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I need to send a slow (steady state most of the time) TTL signals over 10m - 1km (max) distance.

  • Shielded CAT6 wire will be used and I can't use radio based solutions.
  • Wire is only used for that purpose (eg. not shared with anything else, all 4 pairs are available)
  • Electrical isolation is needed because sender and receiver side may have different ground potentials.
  • I want to avoid protocols/standards that needs additional complexity at receiver side. A receiver IC that outputs the signal is OK but I don't want to have a uC for decoding packets over something like RS485/CAN etc.

Since I have twisted pair cabling, using differential line drivers seems to be most appropriate solution for these requirements. Here is my confusion begins..

  1. Most of the line drivers I found labeled as "RS485 transceivers". The thing is, I don't intend to use RS485 protocol. Is there any side effect for using these in my application? I can't see any but I want to be sure.
  2. Again, many line drivers works with single 5V supply. That means it can only generate ~+-2V differential signals. With long wires like I'm using, this can be a problem. Is there alternatives that can generate/receive higher Vpp signals (for example +-12V for improving signal), if any what is the keyword for finding them?
  3. All these solutions will also need a shared ground between sender and receiver. Since I'm going to have multiple receivers, even if I isolate ground from sender side (seperate logic / line driver supplies with optocouplers in between for signals), the sender board will need to connect all receiver grounds to a common point. How can I improve this situation and electrical isolation?

Thanks.

Note: In an other question, I stated to want control a relay with that and got related answers. This question focused on transmitting signal over the wire.

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    \$\begingroup\$ Use a current loop, like an opto-coupler at the receiver end. \$\endgroup\$ – Trevor_G Nov 16 '17 at 11:23
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    \$\begingroup\$ @RohatKılıç 485 still needs a common ground though. Or close to it. \$\endgroup\$ – Trevor_G Nov 16 '17 at 11:33
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  1. RS485/RS422 hardware layer is based on differential line transceivers. The 5V is chosen, because of high speed transfer. Note that a 12V level signal with same slew rate needs more time to "reach" the HI or LO signal level, so small voltage signal is needed for high data frequency. The protocol layer is something totally different story.

Typically a RS485 communication can go up to 1.2km @ 9600 bps, twisted pair, with a conductors diameter >=0.64 mm (AWG 22)- courtesy of Siemens.

  1. You won't find differential signals at high voltage, except some current loop, also used for data communication over distance. This is now obsolete in industry and replaced with RS485, because at time there were no such RS485 transceivers, I guess. The current loop was made with an optocoupler and a current mirror.

schematic

simulate this circuit – Schematic created using CircuitLab

  1. With an isolated RS485 comm. you would need to supply the RS485 transceivers with a galvanically isolated, floating PSU, like DC/DC converter 5V/5V. You have to bias the signal pairs with Vcc and GND potential through bias resistors, and of course the termination resistor at both ends. In such way, the PSUs are equalized to common potential voltage. There is no need to connect the earth on them, and of course it has to be avoided to connect the GNDs of both PSUs throuh a third conductor like shield. The shield is earthed on one end to sink the environment interference.

enter image description here

At each side you put a bias and termination resistors.

schematic

simulate this circuit

Link to ready made transceiver with bias and terminating resistors.

enter image description here

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Use the MIDI physical layer!

Opto coupler at the receiver (with a few resistors and a diode for protection) and a drop dead simple 5mA current loop transmitter, easy, simple, isolated and utterly reliable for this sort of thing.

There will be loads of examples on line showing how to do it.

Regards, Dan.

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  • \$\begingroup\$ So, adding an optocoupler to directly the data lines of the RS485 isn't bad? \$\endgroup\$ – abdullah kahraman May 2 '18 at 4:41
  • \$\begingroup\$ Midi is NOT RS485! It is defined as a current loop and is intended to work this way, it is also point to point which RS485 is not, so the loading from the opto coupler is a non issue. \$\endgroup\$ – Dan Mills May 2 '18 at 6:31
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Please find below answers to your questions.

Q1: Most of the line drivers I found labeled as "RS485 transceivers". The thing is, I don't intend to use RS485 protocol. Is there any side effect for using these in my application? I can't see any but I want to be sure.

Ans1: These are called transceivers because they follow the physical layer specification of RS485. Mean the voltage level output of the ICs will be of RS485 standard. But if you can send the data on variable speed in the form of 1 and 0, then why can't you send very low speed control signal of 1 and 0. Also, these transceivers are just voltage converters and they do not contain any intelligence of making any kind of logic operations on your inputs.

Q2: Output +/-2V, With long wires like I'm using, this can be a problem.

Ans2: This can be a problem when you are making ground of transmitter and receiver. The output signal you will get will be differential signal and it is designed to overcome the technology drawback for such long distance application.

Q3: All these solutions will also need a shared ground between sender and receiver.

Ans3: No, you will not need to connect GND of any of the transmitter or any of the receiver. If you want to isolate the communication part, you need to power these transceiver ICs with isolated power supply on both transmitter and receiver side. Also, you need to provide isolation using opto-isolators on the input and output signals.

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