uses flip-flops to establish which player’s turn it is, illuminating the appropriate color for that square. A set of 3-input AND gates monitor all possible combinations of winning patterns. The outputs of those gates are OR’d down to just one output which is used to light up the ‘Winner’ LED with the right color. If all boxes are lit up and none of those combinations is satisfied the game is declared a draw. This can all be seen in the demo embedded below the fold.

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Pull-string most useless machine

The mechanical simplicity of this pull-string controlled most useless machine is delightful. You can see the metal gripper which is reaching up to tug on a light-fixture-style pull chain. This is how it turns itself off after you’ve pulled the string to power it up.

The device is [Alex555's] entry in the 7400 Logic competition. We do hope that he ends up posting a schematic because we’d love to see the gritty details of how it works. After the break you can watch two doors open, allowing the arm to raise up and the gripper to grab the chain. This takes just four servo motors, which are controlled by the signal from a 555 timer and some accompanying hardware.

Apparently the chain is a fake, as the servos didn’t provide enough force to actuate that type of switch. It’s not a surprise as those pull chains do require quite a tug. An optical sensor was used to trigger the movement when your hand reaches for the chain.

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