Additional Installation Tips
7.7 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
2
3
4
+5 V
0 V
1 K
1 K
120 ohm
120 ohm
120 ohm
+5 V
+5 V
0 V
0 V
0 V
0 V
0 V
0 V
1 K
1 K
1 K
1 K
VCC
VCC
(120 ohm)
Inside Panel PC
Panel PC
RS422
CAB8
CAB8
Bus
RS485
Shield
Bus termination
5358
2
14
15
6
19
7
8
1
2
3
4
5
6
7
8
2
15
3
16
17
4
14
8
7
55
50
17
1 2 3 4 5 6 7 8
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. See 1
in drawing above.
The Panel PCs have another type of recipient, so-called built-in “Fail Safe”, where
simple bus termination resistance is sufficient. See 2 in drawing above.
If other nodes on the RS485 network require pull-up and pull-down and the Panel
PC is at one end of the loop, one of the following procedures can be carried out:
•
Connect two 1k/0.25 W resistors in the 25-pole D-sub contact. See 3 in
drawing above. Set jumper pins 6-19.
•
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. See 4 in
drawing above.
Beijer Electronics, MAEN043
31
