Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 64)

DH and DH+ Communication
Chapter 4
4-4
Stations communicate with each other by sending two types of messages
over a link:
Type of messages: Function:
command messages either gives (writes) data to, or requests (reads) data from one
station to another.
reply messages is a station's response to a command message.
You program command messages into the scanner. Execution of a
message command is controlled by the message (MSG) instruction in the
ladder program. When a scanner receives a command message from
another station, the module automatically generates the appropriate reply
message.
The local station is the one currently initiating some actions, or the one we
are currently doing something with. All other stations are then remote.
We can also describe stations in terms of their relationship to a message.
The station that: Is the:
sends the message transmitting station
gets the message receiving station
transmits a command message command station
transmits a reply message reply station
The scanner can receive solicited and unsolicited messages depending on
whether the message is a command from a remote station or a reply to a
command from the local station.
If the local station issues the command message, the corresponding
reply message is said to be solicited because the local station has
solicited, or requested the data contained in the reply message.
If a remote station issues the command message, that message is said to
be unsolicited.
Either station initiates the data transfer by issuing a command message.
For a local station receives data
from a remote station during a
and a local station sends data
to a remote station during a
solicited messages read operation write operation
unsolicited messages write operation read operation
Communication Terms
Solicited and Unsolicited
Messages