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4
Wiring the Device
4.2
Wiring Suppression Circuits
Equip inductive output loads with suppression circuits
(snubbers) that limit the voltage rise on turn off. Use the fol-
lowing guidelines. Note that the effectiveness of a given
design is dependent on the application, and its adequacy
must be verified. Be sure that all components are rated for
use in the application.
4.2.1
AC Relay Output Suppression
When using a relay or AC output to switch 115 V/230 VAC
loads, place RC snubbers across the relay contacts or the
AC switch as shown in Figure 4.2 below. Use a metal oxide
varistor (MOV) to limit peak voltage. Ensure that the working
voltage of the MOV is at least 20% greater than the nominal
line voltage.
R
C
S7-I/O
Figure 4.2 AC Load with Snubber Across Relay
4.2.2
DC Transistor Output Suppression
The S7-I/O DC outputs contain zener diode snubbers that
may be adequate for your application. Use external sup-
pression diodes for large or frequently switched inductive
loads to prevent overpowering the internal diodes.
Note: Do not use RC snubbers.
See Figure 4.3.
12
MOV
Inductor
+VDC
S7-I/O
+VDC
S7-I/O
Figure 4.3 DC Loads with Diodes Across Load
4.2.3
Communications Wiring
The S7-I/O device is designed to be connected to the
ACCESS system by means of a SEAbus RS-485 connec-
tion. This is a shielded twisted-pair connection that uses the
communications port at the bottom of the S7-I/O base unit.
The preferred method to connect ACCESS devices is a
loop, so that a break in the cable will not prevent communi-
cations. Refer to the ACCESS Systems Installation Guide
(Manual No. SG-6028) for detailed instructions for wiring
devices.
RS-485 connections are made with shielded twisted pair
cable. The data wires are labeled "Transmit/Receive +" and
"Transmit/Receive -," and are wired in parallel. Connect the
shield at one end of the communications link only.
The pinouts of the cable provided with the S7-I/O device is
shown in Figure 4.4.
Siemens Energy & Automation, Inc.
Inductor
Inductor
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