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SD39RIO-1
2.5.6
Remote I/O Rack IOBUS Shunt
This section describes the IOBUS shunt and how to use it in conjunction with a Remote I/O Rack. Since the
layout of an IOBUS also involves other types of racks (i.e., MODULRAC or SIXRAC) refer to their
Installation and Service Instructions for using IOBUS shunts with them.
IOBUS shunt installation must precede the installation of I/O modules. A Remote I/O Rack provides IOBUS
connections on each end of its backplane as shown in Figures 2-4 and 2-9. On the right side of the Remote
I/O Rack's backplane, there is one IOBUS shunt socket designated W4. The W4 designation is printed on the
backplane underneath the removable IOBUS shunt. The shunt is secured to the backplane via its captive
mounting screw. The shunt must be oriented to either continue the IOBUS to another rack or terminate the
IOBUS at an end of the backplane on the Remote I/O Rack.
The following details are provided to assist in determining the orientation of the IOBUS shunt:
•
The removable IOBUS shunt is a two-position device. The shunt's label indicates its two possible
orientations as shown in Figures 2-4, 2-9, and 2-21.
•
If the shunt is oriented so "I/O BUS CONTINUE" is positioned on top, the IOBUS will be continued
from the Remote I/O Rack's backplane to another rack.
•
If the shunt is oriented so "I/O BUS TERMINATE" is positioned on top, the IOBUS is terminated at the
right side of the Remote I/O Rack. The shunt terminates the IOBUS in its characteristic impedance.
2.5.7 IOBUS Attenuators
When the IOBUS is 30 meters or longer in length, one to three attenuators must be installed. The total length
of the IOBUS determines the quantity of attenuators required. One attenuator (part number 16405-1) is
supplied with each Remote I/O Rack. The use of attenuators is a precaution to prevent signal reflections from
interfering with IOBUS communications in the event one side of the IOBUS (A or B) is unintentionally
disconnected or severed in a plant or process environment.
Physically, an attenuator resembles a shunt as shown in Figure 2-21. Either type can be secured to a rack's
backplane using its captive mounting screw; however, they differ electrically. Unlike a shunt, an attenuator is
only to be installed in one orientation (i.e. the I/O BUS CONTINUE orientation). When an attenuator is to be
installed on a specific Remote I/O Rack, it replaces a shunt in the W4 socket located on the right side of a
rack's backplane.
January 2001
INSTALLATION
2-31
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