Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 57)

Chapter 4
Ladder-Program Interfaces
4-10
The PLC-5/VME can automatically read and write every ladder scan
to the the VMEbus without ladder-logic programming. You can configure
this function using your programming software or the ladder program
itself. See your programming software documentation for specific
information about where and how to configure this function in the
software.
Important: If you use ladder logic to make changes to your VME status
file, you must set word 28, bit 8 to 1 to apply the changes to your VME
processor.
You can only enable these operations when the PLC-5/VME processor is in
Run mode. You can specify up to 1000 words as the transfer length.
These words must be contiguous elements in files, but the transfer can span
files (see Figure 5.1).
The PLC-5/VME processor does not have the same programmable
synchronization control as the 6008-LTV processor.
The 6008-LTV processor allows:
copy transfer before or after the I/O update during housekeeping
transfer to 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 the ladder scan until
the transfer is complete.
The PLC-5/VME processor allows 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 comes from the
previous ladder scan. If the new data is not ready from the VMEbus, then
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.
Continuous Copy
to/from VME