Rockwell Automation 1747-SDN SLC 500 DeviceNet Scanner Module User Manual - Manual (Page 147)

Publication 1747-UM655B-EN-P - June 2007
Explicit Message Program Control 147
•
TXID (transaction ID) – when you create and download a
request to the scanner, the processor’s ladder logic program
assigns a TXID to the transaction. This is a one-byte integer in
the range of 1…255. The scanner uses this value to track the
transaction to completion, and returns the value with the
response that matches the request downloaded by the
processor. The ladder logic program monitors rollover and
usage of TXID values.
•
Size – the size of the transaction body in bytes. The transaction
body can be as many as 29 words (58 bytes) in length. If the size
exceeds 29 words, an error code will be returned.
•
Port – the DeviceNet port (zero) where the transaction is routed.
•
MAC ID (node address) – the DeviceNet network address of the
slave device where the transaction is sent. This value can range
from 0…63.
The port and MAC ID attributes coupled together identify the
target slave device. The slave device must be listed in the
module’s scanlist and be online for the Explicit Message
transaction to be completed successfully.
•
Service – for each Explicit Message Request and Response, the
service attribute contains the service request and response codes
that match the corresponding request for the TXID.
Transaction Header
(3 words)
TXID CMD/Status
Port Size
Service MAC ID
Word 224
Word 225
Word 226
Transaction Body (29
Words)
One Word = Two Bytes = 16 Bits
Word 255
15 0