Rockwell Automation 22-COMM-D DeviceNet Adapter - Manual (Page 61)

Using Explicit Messaging 6-5
The following table identifies the number of transaction blocks within a
scanner that are reserved for Explicit Messaging.
PLC / SLC Explicit Message Requests
Scanner Number of
Transaction Blocks Words in Each
Transaction Block Refer To
1747-SDN 10 32 Figure 6.3
1771-SDN 10 32 (two blocks can
be moved at once) Figure 6.2
Word Description
0 Command (Least Significant Byte)
The Command is a code that instructs the scanner how to administer the
request during each download.
00 = Ignore transaction block (empty)
01 = Execute this transaction block
02 = Get status of transaction TXID
03 = Reset all client/server transactions
04 = Delete this transaction block (available only for SLC)
05 – 255 = Reserved
TXID (Most Significant Byte)
The Transaction ID is a 1-byte integer between 1 and 255. It is assigned in the
ladder logic program when the processor creates and downloads a request to
the scanner. The scanner uses it to track the transaction to completion. It
returns this value with the response that matches the request downloaded by
the processor.
1 Size (Least Significant Byte)
The size of the service data is in bytes. Service data includes the words for the
class, instance, attribute, and any data. The maximum size is 58 bytes (29
words).
Port (Most Significant Byte)
The port that is used by the message is always zero (Channel A) on an SLC
scanner. It is zero (Channel A) or one (Channel B) for a PLC scanner.
2 Address (Least Significant Byte)
The node address of the slave device to which the transaction is sent. For the
Explicit Message to be successful, the slave device must be in the scanlist of
the scanner, and it must be online.
Service (Most Significant Byte)
Available services depend on the class and instance that you are using.
Refer to
Appendix C , DeviceNet Objects .
3 Class
Refer to
Appendix C , DeviceNet Objects , for available classes.
4 Instance
Refer to
Appendix C , DeviceNet Objects , for available instances.
5 Attribute
Refer to
Appendix C , DeviceNet Objects , for available attributes.
6 – 31 Request Data
This is data used for the message. For example, it may be the value written to a
parameter.