Rockwell Automation 1756-DH485 ControlLogix DH-485 Communication Module User Manual - Manual (Page 44)

Publication 1756-UM532A-EN-P - May 2006
3-12 Use Data Highway 485 Network
Application Timeout When an error occurs while sending a message to a remote link, it appears to
the sending station as an application timeout because error messages are not
routed back. When an error occurs during routing, it may be dropped.
For example, if an SLC 5/03 controller sends a message to another SLC 5/03
controller, and the second SLC 5/03 controller’s buffers are full, three
things happen:
• The SLC 5/03 controller refuses the message because the buffers are
full.
• The originator detects an application timeout when no reply is received.
• The originator increments its error count.
The SLC 5/03 controller can retry to send the message later. This figure
shows an example of an application timeout.
Example of an Application Timeout
DH485 DH485
ControlLogix Chassis
44068
Routed messages sent
over the backplane.
1756-DH485 Module
SLC 5/03 Controller
1756-DH485 Module
SLC 5/03 controller detects an
application timeout and
increments an error bit.
DH-485 Link 1
DH-485
Link 2
SLC 5/03 controller
buffers are full.
AIC+
AIC+
AIC
AIC