Campbell Scientific MD485 RS-485 Multidrop Interface - Manual (Page 20)

MD485 RS-485 Multidrop Interface
The RS-485 specification also recommends connecting a 100 ohm resistor of at
least 0.5 W in series between each node’s signal ground and the network’s
ground wire. This resistor is in the MD485, between the terminal block
connection marked with the ground symbol and the MD485 power ground.
This way, if the ground potentials of two nodes vary, the resistors limit the
current in the ground wire.
See Figure 1, Figure 3, and Section 3.3 for more information on ground
connections.
8.3 Protection and Isolation
The MD485 incorporates gas tubes and multilayer varistors on the RS-485 port
for protection against ESD and surge. The MD485 passes IEC61000-4-2, test
level 4 for both contact and air discharge. It also passes IEC61000-4-5, test
level 3 for surge immunity.
If large ground potentials exist, optical isolation may be desired. One
recommended optical isolator is B&B’s Model 485OP, which provides 2 kV
isolation. If most of the nodes in the network close together (limited ground
differences), and one node is located at a distance (large ground difference),
then a single optical isolator can be located on the distant node. If all of the
nodes are distant from each other, optical isolation will be needed at every
node.
The Model 485OP can also be used as a repeater to extend the network.
8.4 Termination
The RS-485 spec says to use termination. For high baud rates (>115k) and long
cable runs, this is true.
In most equipment though, with maximum speeds of 115 kbit, it is
unnecessary. Adding termination dramatically increases power consumption,
and complicates system design. It rarely solves problems when used in the
kilobit data range.
If power is not a concern, and signal integrity is questioned, the network can be
terminated with a 120 ohm resistor at the extreme ends of the line. There
should be no more than two terminations in a system that does not use
repeaters.
8.5 Summary
• Use CABLE2TP-L 2-twisted-pair cable with shield and Santoprene jacket
for most installations.
• Connect the MD485’s ground lug to earth ground with an 8 AWG wire.
This connection should be as short as possible.
• Connect the “signal ground” wire between MD485s.
• Ground the shield at one end only (to earth ground).
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