Rockwell Automation 2755-SNx Adaptascan Bar Code Readers - Manual (Page 190)

10–28 Communicating with an SLC 5/03 Processor on a DeviceNet Network using Explicit Messaging
Publication 2755-6.8
The following figure describes the format and mapping of
transaction blocks for request and response messages in the scanner
module.
service
port
TXID command
size
MAC ID
Service Data
word 224
Transaction
Header
(3 words)
word 225
15 0
Class
Instance
Attribute (optional)
Transaction #1
Format of 32-word M0 Transfer File
for Explicit Message Request
service
port
TXID status
size
MAC ID
Service Response Data
word 224
Transaction
Header
(3 words)
word 225
15 0
Transaction #1
Format of 32-word M1 Transfer File
for Explicit Message Response
Processor and Scanner Module Manage Messages
File transfer operations between the processor and the scanner
always originate in the processor. The scanner module can only wait
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 scanner module, a ladder logic program in the processor polls
the scanner 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 scanner 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 scanner
module a number of times before the scanner returns a Response
Transaction Block.
The scanner module recognizes I/O data and control as higher
priorities over explicit messaging on DeviceNet.