The RS-485 differential line consists of two pins:A aka '-' aka Data - (D-) aka TxD-/RxD- aka inverting pinB aka '+' aka Data + (D+) aka TxD+/RxD+ aka non-inverting pinSC aka G aka reference pin. Ground
A ground wire is mandatory !Differential links provides a gain (x2) and noise immunity.But it is imperative that the receiver and transmitter share the same reference voltage OV ie GNDThe only exception are in the case when there is use of a flux, magnetic with transformers or optical with fibre or optocoupler
But it is imperative that the receiver and transmitter share the same reference voltage OV ie GND
WireRS-485 is designed to be a balanced system. Simply put, this means there are two wires, other than ground, that are used to transmit the signal.
Signal ground, don't forget it. While a differential signal does not require a signal ground to communicate, the ground wire serves an important purpose. Over a distance of hundreds or thousands of feet there can be very significant differences in the voltage level of "ground." RS-485 networks can typically maintain correct data with a difference of -7 to +12 Volts. If the grounds differ more than that amount, data will be lost and often the port itself will be damaged. The function of the signal ground wire is to tie the signal ground of each of the nodes to one common ground. However, if the differences in signal grounds is too great, further attention is necessary. Optical isolation is the cure for this problem.
All three devices are grounded but there is no indication that the grounds themselves are connected together.
Yes, but the ground symbol, over a long distance, suggests to me that they are locally grounded.
The balanced A/B pair is to cancel out induced voltages on a long cable run, right?
So this is negated by supplying ground over a long cable run.
For successful design and deployment of RS-485 networks I strongly recommend that you read and comprehend these application note documents:Application Note 847 FAILSAFE Biasing of Differential Buseshttp://www.ti.com/lit/an/snla031/snla031.pdfAN-1057 Ten Ways to Bulletproof RS-485 Interfaceshttp://www.ti.com/lit/an/snla049b/snla049b.pdfB&B Electonics RS-422 and RS-485 Applications eBookhttp://www.bb-elec.com/Learning-Center/All-White-Papers/Serial/RS-422-and-RS-485-Applications-eBook/RS-422-RS-485-eBook-graphics-embedded.pdf
Although the potential difference between the data-pair conductors determines the signal without officially involving ground, the bus needs a ground wire to provide a return path for induced common-mode noise and currents, such as the receivers' input current. A typical mistake is to connect two nodes with only two wires. If you do this, the system may radiate high levels of EMI, because the common-mode return current finds its way back to the source, regardless of where the loop takes it. An intentional ground provides a low-impedance path in a known location, thus reducing emissions.