Allen-Bradley 1404-M4, User Manual

Page: 92 / 356

Publication 1404-UM001D-EN-E - October 2004
4-32 Communications
In the SLC 500 and PLC-5, you assemble the explicit message header
in an integer file and transfer it to the scanner module. When the
response is received, you transfer the response from the scanner to
another integer file. The message header consists of 6 words
organized as follows:
Word 0 contains a transmit identifier (TXID) and command byte.
Assign each explicit message a unique TXID in the range of 0 to 255
decimal (0 to FF hex). The TXID is used to identify the response to
this message request. Valid command codes are:
•
1 hex = Execute transaction block. Use this command first to
start the explicit message.
•
4 hex = Delete transaction from response queue. Use this
command after you copy the response from the scanner to
remove the response from the scanner and enable further
explicit messages.
Word 1 contains the DeviceNet scanner port number and the
transaction body size in bytes. The SLC 500 scanner uses only port 0; a
PLC-5 DeviceNet scanner has two ports, 0 and 1. For a read request,
the transaction body size is 3 words (see table above) therefore 6
bytes. For a write, the body size is the data size in bytes plus the
6-byte path (class/instance/attribute).
Word 2 contains the DeviceNet service code and the MAC ID or node
number of the server device, in this case, the Powermonitor 3000.
Valid service codes for use on Class 4 assembly instances are:
Table 4.6
Message Word High byte Low byte
Header 0 Transmit ID Command
1 Port Size
2 Service MAC ID
Body 3 Class
4 Instance
5 Attribute
6 Data to write if applicable
7
. . .
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