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I am working on a system that gets data from a sensor via a TTL connection. Currently, the data that is gathered from the sensor is sent to another part of the system via a set of modems. The distance between the modems can range anywhere from a few hundred feet to up to 4000 feet or more. I'll try to provide a rough schematic:

 ------  TTL  -----   TTL  -------     CARRIER SIGNAL     -------  TTL  -----  
|Sensor|=====|PCB A| =====|MODEM A|======================|MODEM B|=====|PCB B|
 ------       -----        -------                        -------       -----

So, my question is this: what other alternatives are there other than the use of modems to send the data from the sensor to PCB B? To me, it seems using a modem for a single connection is overkill, especially because the modems have to connect to establish a data link (which is time consuming for this application). I had thought of using a Digital to Analog to Digital conversion scheme, but I am not sure what repercussions (if any) there would be to this at distances up to 4000 feet. I imagine a direct TTL connection across that distance would dissipate the signal fairly quickly. Any insight or suggestions (or corrections) are appreciated.

EDIT: Current data rate is 19.2 kbps. As for the modems, we have actually used two types. One was a spread spectrum modem and the other was a standard dial-up modem. There are only two wires (essentially a tip and ring) connecting the two modems as of now and the system is half duplex.

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    \$\begingroup\$ What kind of data rate do you need? If it's not terribly high, a current loop would probably be the first/most obvious choice. \$\endgroup\$ – Jerry Coffin Nov 12 '14 at 2:34
  • \$\begingroup\$ Currently, it is at 19.2kbps, but we could probably decrease it somewhat if needed. \$\endgroup\$ – Josh Nov 12 '14 at 2:54
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You could probably use RS-485 for the task if you don't want to use this modems. It would help to post more information about what exactly this modems are.

RS485 can help you as you could convert TTL signals into a twisted pair driven by a RS485 transceiver, as it works up to 4000ft.

I could not request more information before making up an answer because I can't put up comments.

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For distances up to as much as 10K feet you can consider the use of a differential signal pair that has drivers run off a 5V power supply. There are chips specially designed for this application. If your communication needs to be bi-directional then look into RS485 transceiver chips from companies such as TI, Linear Tech or Maxim. That would take a pair of signal wires and a GND wire to control the common mode difference between the various devices.

If you only needed uni-directional data transmission it could be done with RS422 differential drivers and receiver chips.

Both schemes will need twisted pair wiring for the differential pair with the suitable set of line termination resistors installed at each end that are matched to the line impedance.

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