1747-6.22, Backup Scanner User Manual

Programming Techniques 9-177
Data Transfer Schemes This manual provides two possible methods for transferring data from
a primary to a secondary processor
The first programming alternative transfers a data block per program
scan. With this method, the application program defines the order that
is used for transferring the data blocks. This programming scheme
results in a lower overall data transfer throughput and has a minimal
impact in the program scan time.
The second programming alternative transfers each data block at its
maximum possible throughput. This method is shown in the ladder
program example earlier in this chapter. With this method, the
application program tries to make all the possible data transfers in
each program scan, according to the DTSW bits.
Data Transfer Method 1
With this alternative, the primary SLC 5/0x uses a counter to select
the 1747-BSN module to which the data block is sent. The primary
SLC 5/0x also uses a second counter to select which block is sent to
the BSN module. If the system has only one 1747-BSN, only one
counter is used. The following procedure is used in the primary SLC
5/0x for sending the data blocks:
1. Read all the DTSWs from the respective 1747-BSN modules.
2. Clear all DTCW bits corresponding to the data blocks that are
marked as already transferred to secondary system in the
DTSWs. Send the data blocks to their respective 1747-BSN
modules.
3. Look at the counters and DTSWs to check whether the next data
block that is sent is able to receive new data from the SLC 5/0x:
• If the next data block is not able to receive new data, increment
the data block counter and check whether it is time to select the
next 1747-BSN module.
• If the next data block is able to receive new data, increment the
module counter, restart the data block counter and repeat step 3.
(Repeat this step until all possible data blocks are scanned.)
4. Copy the data block from the SLC 5/0x to the 1747-BSN module
indicated by the module counter.
5. Advise the 1747-BSN module that the data block is ready
through the DTCW (set the bit corresponding to the data block in
this word).
6. Increment the data block counter and check whether it is time to
select the next 1747-BSN module. If it is time to select the next
module, increment the module counter and restart the data block
counter.
7. Start step 1 in the next program scan.
Publication 1747-6.22