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Help me , running long cables ...

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rishi254

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Hello Sir

I have a project to log the keypresses from an hall of 60m X 50m area. There are around 500 switches in this area(actually there r 500 users sitting in this hall, each table has one toggle switch,reset switch).

Tables are arranged as 10cols X 50 rows.

I was thinking for a simple matrix logic as its extremely simple and would be cost effective in terms of cabling costs as well as other hardware required.

I have a requirement of running 60 m cables with ttl signals.Is it possible with using BUS Buffers/transreceivers?

ON the computer end i will use 60 bit digital I/O board and write the software which will scan rows and columns continuosly.

Will be feasible or there is any better alternative...

Please , can u suggest me on this.

Thanks alot
Best Regards
Rishi Bhanot
 

House_Cat

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I'm not sure you can solve your problem with just TTL. You are going to need to condition the keypress pulses before you can do anything digital with them.

The first considerations that come to mind with the information you have provided are:

1. Timing (propagation delays are going to be a consideration for computer scanning 60m input lines)
2. Losses (TTL levels typically input 2v minimum for 1 and less than .8v for 0)
3. Noise (60m of twisted-pairs/wire/coax is going to make a wonderful antenna for flourescent lighting noise, neon sign noise, stray RF pickup, etc.)
4. Reflections (you will need to terminate each line carefully, or reflected pulses will give you false readings)
5. Logic collisions (if you figure out the timing problem trying to scan so many long-lead inputs, how will you resolve collisions? i.e. two signals arriving simultaneously?)

Without thinking too long, or too hard about your problem - I would consider using a data transciever on each line to condition what will undoubtedly be noisy, poorly formed, signals. The old standby RS232 was designed to drive 50ft of cable at a pretty good rate, but I've seen low data rate reliably transmitted/received over 250ft. Perhaps you could use some cheap RS232 transceiver chips to give yourself a fighting chance at the 180ft you are trying to use.

You still need to figure out the timing/collision problem.
 

Pazam

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I don't think your concept will work.

The line losses and noise issues are just too big to neglect.

What you can do is use a very very low cost microcontrollers (connected to the switches you've mentioned) and create a net.

I recommend using a well shielded differential communication.
You can use an RS-485 like net. For example use two wires for RX
and two wires for TX, and use the microcontrollers pins as differential driver.
Don't forget to put power lines :)
 

marie65

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Use an isolated power source of 24 DC voltage for the matrix and end with optocouplers before your input digital board. Use around 5 mA for each line.
 

r_e_m_y

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Using a matrix 10x50 for scanning your 500 switches may generate false aquisition if there are simultaneous switches on.
Don't forget to put a diode on each switch to avoid current return in the non activated lines.
And if you want to reduce wires, a 20x25 matrix is better, but the scanning time may be longer.
I think that the solution given by marie65 (24VDC supply) is the cheaper one.

I hope this helps you.

:turn-l:
 

Pazam

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TI has a very good application report about RS-485.

The web site is: www.ti.com.
The document is called: 422 and 485 Standards Overview and System Configurations.
The document number is: SLLA070C

Good luck
 

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