Rockwell Automation DeviceNet Communications Module - Manual (Page 65)

Using Explicit Messaging 7-5
Table 7.2 identifies the number of transaction blocks within a
scanner that are reserved for Explicit Messaging.
Table 7.2 – Number of Transaction Blocks Reserved for Explicit Messaging
Scanner
No. of Transaction
Blocks
Words in Each
Transaction Block Refer To
1747-SDN 10 32 Figure 7.3
1771-SDN 10 32 (two blocks can
be moved at once) Figure 7.2
Table 7.3 – PLC / SLC Explicit Message Requests
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.