Multiplexer for data bus?

[sci4me]

Hey guys! So I am working on a hobby project using 74LS (and related) TTL logic chips. I need (de)multiplexers for 8 bit data busses.

What is the best way to do this? (using through-hole components by the way). I figure I could use and gates and decoders right? But that's a LOT of chips to add just for a multiplexer...

Is that the best way to do this? Are there actually plexers with that wide of data bus?

Or I suppose if there are plexers with 2 or 4 that would still be a lot better.

I will probably need multiple different plexers with different numbers of select bits, but I know I will need an 8 bit multiplexer with 2 select bits. I may also need one with 3 select bits... we'll see.

What's the best way for me to do this? Thanks guys!

[T3sl4co1l]

The whole point of a bus is to not need a *plexer.  Surely you're going about this the wrong way?

Tim

[sci4me]

I'm not sure I understand what you just said...

I'm a n00b so can you elaborate on that a bit please?

When I say bus, I mean that I have an multiple 8-bit registers and I want to select an individual one to go onto the bus. Am I just saying what I mean incorrectly or...?

[f5r5e5d]

common practice with a "bus" is to use tri-state output registers or buffers/drivers

all of the output bit lines are wired common to the respective bus bit lines and then you only enable the drivers for the register/buffer chip you want to drive the bus

[sci4me]

Aah right, so I would have a tri-state buffer and use an encoder to enable it right?
Something like a 74LS367?

[macboy]

74xx245 is commonly used. 8 bits wide, bidirectional and tri-statable.

[sci4me]

the 74xx245 is a transceiver, but what if I don't need to be able to do both directions? Should I still use that or...?

[macboy]

the 74xx245 is a transceiver, but what if I don't need to be able to do both directions? Should I still use that or...?

There is no issue with that. Just tie the direction pin Hi or Lo and control CS as needed.

[sci4me]

I mean, I know that works, I was just wondering if it may be better to have something that's just 1-directional...
simply for the purpose of it makes me feel like I'm wasting half a chip to tie the direction Hi or Lo.

Anyway, thanks! I really appreciate all the help you guys have been giving me!

And thanks for clearing up the tri-state stuff. It makes so much sense to me now - very obvious how useful that is.

[Paul Moir]

There are unidirectional ones like the 74LS244, but it has an annoying pinout so people use the 245.  Some other annoying pinout chips get fixed like the 74LS373 octal latch has a saner version in the 74LS573.

[bgm]

Common thing to do is to use a latch when decoding multiplex address bus decoding. 

This is when for example you are dealing with a micro that uses the same pins for both data bus as well as the address bus. 

As has been mentioned, typically they have a control pin as well which indicates if these shared pins are being used for the address or data at a point in time.  The other source of input for decoding this is typically the clock as well - usually it will be a different stages of the clock cycle when the pins are being used for address vs data so you will be latching the data (sometimes falling edge, sometimes rising edge). 

Yes, the 74LS573 has been mentioned and that is a pretty common part for this sort of thing. 

/BGM