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. The secondary

processor program is not synchronized with the primary program.