Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 26)

1–9 Network Layers
Publication 1770Ć6.5.16 - October 1996
Messages
See Chapter 7, “Communication Commands,” for:
•
a description of the command messages for each type of
PLC processor
•
information on how to program the application layer fields of a
message packet for an asynchronous link
All messages on a network have the same fundamental structure,
regardless of their function or destination. If you could freeze a
message packet while it is in transmission, you would see:
Bytes Contents
protocol
Information used by the application and dataĆlink layers of your software to
get the message to its destination:
• If a transaction originates from a PLC processor, the interface module
automatically fills the protocol bytes.
• If the transaction originates from a computer, your computer software
must supply the necessary protocol.
data
Information supplied by application program at the source and delivered to
the application program at the destination.
The following sections describe bytes that you define using the
application-layer protocol bytes in your message packet.
For a detailed description of how to use these bytes for each type of
command, see Chapter 7, “Communication Commands.”
To define this See this page
command and reply message 1-9
message priority 1-10
delivery order of commands 1-10
types of commands 1-11
error codes 1-11
Command and Reply Message
A network transaction consists of a command and a reply.
The two parts provide extra data integrity by making sure that a
required action always returns a reply with some sort of status, either
zero status for a good reply, or non-zero status as an error code.
The application-layer protocol distinguishes a command from a
reply. The data area of a command and its corresponding reply
depend on the type of command.
"
Message Packet Structure