Multiplex / Demux

[Pauldf]

Hi,
I am looking to build a device which can transmit 28 discrete inputs down an 8 core cable.
In essence what I want to do is replicate the time display on a CD player in a remote location, around 30m of cable away. What I was thinking of doing was piggybacking the 4 off 7 segment displays in the CD player and feeding these 28 inputs to a pair of 16 line multiplexers (74150).
The 4 data selects can be created by a counter and a fairly fast clock (not too fast but quick enough so the displays look steady), these used in parallel to create the data selects for both the multiplexers.

These 4 data selects, two data and 0v lines can then be sent down the 8 core cable to a decoder (74154) where I can use this to drive 4 off 7 segment displays.

Will the above work or is there an easier way of doing this? Will TTL work over 30m of CAT5?

Cheers

Paul

[piratepaul]

Sounds interesting... I will need to think about it... hmmm.
I will look it up and be back tomorrow night.

Dont go away.

[piratepaul]

2 mux output wires, 4 select wires, 2 power + and - as far as I can see...
Piggi back should be ok, but be carefull wher you take the power from.
I have not worked out the drop in the cable which is the only poss flaw... if so thicker cable.

[Pauldf]

Thanks for that,
For the counter select side of things I was thinking of using a 555 astable driving a 7493 chip (divide by 2 and divide by 8) to give a binary count up to 16, the only thing I am unsure of here is what happens on the 17th clock pulse? Will the chip hold at 1111 or will it reset to 0000? I'm thinking it will go to 0000 which is what I want but best to double check.
I'll have to open up a CD player and have a play about inside to see if the display is source or sink as this will have an effect on the project, mainly the selection of the 7 segment displays at the far end.

I was thinking about using a separate PSU at the remote end to power the 7 segments hence only using 7 of the 8 cores in the cable, maybe doubling up the 0v rail.

[piratepaul]

I looked up the chip, it will roll on and on.
I dont understand what you mean (divide by 2 and devide by 8) it will count 0000-1111-0000.

Nice talking to you.

Thanks.

[piratepaul]

I posted that in haste, it needs to be a mod 28 counter ... I think you will need to add more circuits to make a mod 28 with this chip(s)... there are other ttls available which do not need the extra circuits.

Back tonight.

[Pauldf]

I'm not sure what a mod 28 counter is, the way I understand the 7493 chip is it has two counters, one that divides by two and the other that divides by eight (with connections to the two, four and eight stages). Connecting the output of the /2 to the input of /8 gives an overall /16, the four outputs giving a binary count of 0000 to 1111.
Cheers

Paul

[piratepaul]

Yea, you are counting to 1111 twice... 2 chips.
If the mux's where running through 1 wire ... you would need to count to 11100 then return to 00000.
00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
10000
10001
10010
10011
10100
10101
10110
10111
11000
11001
11010
11011
11100
11101
11110
11111
I had to write it out to remind myself, when it counts to 11100 you truncate 11100 happens for a few nano seconds so does not count... the effective count is 11011 then return to 00000 I was thinking of 1 mux when I said 28 ... but then you would need 5 select wires... my mistake.
There are 2 wires for the mux so you need to count to 1111 which does not happen... you need a logic which when the counter counts to 1111the NAND is 00 (ie some extra gates) at its input which truncates the counter to 1110.
The counter is edge triggered or level triggered, it can be leading edge or trailing edge...if leading edged... you count to 1110 and truncate, if it is trailing edge or level triggered you count to 1111 and tuncate.
You might be able to do this simply with the 7493... you do not give the letters that come with the chip.

I think I have got that right, I will check and make sure when I have time ... I have had a long day... I realy is good to talk to someone with a similar interest, you obviously know your stuff... I have read fundamentals of digital electronics (ebay) and recomend it... if you get that book and use the chips in it we can talk the same language.

I dont want to give a time when I will be back but I will be in a bit.

[piratepaul]

OK, I was very tired just then, this is eating my mind, a mod 10 counts to 10... a mod 6 counts to 6 etc
The posetive going or negative going clock edge trips the counter to its next binery state, the numbers I just gave are prob one or two out, I am not going to go back because I will confuse myself...

new circuit ... new chips.

A mod 28 mux call it the data mux...
Two counters on one chip. First counter a mod 28 = 5 flipflops... second counter a mod 5 =3 flipflops... the 555 clocks the mod 5... the mod 5 clocks the mod 28
The mod 28 counts the data mux ...
The mod 5 counts a select mux. The frequency of the select mux is 5 times the frequency of the data mux.

3 wires...
Data mux wire
select mux wire
0V wire.

It is poss to use 2 wires only ... 0V and mux.

The 7493 has 4 flipflops, it has QA, QB,QC and QD outputs, left as it is it will not roll, 0000-1111 stop. Join QA to QD and it rolls on and on.
There are TTLs which have NOTQ and Q output pins, with these chips it is poss to make any modulus (mod).

It is poss to create this circuit using point to point lazer ... no wires.
Or radio no wires ... no wires is pushing it a bit... but 2 is poss... I will have a think about it.

Tar&stuff.

Back in a bit.

[piratepaul]

OK 2 wires... The 7493 def rolls, if you do what I just said it will damage it, I should have said feed QD back to set or reset... if any thing... bbbuuut it rolls... rolllllssss you hear... achmm.

another counter ... a mod 2 = 1 flipflop is clocked by the 555, the mod 2 clocks the mod 5, the mod 5 clocks the mod 28.

The 28 input data mux and the 5 input select mux outputs are fed into the 2 input mux's inputs (call it the output mux) the output, of the of the output mux is fed into the 2 input demux via the wire +0V... etc. etc. etc.

Next we must choose chips, pin connections and draw a diagram.

I am having probs with my laptop, I have been downloading drawing programs which dont work virus mall ware what ever, I have no working camera etc.
The laptop is running very very slow. I have sent off for a clean up disk and a copy of the latest linux... I hope to be up and running with the diagrams on Monday but when ever it happens.

Tar&stuff.

Back in a bit.