FieldServer FS-8700-24 instruction manual Appendix A. Advanced Topics Appendix A.1. Write-throughs

FS-8700-24 ControlNet Driver Manual Page 11 of 12

Appendix A. Advanced Topics

Appendix A.1. Write-throughs

Some FieldServer drivers support “write-throughs”. This is the ability of a Read Map Descriptor to automatically create a temporary write Map Descriptor to update the value of an external R/W (Read or Write) memory location. Since the ControlNet card contains separate Read-Only (Output) and Write-Only (Input) Buffers, write-throughs are not supported on ControlNet messaging, but are supported for PCCC messaging. This is because PCCC messages are embedded within ControlNet messages allowing for the receiving device to process the embedded write message.

Appendix A.2. Map Descriptor functions

The diagram below explains the difference between the use of WRBC (Write Block Continuous) and RDBC (Read Block Continuous) Map Descriptors. Note that WRBC Map Descriptors write data from the FieldServer’s Data Arrays into the output data memory area of the card whereas RDBC Map Descriptors place data from the card’s input data memory area into the FieldServer’s Data Arrays. The IO data memory areas are kept in RAM on the card and are defined by Connection Control Blocks on the card.

FieldServer

Data Array Data Array

WRBC Mapdesc RDBC Mapdesc

SST 5136-CN Card

ControlNet Device

Connection

Control

Block 01

Connection

Control

Block 128

Input data memory

area

Output data memory

area

.

.

.

Input data memory

area

Output data memory

area

Output data memory

area

Input data memory

area

ControlNet

Network

Note:

1.For ControlNet messaging the target connection is specified by Node_Name & CN_Instance.

2.For PCCC messaging the target connection is specified by Node_Name & Block_Number.

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