Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 108)

Performance and Theory of Operations
Chapter 7
7-3
The PLC scan-time impact for either a read or write transfer with the
dual-port memory can be calculated as:
Transfer time = 22.41
µ
s + (2.332
µ
s)N + 0.83
µ
s(N–1)
Where: Is the:
22.41
µ
s dual port set-up time
(2.332
µ
s)N dual port access per word
0.83
µ
s(N–1) VME coprocessor loop time
N number of words to transfer
Note: a transfer of 100 words = 337.78
µ
s
Ladder-Logic Method
The ladder-logic method uses four VME commands:
Copy to VME
Copy from VME
Send VME interrupt
Check VME status file
When scanned by the ladder-logic processor, the message instruction sets a
flag for the VME coprocessor. Once the coprocessor sees the flag, it
examines the command to determine the requested function and the
address of the data; it then executes the transfers.
There are some additional considerations when using this method:
It adds additional throughput time to the completion of the message
instruction because the coprocessor has to examine the
requested command
Because it is a standard message, the VME messages have to compete
with the other message-instruction activity in the PLC processor, so
timing is subject to many variables. For example, having a
programming terminal attached to the PLC processor increases the
transfer time.
Once the data is exchanged with the dual port, the ladder-logic processor
starts or is allowed to continue scanning the program and the VME
coprocessor asynchronously passes the data between the dual-port and
VME memory.
When using the message instruction, the ladder scan is impacted. The
message instruction transfer is totally asynchronous with the ladder scan
and may start and complete anywhere within the scan. It possibly could
take more than one scan to complete in a very fast program or could
happen more than once per scan with a continuous message in a very
long program.