Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 5)

Backup Scanner Module 5
Publication 1747-5.38
Overview
The 1747-BSN Backup Scanner Module provides a high-speed communication
channel between two modular SLC 500™ (5/02 or higher) processors. The 1747-
BSN backup system uses a set of modules, with one or more 1747-BSN modules
residing in the primary system and one or more complementary modules in the
secondary or backup system. The primary system controls the operation of
remote I/O, while the secondary system monitors communications via the high-
speed serial link (HSSL) and is available to take over control in the event of a
fault in the primary system.
The backup scanner has the capability to switch between two communication
channels. The first channel is configurable as Remote I/O (RIO) or Data
Highway Plus™ (DH+). The second channel is used to switch one RS232/485
channel in order to provide connection for electronic operator interfaces.
A backup system using the 1747-BSN modules supports up to eight BSN
modules in each processor chassis, connected by a local status link (LSL). Only I/
O residing in remote chassis is backed up. Local I/O is not backed up. Examples
of RIO and DH+ system configurations are shown in Figures 1 and 2 on page 6.
Note: During the transfer of control from one processor to
another (switchover), the output modules in the remote
chassis maintain their last state until the secondary
processor program takes control. The secondary
processor program is not synchronized with the primary
program.