Rockwell Automation 1747-BSN Backup Scanner Module - Manual (Page 59)

Publication 1747-UM010B-EN-P - September 2003
Operating Your SLC 500 Backup System 4-7
How the Backup System
Operates Some amount of support ladder program is necessary to make both
modules operate properly in a backup system.
Both processors have a transmitter program and a receiver program.
At system startup, the processor reads the Module Status Word (MSW)
and the System Status Word (SSW) from the 1747-BSN to determine its
state. If the processor is in the primary state, it executes the transmitter
program. If the processor is in the secondary state, it executes the
receiver program.
Primary Program
Each data block that can be transferred from the primary to secondary
SLC 5/0x is identified by its address in the M0/M1 files. The
application program must transfer each block using the following
procedure.
A MOV instruction is used to get the Data Transfer Status Word
(DTSW) and to see if the block can be transferred from the SLC 5/0x
to the 1747-BSN (if the data block status bit is clear). When this bit is
clear, the new data block must be copied to this corresponding M0
address. Then the data block control bit in the Data Transfer Control
Word (DTCW) must be set to advise the 1747-BSN that this new data
block is ready to transfer to the secondary system. This bit must be
held set until the corresponding bit in the DTSW is set by the
1747-BSN. The bit must then be cleared.
Secondary Program
For each block that can be received by the secondary SLC 5/0x, its
application program must do the following procedure.
A MOV instruction is used to get the Data Transfer Status Word
(DTSW) and to see if new data for the block is ready for reading in
the secondary 1747-BSN module. When this bit is set, the new data
block must be copied from the 1747-BSN M1 file. Then its
corresponding bit in the Data Transfer Handshake Word (DTHW)
must be set to advise the 1747-BSN that the data block was already
received. This bit must be held set until the corresponding bit in
DTSW is cleared by the 1747-BSN module. The bit must then be
cleared.