Urgent Decoder 4x16

Thread Starter

little princess

Joined Nov 26, 2006
3
Hi

I need to connect 2 decoders 3x8 (SN74LS138) to get a decoder 4x16 but I don't understand wich enable pin should be connected to join the IC. Could somebody help me?

Thanks
 

Papabravo

Joined Feb 24, 2006
12,301
If the lines you are decoding are D, C, B, A; then connect C, B, A to the pins with that label on both of the 74LS138 chips. Connect D to G1 on one of the decoders, and on that same decoder connect G2A and G2B to logic low(GND). On the other decoder connect D to either G2A or G2B, and connnect G1 to logic high, and which ever of the G2 inputs you did not connect to D should go to logic low(GND)

Both decoders should be enabled all the time and you have a 4 x 16 decoder.
 

kubeek

Joined Sep 20, 2005
5,587
Hi

I need to connect 2 decoders 3x8 (SN74LS138) to get a decoder 4x16 but I don't understand wich enable pin should be connected to join the IC. Could somebody help me?

Thanks
You should connect the three input bits in parallel, and the fourth to one decoder and its negation to the other enable pin to switch the lower eight and higher eight outputs.
 

jack11509

Joined Mar 4, 2018
1
This thread is 12 years old. I was four when it was created. However, whoever you guys are, you just saved my ass in my logic design class. So thank you.
 

WBahn

Joined Mar 31, 2012
24,572
This thread is 12 years old. I was four when it was created. However, whoever you guys are, you just saved my ass in my logic design class. So thank you.
That's only true if you actually LEARNED something, instead of just finding a solution that you can copy down and turn in and move on to the next problem. If the latter is the case, then all that happened is you've dug yourself a deeper hole.

To see which is the case, you might consider the following (no need to post anything, just take out a piece of paper and see if you can do it).

Without looking at any notes, can you build up a 4-to-16 line decoder using just a bunch of 2-to-4 decoders?

If you can, then you are in good shape. If you can't, then you only think you've learned something and aren't in good shape. Don't proceed until you can. We can help you get there, but you'll need to do most of the work.
 

kubeek

Joined Sep 20, 2005
5,587
This thread is 12 years old. I was four when it was created. However, whoever you guys are, you just saved my ass in my logic design class. So thank you.
You´re welcome. Also, jeez am I here for that long? I´m getting old :D
 
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