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