Siemens 505-2571 - Manual (Page 21)

Module Operation 3-2 SIMATIC 505-2571 Program Port Expander Manual
The PPX:505-2571 is designed to work with operator interface devices which
adhere to the Non–Intelligent Terminal Protocol (NITP). Up to four
supported devices may concurrently communicate with the controller. The
PPX:505-2571 will provide buffering for the commands from the operator
interface and responses from the controller. Commands are sent to the
controller in the order they are received; no port has priority over any other
port. Only one command per port can be pending at a given time; a response
must be sent to the operator interface before a subsequent command will be
accepted. With a few minor restrictions, the PPX:505-2571 port should
function the same as the controller program port.
NOTE: The ACTIVE indicator should remain on at all times after power up.
Should the ACTIVE indicator go out or exhibit a blink pattern, refer to the
troubleshooting section. When data is sent to a port the RCV indicator for
that port should light briefly. Similarly, when data is sent from a port the
XMT indicator should light briefly.
This release of the PPX:505-2571 does not support Transparent Byte
Protocol (TBP). In addition, the PPX:505-2571 does not support an ASCII
print data stream.
Because the PPX:505-2571 is a Special Function module, the controller scan
time will be longer when the module is logged into the controller. The effect
on scan time depends on several factors, including the type of request the
controller is processing and the number of requests processed per scan.
Some requests, such as a Read or Write memory location, can be processed
relative quickly. Others, such as Find, consume significant controller CPU
resources.
The SIMATIC 545 CPU can process a maximum of eight requests per scan.
It is possible for the user, via Aux 16, to set the number of requests (task
codes) that are processed each scan, for a minimum of one to a maximum of
eight. A lower number should normally result in a faster controller scan
time; the amount of increase depends on the type of request. However, since
more requests can be processed each scan, this may improve data
throughput through the PPX:505-2571.
Effect of Controller
Scan Time