Rockwell Automation 1775-MX_S4A,D17756.3.1 User Manual PLC-3 BACKUP CONC(OR.DU1 - Manual (Page 17)

Installing a Backup System
Chapter 2
2-2
Important: The number one scanner module (thumbwheel set to 1) in
both primary and backup processors must be a 1775-S4A or -S5 for PLC-3
based processors, or a 1775-SR or -SR5 for PLC-3/10 based processors.
Figure 2.1 on page 2-3 shows typical connections for a PLC-3 backup
system using scanner backup. Figure 2.2 on page 2-4 shows typical
connections for a PLC-3 backup system using MX backup. Figure 2.3 on
page 2-5 shows typical connections for a PLC-3/10 backup system using
scanner backup.