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Working on a RS485 IC circuit.

It is a point to point communication between two devices connected to each other via MAX485 IC and CAT6 Cable.

At 1 or 2 m length of cable, the data transfer seems fine. But above 5m the logic 0 transmission is not good. The logic 1 transmission is good.

Used Fail Safe Resistors of 10k/20k on the A & B Data lines. Also terminated the line with a 120 Ohm resistor.

Currently, I am just giving a logic 1 & logic 0 through the arduino. All common ground and other considerations checked. All fine. At 1 or 2m it works fine but above 5m the thing is not working reliably.

I just need to send some logic states over the cable. Data rate : about 10 bits per second. Length of cable requirements: 10m, 15m & 20m.

What can be done?

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    \$\begingroup\$ Do you have a schematic? Did you measure the receiver end of the cable with a scope or multimeter? \$\endgroup\$ – sstobbe Oct 18 '18 at 15:55
  • \$\begingroup\$ with no schematic, scope data, hard to diagnose, but: 1: Make sure Drive Enable (DE) is high, DI is the data to be transmitted, not the other way around if DE is low, you won't see much of anything, though with those high pullup/down resistors you may actually be getting some bleed-through to be able to pick stuff up, given the distance and data speed you're talking about. \$\endgroup\$ – isdi Oct 18 '18 at 16:25
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    \$\begingroup\$ 2: With a 10k or 20k pullup/pulldown your idle voltage will be really small, affecting your receiver, this is not a problem if the link is simplex (one-way data transfer) but a half-duplex line will have problems. Use 750 ohm to 1K pullup/down resistors, not 10K/20K. \$\endgroup\$ – isdi Oct 18 '18 at 16:25
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Used Fail Safe Resistors of 10k/20k on the A & B Data lines. Also terminated the line with a 120 Ohm resistor.

This feels like you have a big voltage division going on, which can certainly cause an issue (though you'd have to have inverted the logic somewhere to have zero and not one problems).

A good circuit diagram would be of help.

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    \$\begingroup\$ This isn't an answer. The resistor theory doesn't explain the situation unless something is very wrong in the wiring. You are right that a circuit diagram would help, but requesting that is the role of a comment, not an answer. \$\endgroup\$ – Chris Stratton Oct 18 '18 at 15:49

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