Communication methods, End-of-scan transfer – Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual User Manual
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Performance and Theory of Operations
Chapter 7
7-2
For the check-VME-status-file command,
If you want the VME:
Set the NOCV bit to:
to check the VME status
file for changes during
every program scan
zero.
Important:
This causes the performance of message instruction
transfers to degrade by a factor of 2 or greater.
not to check the VME
status file
one.
You can still check the VME status file by issuing the “check VME
status file” command using the message instruction “CSF.”
See Chapter 4 for information on the check-VME-status-file command.
Because the PLC processor uses the message instruction for VME
transfers, it competes with other message-instruction activity going to the
communication processor that handles this traffic in the PLC processor.
For example, having a programming terminal attached to the PLC
processor can have a large impact on VME transfers using the message
instruction. If the PLC processor is attached to a large DH+ network, with
many transfers going to the PLC-5/VME processor, then this will impact
it as well.
The PLC processor initiates the communication with the VME host CPU in
two ways:
configuring an end-of-scan transfer
through ladder logic and the message instruction
Both methods use the dual-port memory as an intermediate buffer between
the PLC processor and VME coprocessor.
It is important to note that the two methods exchange data with the dual
port in different ways. The transfer can be either during the housekeeping
section of the PLC scan or the ladder logic section of the scan. The effect
on total PLC scan is the same but it is important to make the distinction
between them.
End-of-Scan Transfer
This method uses the copy-to-VME and copy-from-VME end-of-scan
transfer commands. You can execute one or both commands in a single
ladder scan and transfer up to 1000 words each.
When using this method, the housekeeping time is impacted. This is
because the transfer delays the ladder processor until the transfer with the
dual-port is complete; therefore, it is synchronous with and occurs only
once per scan.
Communication Methods