# Building a 2-bit adder using discrete DTL logic

#### brahms

Joined May 9, 2014
32
Sir Allen, i have watched some videos explaining about Full Adders and Half Adders. Now I quite understand how it works.. but here, I found a 2 bit-half adder on the net so i simulated it on my own. The circuit uses 4 XOR gates, 3 AND gates and 1 OR gate. Now the problem is I do not know how to convert these gates into DTL or RTL circuits..

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#### absf

Joined Dec 29, 2010
1,949
ok, let me show you how to make an XOR gate using transistors in DTL.

The first schematic is the basic NAND, NOR, NOT and AND-OR-NOT gates in DTL.

In the second picture, The top left is a XOR gate symbol, Top right is the XOR broken down into simple inverter, AND and OR gates.

On the bottom is the XOR in DTL logic using transistors. You'll need to make 4 of these in order to make your 2-bit adders.

I have simulated it on proteus ISIS and it works.
Allen

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#### brahms

Joined May 9, 2014
32
I have experimented things out, and came out of this simpler XOR gate. Try it out and it sure works hehehe

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#### brahms

Joined May 9, 2014
32
Plus this 1 sir, i found it on the net but I can't make it work when i replace the inputs and outputs by switches and LEDS..

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#### absf

Joined Dec 29, 2010
1,949
I have experimented things out, and came out of this simpler XOR gate. Try it out and it sure works hehehe

Did you take voltages on the output when it is high or low? Yes the circuit works when connected to LED and with 9V power supply. But are the voltages compatible with that of the DTL/TTL circuits.

May be you can make a simple DTL inverter gate or just use a 7404 to connect to the output of your XOR gate with the LED relocated to the output of the 7404 and see if the LED still lights.

If it works then you'll stand a good chance that the XOR would work with the rest of the DTL gates.

Allen

#### brahms

Joined May 9, 2014
32
Here's my progress report sir, I'm currently finished at the first XOR gate and it's first output. But I'm now having problems in the next XOR gates and the next one at the right side of it.. (corresponding from the picture above 2 bit half adder)

I think my circuit is crudely designed.. and the Vcc of the transistor, i don't know where to properly connect it. When i just connect it in the 9v rails, other circuits are affected...

Thanks for the help Allen!

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#### absf

Joined Dec 29, 2010
1,949
Your XOR gate using 4 transistors was designed as an independent circuit to demostrate its function on a LED. Here are the voltages readings that I took across R4.

Rich (BB code):
=====================================================================
Battery=9V        Battery=9V       Battery=9V           Battery=5V
Without LED      LED                    LED in series      LED in series
connected         connected        with 470Ω              with 470Ω
=====================================================================
H=7.82V              H=2.35V            H=6.53V                H=3.43V
L=3.93V               L=2.13V             L=2.23V                 L=1.77V
=====================================================================
From the reading you can see that with the LED in, the differences between H and L is very little. Besides, the Low may be too high for the next stage DTL to take it as High instead of Low.

If the basic XOR gate is foul, how do you expect the combination of it to work with other gates?

You have to do some studies on the basic building blocks first before you can start to construct your final adder.

Allen

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#### absf

Joined Dec 29, 2010
1,949
Here is a simpler XOR in DTL composed of a NAND, an OR and an AND gate. It used just 6 transistors.

You can reduce it to 5 transistors if you use 3x NAND and 2x inverters.

Allen

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#### brahms

Joined May 9, 2014
32
I think I really need to. Hehe, i know how to measure voltage amperes and other quantities. But they didn't taught us how to build a proper circuit.

How should it be Sir? The measurement in Low and High? And what are they? Can you suggest a link in where I can study them? =) I might got it wrong when I searched for it cause i don't know it

I'll study the circuit you gave me Sir, it's kinda still hard for me to know it all at first look. =)) I'll get back here later for some updates!

Thank you so much!!!!!!

#### brahms

Joined May 9, 2014
32
Sir Allen, may I ask you to rebuild the circuit without using the ground in the resistors and transistors, and really connecting it to the battery? I can't make mine work..

#### absf

Joined Dec 29, 2010
1,949
No problem,

Here you go. See the attached.

Allen

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#### absf

Joined Dec 29, 2010
1,949
I think I really need to. Hehe, i know how to measure voltage amperes and other quantities. But they didn't taught us how to build a proper circuit.

How should it be Sir? The measurement in Low and High? And what are they? Can you suggest a link in where I can study them? =) I might got it wrong when I searched for it cause i don't know it

I'll study the circuit you gave me Sir, it's kinda still hard for me to know it all at first look. =)) I'll get back here later for some updates!

Thank you so much!!!!!!
If you got a Logic Probe. It will tell if the voltage you measured is High or Low or Open Circuit in TTL or CMOS mode. But I dont know about the legal levels for H and L in DTL. Need to do more reading on DTL before I can answer your question.

Inside the CMOS and TTL datasheets, the voltages for legal "H" and "L" and "indeterminate" were also listed. I'm very sure Wikipedia would have all these info. Do a search for it and you'll find out the answers there yourself.

Allen

PS here is a chart for the voltage levels for different families of logic chips.
Unfortunately DTL was not included thet

Logic thresholds for logic families

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