ProSoft Technology PS-QS-1x10-0781 - Manual (Page 53)

FieldServer Configuration Manual Page 53 of 90
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8 PORT EXPANDER MODE - PEX MODE
Under certain conditions the FieldServer can be configured in a Port Expander Mode where statically configured
Map Descriptors are not required to retrieve data from a Server Node.
8.1 How Port Expansion Works:
When the FieldServer receives a poll from the Client Node, it scans its internal tables looking for a Map Descriptor
that matches the poll. If such a Map Descriptor is found, the FieldServer responds with data from the appropriate
Data Array. If no Map Descriptor is found, the FieldServer scans the list of configured Nodes and creates a Map
Descriptor (cache) to fetch the data from that Node and returns this data to the Client. The FieldServer will
continue to retrieve data from the Node for future polls from the Client Node. If the Client Node does not access
the data for longer than the time configured under Cache_Time_To_Live, (Refer to Appendix B.4) then the
FieldServer will stop reading the data and remove the Map Descriptor (cache).
8.2 Advantages of Port Expander Mode
Configuration is simpler - the FieldServer automatically creates and deletes Map Descriptors as required. If
mapping changes are made to a Client, the FieldServer usually does not need to be reconfigured.
8.3 Limitations of Port Expander Mode
Port Expander Mode does not work with all combinations of drivers.
If the FieldServer is used as a Pure Port Expander (Single Protocol) there is no restriction at all, e.g. Modbus RTU
Port Expander.
The following families of drivers support Port Expansion within the family:

Modbus RTU

Allen Bradley PCCC

Metasys® 10
8.4 Port Expander Write Options
Three possible scenarios exist for Writes in Port Expansion Mode:

A Temporary Read Map Descriptor already exists for the point being written.

A Temporary Write Map Descriptor already exists for the point being written.

No Temporary Map Descriptor exists for the point being written.
In the first two cases data is simply witten through the FieldServer to the Server using the existing Temporary Data
Arrays. Iin the third case, temporary Map Descriptors are created.
It is possible to configure the FieldServer to send an immediate acknowledgement of a write instead of waiting for
acknowledgement of successful receipt from the Client. The Node parameter Write_Ack_Option
11 needs to be
configured. Refer to Appendix B.4.
10 Metasys is a registered trademark of Johnson Controls, Inc
11 This setting only affects writes to points not configured/existing as read cache Map Descriptors. Writes to existing points on the FieldServer
are acknowledged immediately