Rockwell Automation 2755-SNx Adaptascan Bar Code Readers - Manual (Page 244)

12–23 Communicating with a PLC-5 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 0
Transaction
Header
(3 words)
word 31
15 0
Class
Instance
Attribute (optional)
Transaction #1
service
port
TXID command
size
MAC ID
Service Data
word 32
Transaction
Header
(3 words)
word 63
Class
Instance
Attribute (optional)
Transaction #2
Format of 64-word Block Transfer Write
for Explicit Message Request
service
port
TXID status
size
MAC ID
Service Response Data
word 0
Transaction
Header
(3 words)
word 31
15 0
Transaction #1
service
port
TXID command
size
MAC ID
Service Response Data
word 32
Transaction
Header
(3 words)
word 63
Transaction #2
Format of 64-word Block Transfer Read
for Explicit Message Response
How the Processor and Scanner Module Manage Messages
Block 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 a Block Transfer Read 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.