Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 65)

Chapter 5
Commands
5-4
Copy Synchronization
The PLC-5/VME processor does not have the same programmable
synchronization control as does the 6008-LTV processor.
The 6008-LTV processor allows the copy transfer to:
happen before or after the I/O update during housekeeping
be asynchronous or synchronous with the ladder scan
In other words, the ladder scan would keep going (regardless of whether
the VME transfer finished or not) rather than holding until the transfer
is complete.
The PLC-5/VME processor allows the copying of data between the
VMEbus and the PLC-5/VME’s data table:
during the housekeeping of the ladder processor
concurrently with the I/O update
The data coming from the VMEbus is buffered and was collected during
the previous ladder scan. If the new data is not ready from the VMEbus,
the housekeeping is held up until the new data is available. The data going
from the PLC-5/VME to the VMEbus is transferred into VME during the
next ladder scan, just after housekeeping. There is a separate on-board
coprocessor that handles all VME transfers; and it is this processor that is
sending data to the VMEbus during the ladder scan.
You can read the processor’s input table. Because the transfer occurs
asynchronously with the I/O scan, however, values obtained from the input
table would likely be a mix of most recent values and values from the
previous scan cycle.
See Appendix A for examples of the commands and Chapter 7 for details
about performance and operation.
Error Codes
These are errors reported during the repeated continuous-copy operations
initiated by the continuous-copy-to-VME and continuous-copy-from-VME
commands. The existence of the error can be determined by examining the
copy-to-state and copy-from-state fields in the command control register.
The error code itself can be found in the VME status file.