Beijer Electronics EXTER K20m EN - Manual (Page 27)

Additional Installation Tips
Beijer Electronics, MAEN852A 27
6.7 Cable and Bus Termination RS485
– Use shielded and twisted pair cable.
The pair capacitance may not exceed 52.5 pF/m and area at least 0.25 mm
2
(AWG
24), if you want to use the maximum transfer distance and maximum transfer
speed.
– 0 V, the reference voltage for communication should be included in the cabling.
With two-way communication use two pairs; one pair for communication and
one pair for 0 V.
– The shield must be grounded at one end. The other end is usually grounded, but
with longer distances or when there is a difference in the ground potential, the
shield should be connected to the ground via 0.1 uF/250 V plastic capacitor to
prevent ground current in the braided shield.
A number of manufacturers recommend that the shield be grounded at each node.
Various manufacturers have different systems for bus termination. The RS485
standard does not describe how the “Fail Safe” function would be carried out, just
that the system should be able to handle the error.
Depending on the recipients’ design, the bus wires may be on the same level or
require pull-up or pull-down to ensure that no faulty signals are detected when the
bus is in resting mode (all transmitters are disconnected).
1. Some (older) operator panels had pull-up and pull-down resistance except for
the actual bus termination at 120 ohm, similar to Westermo and Profibus.
2. Newer panels have another type of recipient, so-called built-in “Fail Safe”, where
simple bus termination resistance is sufficient.
If other nodes on the RS485 network require pull-up and pull-down and the
operator panel is at one end of the loop, one of the following procedures can be
carried out:
3. Connect two 1k/0.25 W resistors in the 25-pole D-sub contact. Set jumper pins
6-19.
4. Use CAB8. It offers the option of bus termination with pull-up/-down. It is also
easy to connect the bus cable via the screw terminal block.
1 2 3
4
5358
+5 V
0 V
120 ohm
1K
1K
120 ohm
(120 ohm)
Inside operator panel
+5 V
1K
1K
0 V
0 V
CAB8 CAB8
Operator
panel
RS422 Bus
RS485
0 V
0 V
0 V
0 V
120 ohm
1K
1K
VCC
+5 V VCC 2.65 V
2.35 V Bus termination
Shield