RSTi-OM User Manual
Section 13
GFK-3212A
Jun 2021
Functionality Descriptions
136
13.3
ISDU Handling
The IOLM provides a very flexible ISDU interface that is used by all supported industrial protocols. The
ISDU interface contains the following:
•
An ISDU
request
may contain one or multiple individual ISDU read and/or write
commands
.
•
Individual ISDU command-based byte-swapping capabilities.
•
Variable-sized command structures to allow access to a wide range of ISDU block sizes.
•
A single ISDU request may contain as many ISDU read and/or write commands as allowed by
the industrial protocol payload. For example, if an industrial protocol provides up to 500
bytes read/write payloads, then an ISDU request may contain multiple commands of various
lengths that can total up to 500 bytes in length.
•
For the ControlLogix family of EtherNet/IP PLCs, both blocking and non-blocking ISDU request
methods are provided.
o
The IOLM implements blocking ISDU requests by not responding to an ISDU request
message until all commands have been processed.
o
The IOLM implements non-blocking ISDU requests by:
•
Respond to an ISDU request message immediately after receiving and verifying the ISDU
request.
•
Requiring the PLC to monitor the ISDU request status with read messages. The IOLM will not
return a completed status until all of the ISDU commands have been processed.
13.3.1
ISDU Request/Response Structure
ISDU requests may contain a single command or multiple, nested commands. This subsection
discusses the following:
Single ISDU Command Request
This illustrates a single ISDU command request.
Figure 98: Single Command ISDU Request/Response
