FSK or just serial digital data.

[potcore]

Hello,
It is required to send digital data over two miles of cable which is underground in a remote area.
It has been suggested that the serial digital bit stream is converted to an FSK signal, then sent over the two miles.
1.2KHz is to represent the logic low
1.4KHz is to represent the logic high.

However, i want a design with minimum component count.

I am wondering if i can just convert the digital data stream into RS232 format and send it over the two miles. I appreciate that line capacitance will affect the rise/fall times of the pulses but surely it can't be that much capacitance and it will still be possible?
Also, an amplifier on the transmit & recieve ends should make good any attenuation due to line resistance?

If i have to use FSK, then i will need a demodulator on receiving end. I think the easiest way to demodulate the FSK would be to put the FSK through a comparator to turn it into a series of pulses.....i could then send these pulses into a PIC and count the number of pulses occurring over a time_interval. This would form the basis of distinguishing the high and low frequencies. -I am wondering if anyone has any better ideas for demodulation, (and any thoughts about just using RS232)?

 

[Skogsgurra]

It is certainly possible to use a long cable. But, that wouldn't be RS232 according to standard any more.

Quote: "Maximum cable lengths
Cable length is one of the most discussed items in RS232 world. The standard has a clear answer, the maximum cable length is 50 feet, or the cable length equal to a capacitance of 2500 pF. The latter rule is often forgotten. This means that using a cable with low capacitance allows you to span longer distances without going beyond the limitations of the standard. If for example UTP CAT-5 cable is used with a typical capacitance of 17 pF/ft, the maximum allowed cable length is 147 feet.

The cable length mentioned in the standard allows maximum communication speed to occur. If speed is reduced by a factor 2 or 4, the maximum length increases dramatically. Texas Instruments has done some practical experiments years ago at different baud rates to test the maximum allowed cable lengths. Keep in mind, that the RS232 standard was originally developed for 20 kbps. By halving the maximum communication speed, the allowed cable length increases a factor ten! "

Use the old current loop (no, not the 4-20 mA analogue kind, the TTY version) and force something like 10 - 20 mA through an optocoupler at the receive end. Modulation is simple, just short the transmit end with a transistor for zero and open the short for one. That minimizes the voltage swing and capacitance will not be a problem.

The benefit with FSK is that you can use transformers at both ends for transient protection.



Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Skogs has good points.

What about RS485. Your length is right in its spec'ed distance.

For transient protection you can use typical optical isolation on an end.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[cbarn24050]

google modem.

 

[VEBill]

http://www.blackbox.com

Lots of range extenders based on all sorts of technologies including fibre optic. Expensive, but probably cheaper than building from scratch. Even if you're building, you'll see what is possible.

http://www.blackbox.com/Catalog/Category.aspx?cid=381,1452,1465

 

[danw2]

I saw the current loop scenario work well at about 2 miles at a steel mill. I know Westermo still makes the converters:

http://www.westermo.com/Resource.phx/content/products/current-loop-20-ma/current-loop-20-ma.htx

 

[Skogsgurra]

Yes! Of course! Those current loop converters are used throughout the world for exactly the application potcore described.

They even use the principle I described. I probably saw it there many years ago and thought it was my own idea (apologies to Tore Andersson).

Get a couple of those and your problems are over!

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[bogeyman]

I would chose a pair of modems like we used to have to use to get on'tinternet

 

[watchjohn]

DTMF "Dual Tone Multi Frequency" can also be used to
transmit (very slow) data.

DTMF transmitter and receiver chips are available
for less then 1 U$

They will give you at least 10 codes that can be transmitted
over great lengths as simple audio.

Transceivers (RX and TX in 1 chip) are also available.

Yes...this will probably only work up to a few bits per sec
since switching-speed between tones is probably limited
by the chips internal key-debounce circuits.
There are TTL DTMF chips too they have igital inputs instead of key inputs(pushbuttons) maybe these are a little faster.

WatchJohn

 

[benta]

WatchJohn, the DTMF chips are not speed limited by the debouncing, but by the PLL lockuptime of the receiver. Fastest speed corresponds approximately to what you hear when re- or speed-dialling. No way to change this, unfortunately.

Best Regards,

Benta.