Multiplexer & demultiplexer, some hints please

[ikalogic]

hi, here is a picture attached to that message where you can see the way i understand the use of a MUX and a DEMUX in a crcuit to control 4 channels.
the counters will be built from 2 555 timers each (or i may buy a ready made one, it is not the problem)

THE PROBLEM IS:
i dont know how to sycronize the 2 counters, in such a way that the 2 multiplexer select the same data pin at the same time. In other words i have to start the 2 timers at the same time,.... how to do that?


thx a lot

NB: i know the diagram is missing the enable pins and the strobe,etc,..

I know there are easier way to do it, but i want to learn to do it this way!

 

[Russlk]

You can use Manchester coding to send a clock along with the data, but to synchronize the data "packets" , another signal is needed. If the modulation is FSK, then the carrier is always present. You can turn the carrier off between packets to synchronize.

 

[ikalogic]

i dont understand a lot of what you have said, sorry..i am a beginner, so please try to use more explanation. please!

 

[Russlk]

do a Google search for Manchester code. If you reset both counters each time the carrier goes away, they will be in sync. Perhaps you are attempting too complex a project for a beginner?

 

[ljcox]

There is an IC called a UART that will do all the work for you.

Incidentally, I would not use 555s as a counter. There are various binary counter packages available.

Len

 

[ikalogic]

now, what if i use a 2nd RF channel to sychronize the 2 counters..would this work allright..?

 

[ljcox]

Yes you could use the second channel to send a reset pulse so that both counters are reset before you start sending data. I suggest you use a long pulse, say 200 ms in order to prevent reset due to noise bursts.

However, it could be done with one channel. Simply send the long pulse before the data.

Len

 

[ikalogic]

ok till now it is gr8 but with only one BIG problem: In the receiver, how to make a difference between the 'long' reset pulse and the data pulses??

 

[Nigel Goodwin]

ok till now it is gr8 but with only one BIG problem: In the receiver, how to make a difference between the 'long' reset pulse and the data pulses??

You could use a 555 to detect the longer pulse, but you would need to make the pulse longer than any normal pulse it's going to see. The usual way to do this is some kind of coding on the data, as already mentioned Manchester coding is one kind which is commonly used for this purpose.

 

[ljcox]

Nigel said "You could use a 555 to detect the longer pulse, but you would need to make the pulse longer than any normal pulse it's going to see."

The other way to do it is the feed the incoming data into a shift register and use an AND gate to detect the long pulse.

In order to avoid noise (ie. the occasional wrong bit in your "long pulse" due to noise) you could use an "analogue AND gate". This is comprised of resistors of the same value connected to the Shift Register outputs with their other ends connected together. This is then connected to a threshold detector. Set the threshold so that the TD output changes when say 70% or more of the bits are High.

Len