Siemens 505-2571 - Manual (Page 22)

Module Operation 3-3 SIMATIC 505-2571 Program Port Expander Manual
The requests are processed during the discrete portion of the controller scan
(refer to Chapter 1 of the SIMATIC 545/555/575 System Manual for a
discussion of the timeline). Thus, the scan interval to process the requests
from the PPX:505-2571 (or any other Special Function module) cannot be
lengthened by manipulating the time slices, which affect only the analog
portion of the scan.
To maximize throughput, the PPX:505-2571 should be placed in the local
base with the controller. A PPX:505-2571 module in the local base has the
advantage of 8-bit parallel data transfer to the controller across the
backplane. A remote I/O module must communicate with the controller via a
serial data stream. A local base module also has a few milliseconds
advantage because it is not subject to the remote base timing restrictions.
These advantages add up to an order of magnitude improvement in local
base execution time over remote base.
Whether the effect on scan time is significant or not depends upon your
application. If your controller is lightly loaded and/or task code requests
routed through the PPX:505-2571 are simple, then you may not notice the
effect at all. On the other hand, if the controller is heavily loaded, scan
times are critical, and complex requests are being sent to the controller, you
may have to make some trade-offs between controller performance and the
time required to respond to task code requests. In general, you will have to
make any adjustments based on actual experience.