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 highspeed

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.