Eia/Rs-485 Communications Wiring - Zener MSC-3 Instruction Manual

EIA/RS-485 Communications Wiring

The MSC-3 communications port is EIA/RS-485 compliant and is isolated from ground and other
circuits. This communications port is used by BACnet MS/TP and Modbus RTU protocols.
The communications system (communications cable, MSC-3 and other devices) needs to operate
reliably in a potentially electrically noisy environment. For best performance and to reduce the risk of
network failure, we recommend the following:
Best practice
Best practice for EIA/RS-485 communications requires 3 conductors and a shield. It is frequently
discussed in terms of being a two wire network, but this is not the case.
• Two conductors are used to carry the EIA/RS-485 data as a differential voltage signal. These wires
should ideally be twisted together so that any magnetically induced interference voltage will occur
equally in both conductors and be rejected by the differential nature of the EIA/RS-485 interface
circuit.
• The third conductor is used to keep the common connection (marked as terminal "C" in MSC-3) in
all the communications interfaces at the same potential, that is, keep the common mode voltage
at each interface within the limits specified by the standard.
• The Shield is connected to earth/ground at one end
capacitive coupling to nearby cables and other electrical noise sources.
One arrangement that meets these requirements using generic materials is as follows:
• Use a standard two pair shielded instrumentation cable. Internally, this will have a total of 4
conductors, physically arranged as two twisted pairs surrounded by an aluminium wrapper as a
screen. A bare "drain wire", in contact with the aluminium wrapper, makes an electrical connection
to the screen.
• One pair is used for the data signals. The other pair is used in parallel as the common wire. The
screen (drain wire) is connected to ground at one end only.
Cable jacket
Some cables may
also have a braided
screen (better, but
more expensive)
Terminating resistors
For best performance with long cable runs and high data rates a terminating resistor should be fitted
at each end of the cable run. Values of 100 or 120 Ohms are commonly used, connected between
the A and B terminals of the first and last devices on the cable run. For convenience, the MSC-3
EIA/RS-485 interface incorporates a 120 Ohm terminating resistor that is controlled by menu item
G167 TERMINATOR. See page 112 for details. A terminator should not be fitted to, or selected at,
intermediate devices on the communications cable.
2. Why one end only? Because there will be voltage differences between various ground points in an electrical
installation that contain significant amounts of electrical noise and occasionally significant power frequency
voltages during electrical fault events. We don't want these voltages to cause a current in the communications
cable screen (because it would then induce a voltage in the cable conductors inside), so we ground the screen
at one point only.
only and provides protection against
2
Foil screen
Drain wire for
screen connection
IM00120 IM00120
MSC-3 Instruction Manual
Use this pair for
the A & B wires
Use this pair, shorted
together, for the
common (C) connection
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