Is RS-485 a two-wire ore a three-wire system? It is most definitely a three wire system
(four plus one wire with respect to full-duplex operation).
The TIA standard (ANSI/TIA/EIA-485-A, page 15, A.4.1) requires the presence of a common return path between
all circuit grounds along the balanced line for proper operation.
The TIA standard defines a maximum common mode voltage range from -7V to +12V
on the signal lines A and B, measured against C (common ground). A TIA/EIA-485
system however with only two wires A and B (C generator and C receiver commons
not connected) can result in an unpredictable common mode voltage superimposed
on the interface lines A and B, caused either by electrostatic charging or
A 2-wire system often may work though due to
idle-line fail-safe resistors at the receiver inputs,
which can be considered as a loosely coupled common ground. Nevertheless this
operation mode cannot be recommended - what is working flawless in the laboratory
may not work reliable under real conditions in an industrial environment.
Where do we get the third wire? Many times the outer cable shield is used as the
third (fifth) wire. However, EKF recommends to use a two pair cable (three pairs
for full-duplex operation), with one or both wires of the additional pair as the
dedicated common ground. Connect these additional wires directly to the
pins 3, 6 & 5, 8 of the DB9 connector for proper grounding.
The optimum cable solution would comprise an inner shield for each signal twisted pair.
The inner shield can then be used for establishing the common ground between
TIA/EIA-485 nodes (connect to pins 3, 6 & 5, 8 of the DB9 connector).
An additional outer cable shield, that may cover the inner signal and ground cable pairs,
should be connected to the pin 1 of the connector (it is equivalent and sufficient to
connect the shield with the metallic shell of the DB9 connector). This shield should
be grounded at one point only (isolate the shield at the opposite cable end in order
to avoid any contact with the connector hood).
You may read a helpful
related to this topic.