1747-6.22, Backup Scanner User Manual

9-178 Programming Techniques
Publication 1747-6.22
To receive the data blocks, the secondary SLC 5/0x application
program uses the following procedure:
1. Read all the DTSWs from the respective 1747-BSN modules.
2. Clear all DTHW bits corresponding to the data blocks that were
previously read and that no longer have a data ready bit set for
them in the DTSWs. Send all the DTHWs to their respective
1747-BSN modules.
3. Look at the counters and DTSWs to check whether the next data
block that is received is ready in the 1747-BSN module
• If the next data block is not ready, increment the data block
counter and check whether it is time to select the next 1747-
BSN module.
• If the next data block is ready, 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 1747-BSN module to the
SLC 5/0x.
5. Advise the 1747-BSN module that the data block was already
read through the DTHW (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.
The worst scenario with this transfer method is a system that has eight
1747-BSN modules in which each one is transferring 16 data blocks.
The total number of data blocks is 128 and the backup system spends
128 program scans to transfer all the data blocks.
Data Transfer Method 2
With method 2, the application program in the SLC 5/0x considers
each data block independent from the other data blocks. This is
shown in the ladder program example earlier in this chapter for one
set of BSN modules. Reproduce this logic for additional sets of BSN
modules, being sure to update the slot number in the M-file addresses
for each set. 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.