Siemens 505-2571

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

SIMA

TIC 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.

o 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.