You will all no doubt be familiar with the 74 series logic integrated circuits, they provide the glue logic for countless projects. One of the more famous of these devices is the , a cascadable 4-bit arithmetic logic unit, or ALU. An ALU is the heart of a microprocessor, performing its operations. The appeared in many lates and earlys minicomputers, will be familiar to generations of EE and CS students as the device they were taught about ALUs on, and can now be found in some home-built retrocomputers. Why on earth you might think would an ALU need to do that? The answer lies in the way it performs carrying while adding, a significant speed-up can be achieved over ripple carrying along a chain of adders if it can be ascertained whether a bit addition might generate a carry bit.
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Why on earth would you want an oversized replica of an outdated logic chip from nearly five decades ago, we hear you ask? The answer lies in education. If you were to embark on learning about the internals of a microprocessor by taking a modern example such as the one that powers the device on which you are reading this, you would find it to be a daunting task.
The starting point for the student of microprocessor internals often lies in the past. The technology of the early s holds the fundamentals from which a modern processor can be understood, but remains simple enough to grasp in its entirety as a beginner.
And for decades the , as an all-in-one 4-bit ALU on a chip that you might have found in a minicomputer at the turn of the s, represented the most convenient way to teach the operation of these devices. Electronic engineers and computer scientists of all ages will have encountered them as they gained their qualifications. The PCB version of the faithfully follows the original, but with modern 74LVC gates laid out as they would be in the circuit diagram of the chip, and LEDs to show logic state at the different parts of the circuit.
Thus when it is used to teach ALU operation it can show every part of the device in detail in a way a real would never have done. I love these types of builds! This is exactly the type of stuff that flip on those light bulbs for people like me. Awesome job! Regarding the project as it is now, fun and very interesting!
I had the same thought as well. Then I was thinking perhaps a small CPLD with toggle buttons for the inputs and a 4 bit up down counter with buttons for up and down for the select. Then it becomes two many pins for a small CLPD so perhaps even 74xx would do it. It would be a challenge to get the current for the gate input LEDs as the fanout is 10 or more in some cases and there are many signal paths. Some with a fan out of up to 10 … would be very high bling factor … lots of LEDs.
Also serially connecting some of the LEDs in the same node to save on buffers? With 87 discreet signals it would have to be a pin CPLD. Some fanouts are over ten. Putting 10 LEDs in series would require to high a supply voltage so a parallel system would be better if buffers are used. The next step would be to understand how people used the AND AM series of bitslice processor devices to design their own custom computers. The s were much more popular than the Intel series.
All of those used the AM family members. Both used stock AMD family bit slice architectures. The AM and some of the other bitslice stuff is next on my list to experiment with and create replica boards for! The circle on the output means not. So a simple NOT gate. The circle is the NOT part. Yes, there are plenty of New Old Stock NOS out there, but there are no current silicon manufacturers who produce new parts. This has lead to a booming business for selling the NOS parts to students who are talking college classes that require a My hope was that these could be used in those classes instead….
This site uses Akismet to reduce spam. Learn how your comment data is processed. By using our website and services, you expressly agree to the placement of our performance, functionality and advertising cookies. Learn more. Simple enough to work your way through the logic The starting point for the student of microprocessor internals often lies in the past.
Report comment. Would be nice to have the original schematic overlaid on silkscreen :. Open collector transistor arrays like the ULN are probably just as pin deficient as buffers. What gates are at the top of the circuit? Leave a Reply Cancel reply. Search Search for:.
Huge 74181 Is A Classic ALU You Can Actually Understand
Why on earth would you want an oversized replica of an outdated logic chip from nearly five decades ago, we hear you ask? The answer lies in education. If you were to embark on learning about the internals of a microprocessor by taking a modern example such as the one that powers the device on which you are reading this, you would find it to be a daunting task. The starting point for the student of microprocessor internals often lies in the past.
Explaining The Operation Of The 74181 ALU
The represents an evolutionary step between the CPUs of the s, which were constructed using discrete logic gates , and today's single-chip CPUs or microprocessors. Although no longer used in commercial products, the is still referenced in computer organization textbooks and technical papers. It is also sometimes used in 'hands-on' college courses, to train future computer architects. The is a series medium-scale integration MSI TTL integrated circuit , containing the equivalent of 75 logic gates  and most commonly packaged as a pin DIP.
Inside the vintage 74181 ALU chip