Manual

6-2 VSM500 DeviceNet Option Board
6.3 Formatting Explicit Messages
There are ten 32-word transaction blocks within the scanner module reserved for
explicit message program control. These transaction blocks accommodate both
downloading explicit message requests and uploading explicit message responses.
The scanner module can accommodate one request or response for each transaction
block. Each transaction block must be formatted as shown in figure 6.1.
Request Response
15 0 15 0
TXID Command TXID Status
Port Size Port Size
Service MAC ID Service MAC ID
Class Service Response Data
Instance
Attribute
Service Data
Figure 6.1 – Explicit Message Format
Data Field Description
TXID The transaction ID is a 1-byte integer in word 31 with a range of 1 to 255. It is
assigned in the ladder logic program when the processor creates and downloads a
request to the scanner. The scanner uses it to track the transaction to completion.
It returns this value with the response that matches the request downloaded by the
processor.
Command One of the following command codes instructs the scanner how to administer the
request during each download:
0 = Ignore transaction block (empty) 3 = Reset all client/server transactions
1 = Execute this transaction block 4 = Delete this transaction block
2 = Get status of transaction TXID 5 – 255 = Reserved
Status One of the following status codes is provided during each upload:
0 = Ignore transaction block (empty) 9 = Scanner out of buffers.
1 = Transaction completed
successfully
10 = Other client/server transaction in
progress
2 = Transaction in progress (not
ready)
11 = Could not connect to slave device
3 = Slave not in scan list. 12 = Response data too large for block.
4 = Slave offline. 13 = Invalid port.
5 = DeviceNet port disabled or offline. 14 = Invalid size specified.
6 = Transaction TXID unknown. 15 = Connection busy.
7 = Unused 16 – 255 = Reserved
8 = Invalid command code.
Port The port used by the message is always zero (Channel A) on a 1747-SDN
scanner.
Size The size of the service data is in bytes. The maximum size is 58 bytes (29 words).
Service The service attribute contains the DeviceNet service request and response codes
that match the corresponding TXID.
MAC ID The node address of the slave device to which the transaction is sent. This value
has a range of 0 to 63. The slave device must be listed in the scanner’s scan list
and be online for the explicit message transaction to be completed.