Part of my 8-bit breadboard computer series. The humble 4to1 multiplexer (or data selector) can be configured as 16 different weird and wonderful logic gates.
By Julian
Youtuber, shed dweller, solar charge controller aficionado
Good morning all…
Youtuber, shed dweller, solar charge controller aficionado
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Use other 4Way as magnitude comparator. Let 00 and 11 pass EQual. Let 01 pass LT control, and 10 pass GT. EQ also serves Borrow, or Carry (but first let ADD XORvert B). Inverse B distorts the truth table, no problem but truth NXOR to get XOR for addition. Transmission gate upgrade will let comparison propagate without ripple. 74CBT3253 dual 4Way fits well with 74LVC2G86 dual XOR on a generic SSOP to DIP28 converter. XOR your current logic result with previous CarryBorrowEQual for a complete ALU. A+B, A-B, B-A, all 16 logics, 6 comparisons, everything but rotate. Jam the chain uniformly high or low LT=EQo=GT for plain logic without arithmetic.
2 = A AND (NOT B), aka AB==10
4 = B AND (NOT A), aka AB=11
11 = A OR (NOT B), aka AB!=11
13 = B OR (NOT A), aka AB!=10
Great discovery about multi-functionality of this chip. Could be great for implementing ALU components. Or home brown FPGA
I wish you could bring this project to the next step – just been watching the playlist!
I had no idea a multiplexer could be used like that! Brilliant!
Regarding the 4 "left-over" states of your truth table: Do you know about the "imply" gate function? The state "LED is lit" relies upon (or IMPLIES) that power be fed to your circuit. If your circuit has no power, but the LED is lit, that's an invalid state. Try "B implies A" and "A implies B" as well as the inverted-output variation of both" for these four lines in your 4-bit truth table.
You are WRONG at the end of your Video! The 2 select lines are common to the multiplexers.
You could only have 2 different permutations for the inputs A and B!
Eg.: one side performs XOR and the other AND, which is an Half adder (how convenient)
Can you post where did you buy the HEX switch? Thanks
There is going to be more right Julian? 🙂
i subscribed because of these stuff 😛 i love watching vids on home brew breadboard computers make more on this please
Bit late to the game, but here's my thoughts.
That's every possible logic gate for two bits. Looking forward to seeing you figuring out the 512 ways (16*16*2 2 for add or subtract) that two stacks of them can combine with the adder. 🙂
True and False are definitely useful – set one multifunction to False (0) to use the other to do straight bitwise logic, and set one to TRUE (-1) and either subtract or add to increment or decrement a number.
I noticed that if you split it down the middle of the X axis the bottom is inverted and mirrored. Neat!
I want to meet you some day….😊
A is A, but B is not B
I like the rotary encoder switch, can't find it on BangGood or Aliexpress. Where did you get it?
Hi I don't see how you can get an 8-bit wide gate (two input 8 bit words) from only four of these beasts since as I understand it the two "sides" of the '253 use the same A B control lines.
What a wonderful Idea!!! I see some usefull ways to use this: a digital filter stage for datastreams, a timing manipulator for a R-C element (kinda for music?)
Why not call:
Function 2 : "A>B"
Function 4 : "A<B" or "B>A"
Function B : "A>=B"
Function D : "A<=B" or "B>=A"
?
Very dodgy relying on TTL inputs floating high Julian. Beware of induced noise