The recommendation is to use shielded cables as electrostatic “pipes” between apparatus and
systems, and to run heavy gauge equi-potential bond wires alongside all shielded cables.
When a shielded cable runs through the metallic wall of an enclosure or machine, it is
recommended in IEC 1000–5–2 that the shield should be connected over its full perimeter to
the wall, preferably using a conducting adapter, and not via a pigtail wire connection to an
earth ground bolt. Shields must be connected to every enclosure wall or machine cover that
they pass through.
Analog and RS232 Cables
Providing an earth ground for both ends of the shield for analog circuits provides the perfect
electrical environment for the twisted pair cable as the loop consists of signal and return, in a
perfectly balanced circuit arrangement, with connection to the common of the input circuitry
made at the module terminals. RS232 cables are handled in the same way.
Multidrop Cables
RS422 twin twisted pair, and RS485 single twisted pair cables also require a 0V link, which
has often been provided in the past by the cable shield. It is now recommended that you use
triple twisted pair cabling for RS422 links, and twin twisted pair cable for RS485 links. This
is because the extra pair can be used as the 0V inter-system link. With loop DC power
supplies earth grounded in both systems, earth loops are created in this manner via the inter-
system 0V link. The installation guides encourage earth loops, which are maintained at a low
impedance by using heavy equi-potential bond wires.
To account for non–European
installations using single-end earth grounds, and sites with far from ideal earth ground
characteristics, we recommend the addition of 100 ohm resistors at each 0V link
connection in network and communications cables.
Shielded Cables within Enclosures
When you run cables between PLC items within an enclosure which also contains susceptible
electronic equipment from other manufacturers, remember that these cables may be a source
of RF emissions. There are ways to minimize this risk. Standard data cables connecting PLCs
and/or operator interfaces should be routed well away from other equipment and their
associated cabling. You can make special serial cables where the cable shield is connected to
the enclosure’s earth ground at both ends, the same way as external cables are connected.
DL205 Installation and I/O Manual, 2nd Edition
A-7
Appendix A: European Union Directives (CE)
A
2
3
A
5
H
7
10
A
10
11
12
13
14
A
B
C
D
RXD
Master
RXD
TXD 0V
+
–
+
–
Slave n
TXD 0V
+
–
+
–
Last Slave
RXD
TXD
0V
+
–
+
–
Termination
100
100
Termination
100
