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At this link there are specs of a converter with the following excerpt:

enter image description here

What does "+/-" sign indicate(mean) above before TxD or DATA?

As far as I know TxD pin/line transmits and RxD receives the serial pulse train.

But what does the extra prefix "+/-" indicate then?

(And are these data lines always isolated from the ground of such converter? What is meant by Isolated RS-485 converter?)

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The +/- means that the transmitted and received signals are 'bipolar' relative to ground.
1 / idle / spacing / logical 1 is usually negative relative to ground.
0 / active / logical 0 is usually positive relative to ground.

Where a polarity is not specified (eg RS422) all signal levels are always positive relative to ground - but in a balanced signalling system such as RS422 the levels on the two data leads will be of opposite level t0 each other (1-0 or 0-1). .

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TERMINOLOGY:

Differential:

Two signalling lines in a balanced pair.
"One goes up and the other goes down."
Both are usually within a single set of supply rails (eg V+ and ground BUT the signal is not ground referenced. The common mode voltage MAY need to remain within the rails or some limit BUT this is not part of the signalling system per se.
Example - RS422

BIPOLAR:

The signal is on a single line referenced to (usually) ground.
It transitions above and below ground to signal (sending or receiving).
It is not balanced, is not inherently noise immune.
Example - R232.

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  • \$\begingroup\$ Thanks, and does isolated RS422 converter mean that its power supply ground is isolated from its data driver ground? \$\endgroup\$ – floppy380 Sep 13 '19 at 9:25
  • \$\begingroup\$ @HelpMee Isolated RS422 would have no Ohmic connection between any primary / input and secondary output sidesEXCEPT a high impednace ground to ground connection may be made to keep the absolute ranges of the two sides about the same. A fully isolated secondary could notionally float relative to receiver to 1 or 10 or 1000 or 1000000 volts. \$\endgroup\$ – Russell McMahon Sep 13 '19 at 9:30
  • \$\begingroup\$ @HelpMee In a non isolated RS422 system such as yu mention on JRE's answer - ground differential can cause link failure if too large. \$\endgroup\$ – Russell McMahon Sep 13 '19 at 9:32
  • \$\begingroup\$ Is 'bipolar' still used? Is there a distinction between 'bipolar' and 'differential'? \$\endgroup\$ – schadjo Sep 13 '19 at 19:00
  • \$\begingroup\$ @schadjo Differential: Two signalling lines in a balanced pair. One goes up and the other goes down. Both are usually within a single set of supply rails (eg V+ and ground BUT the signal is not ground referenced. The common mode voltage MAY need to remain within the rails or some limit BUT this is not part of the signalling system per se. Example - RS422 || BIPOLAR: The signal is on a single line referenced to (usually) ground. It transitions above and below ground to signal. It is not balanced, is not inherently noise immune. Example - R232. \$\endgroup\$ – Russell McMahon Sep 13 '19 at 21:11
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In this context, it indicates that the TxD and RxD signals are transmitted differentially. There is TxD+ and a TxD- line, and it is the relative voltage between them that indicates a logical high or low signal. This differential signalling is used to improve noise immunity, and is common with relatively low speed signals that must endure potentially harsh electromagnetic environments, as well as extremely high speed signals like USB or HDMI.

To the second part of your question, no, it does not mean that the signals are isolated from ground, at least with RS485/RS422. In both of these signalling schemes, the + and - halves of each signal (also called A and B or X and Y) are explicitly referenced to ground. It is actually their voltages relative to ground that are compared to establish the differential voltage (and thence the logical state of the line). RS485 and RS422 do, however, require transceivers to tolerate a fairly high common mode voltage range of -7V to +12 relative to ground, which allows devices to absorb a fair amount of ground voltage offset from one node to another.

An "Isolated RS845 Converter" would be an interface that galvanically isolates the signals on one side from the signals on another side, including ground. This can be accomplished by converting electrical signals to optical signals to cross the isolation barrier and then back to electrical signals on the other side, or by using transformer-based or capacitive coupling. Isolated converters are useful when a set of signals needs to cross a much larger difference in ground potential, or where electrical failures might otherwise result in large fault currents flowing through the signal wires, which would damage or destroy equipment or wiring.

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  • \$\begingroup\$ I suggest +/- means just the opposite of what you say. RS232 is non differential and uses voltages above and below ground. RS422 uses differential signalling without ground reference. See my answer. The +/- nomenclature is shown on that page only against RS232 signals. \$\endgroup\$ – Russell McMahon Sep 13 '19 at 21:26
  • \$\begingroup\$ The specs quoted in the question reference RS422 and RS485. RS485 and RS422 are differential, as I said. I made no mention of RS232. RS485 and RS422 signals are both differential and referenced to ground. \$\endgroup\$ – ajb Sep 13 '19 at 21:56
  • \$\begingroup\$ You are correct re what the specs refer to. I've managed to intermingle two different issues. I will tidy up my answer latterly - Out of time just at present. And/but I (still) disagree with some of your technical explanation - it does not matter too much short term. To say the signals voltages relative to ground is what is compared is 'interesting. If I understand the system (perhaps moot :-) ) then you could as well say it was their voltages wrt +1V that are compared - a comparator "usually" works on the absolute difference of the signals. Semantics may allow one to add wrt gnd but ... \$\endgroup\$ – Russell McMahon Sep 14 '19 at 7:33

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