Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 21)

Installing a Backup System
Chapter 2
2-6
For a PLC-3 backup system, it is important to choose suitable power
supplies and make sure they are connected properly.
To reduce the chance of inadvertent shutdowns due to loss of power, we
recommend using two separate 1775-P1 or -P3 power supplies. Use one
power supply for the primary processor and the other power supply for the
backup processor. Further, connect each power supply to a separate source
of AC power.
Important: In backup systems, never use the processor supply for
powering an I/O chassis. A processor failure could cause the I/O to fail
also.
For detailed information about installing power supplies, refer to the
PLC-3 Programmable Controller Installation and Operation Manual
(publication 1775-6.7.1).
All grounding requirements applicable to a stand-alone PLC-3 or
PLC-3/10 processor also apply to backup systems. In addition, you need to
do the following:
When using a 1775-P1 power supply, connect the system common studs
together on the power supplies of the primary and backup processors .
Use 1-inch braid or 8 AWG wire.
If the primary and backup processors are in separate enclosures, connect
the ground bus plates of the primary and backup processors together.
Use 1-inch braid or 8 AWG wire.
Connect the ground bus plate of either the primary or backup processor,
but not both, to earth ground.
Use star washers to assure a positive ground connection between metal
surfaces and ground lugs.
For detailed information about grounding, see Wiring and Grounding
Guidelines (publication 1770-4.1).
Use shielded cable for I/O channel wiring as specified in the PLC-3
Programmable Controller Installation and Operation Manual (publication
1775-6.7.1) to minimize noise pickup. Also, use shielded cable for
peer-to-peer communication channels, backup communication channels,
and Data Highway/Data Highway Plus channels. We recommend using
1770-CD twinaxial cable (Belden 9463), or equivalent.
Power Supply Considerations
Grounding Considerations
Shielding Considerations