Rockwell Automation 2711 PANELBUILDER SOFTWARE USER MANUAL - Manual (Page 381)

PLC Programming Considerations
Appendix A
A-17
A typical remote I/O block transfer takes anywhere from 55 to 200 ms to
complete, depending on factors such as PLC processor type, program scan
time, I/O scan time, number of block transfers on the same channel, and
number of block transfers in the queue. These typical times could become
much greater if recommended practices are not considered.
Recommended Practices
The most significant factors determining block transfer times are as
follows:
Number of Block Transfers on the Same Scanner Channel
Remote I/O scanners generally will perform only one block transfer for
each chassis (Rack Assignment terminal) in a given I/O scan. Remaining
block transfer requests will be queued. Do not assign more PanelView
terminal block transfers than necessary—in other words a 32 word block
transfer is much faster than two 16 word block transfers.
If many other intelligent block transfer I/O modules are on the same
channel, consider enabling them sequentially and only as often as
necessary. This will improve the response times of those block transfers
that are more critical. PanelView terminal block transfers are normally
programmed for constant operation to meet operator interface needs.
If more than one I/O scanner channel is available, and many block transfer
devices are involved, block transfer times will be improved by distributing
those devices evenly among the channels. Alternatively, non-critical block
transfers could be grouped separately on a less critical I/O channel.
Number of I/O Chassis on the Same Scanner Channel
I/O scan time is mainly determined by the number of chassis on the same
scanner channel, and is also a significant factor in block transfer timing as
well as discrete data transfer times. If more than one scanner channel is
available, it may be desirable to distribute I/O chassis (or terminals) evenly
among the available channels.
Program Scan Time
An excessively long program scan can be a limiting factor in block transfer
timing. This usually only applies to larger PLC-3 applications. In these
cases, the effective program scan can be cut in half by creating a
sub-routine which could be accessed from different points in the program.
The effective update times of the programming within the sub-routine
would be reduced.