Rockwell Automation 2755-SNx Adaptascan Bar Code Readers - Manual (Page 242)

12–21 Communicating with a PLC-5 Processor on a DeviceNet Network using Explicit Messaging
Publication 2755-6.8
Formatting the Explicit Message Transaction Block
Ten 32-word transaction blocks within the scanner module are
reserved for Explicit Message Program Control. The transaction
blocks accommodate both the download of Explicit Message
Requests and the upload of Explicit Message Responses.
The scanner module can accommodate one request or response for
each transaction block and can transfer two blocks for each upload
and download. You must format each transaction block as shown in
the following figure.
service
port
TXID cmd/status
size
MAC ID
Transaction Body
(29 words)
word 0
word 1
word 2
Transaction
Header
(3 words)
word 31
15 0
One word = two bytes = 16 bits
The transaction block is divided into two parts:
•
transaction header – Contains information that identifies the
transaction to the scanner and processor.
•
transaction body – In a request, this contains the DeviceNet
Class, Instance, Attribute and Service Data portion of the
transaction. In a response, this contains only the response
message.
Each of the data attributes in the transaction header are one byte in
length:
•
command/status – For each download, you assign a command
code to instruct the scanner how to administer the request.
Command Code Description
0 Ignore transaction block (block empty).
1 Execute this transaction block.
2 Get status of transaction TXID.
3 Reset all client/server transactions.
4 Delete transaction from response queue.
5–255 Reserved