Rockwell Automation 1756-DHRIO_DHRIOXT ControlLogix Data Highway Plus-Remote I/O Communication Interface Module - Manual (Page 33)

Rockwell Automation Publication 1756-UM514C-EN-P - June 2014 33
Using the Data Highway Plus Network Chapter 2
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 can be dropped.
For example, if a PLC 5/40 processor sends a message to a PLC processor, and
the PLC-5/25 processor’s buffers are full, three things happen:
• The PLC-5/25 processor refuses the message because the buffers are full.
• When no reply is received, the originator detects an application timeout.
• The originator increments its error count.
The PLC-5/40 processor can retry to send the message later. The following
figure shows an example of an application timeout.
ControlLogix Chassis
19770
Routed messages sent over
the backplane.
1756-DHRIO
PLC-5/25 Controller
1756-DHRIO
PLC-5/40 Controller
DH+ Link 1
DH+ Link 2
PLC-5/40 detects an
application timeout
and increments the
error count.
PLC-5/25’s controllers
are full.