Rockwell Automation 1747-SDN SLC 500 DeviceNet Scanner Module User Manual - Manual (Page 148)

Publication 1747-UM655B-EN-P - June 2007
148 Explicit Message Program Control
The following figure describes the format and mapping of transaction
blocks for request and response messages in the module.
Transaction Blocks
How the Processor and Module Manage Messages
File transfer operations between the processor and the scanner always
originate in the processor. The module can wait only for the processor
to download a transaction block to the module or request an upload
of a transaction block from the module.
Once an Explicit Message Request transaction block is downloaded to
the module, a ladder logic program in the processor polls the module
for the transaction block containing the Explicit Message Response for
that request. This is done by the processor with an M1 file transfer on
the module.
Depending on the network load, the scanner could take a few
seconds to complete the request. When a response is loaded, bit 15 of
the module status register is set to 1. The program may have to poll
the module a number of times before the scanner returns a Response
Transaction Block.
The module recognizes I/O data and control as higher priorities over
explicit messaging on the DeviceNet network.
Transaction Header
(3 Words)
TXID CMD/Status
Port Size
Service MAC ID
Word 224
Word 225
Word 226
Service Data
Transaction #1
Word 255
15 0
Transaction
Header (3 Words)
TXID CMD/Status
Port Size
Service MAC ID
Word 224
Word 225
Word 226
Service Response
Data
Format of 32-word M0 Transfer File for
Explicit Message Request
Word 255
15 0
Class
Instance
Attribute (optional
Transaction #1
Format of 32-word M1 Transfer File for
Explicit Message Response