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Operating Your SLC 500 Backup System
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.
Backup System Theory of Operation
A redundant system using the 1747-BSN can be initially defined as
an “Asynchronized Data Transfer” system. While the Input Image
Table is automatically acquired from the RIO link by the secondary
system, the Data Table is transferred by an application program
written by the user. It is recommended that you minimize the amount
of data to be transferred from the primary to the secondary processor
because the program logic generates the same outputs based on the
same inputs.
All timer and counter values must be transferred at least once because
the primary and backup processors may have started at different times
and are not synchronized. Therefore, timer and counter data may be
different in the two processors. For example, after a faulted processor
is repaired and reinstalled as a backup, the timer and counter
accumulated values, as well as the control words, in the two
processors are different.
The number of data table words to be transferred from a primary to a
secondary processor is dependent on the user program and/or user
architecture.
Publication 1747-6.22