Allen-Bradley ControlLogix 1756-DH485, User Manual, Page: 42 / 110

Publication 1756-UM532A-EN-P - May 2006
3-10 Use Data Highway 485 Network
Limitations of Remote DH-485 Messaging
Remote DH-485 messages are encapsulated in CIP messages and sent on CIP
connections when they are sent across ControlNet, EtherNet/IP, and the
ControlLogix chassis backplane. Although this is transparent to the user, there
are resource limits associated with CIP on the 1756-DH485 module.
The 1756-DH485 module supports up to 32 CIP connections. These
connections are made when devices want to send a DH-485 message out of a
1756-DH485 module’s DH-485 channel. The 1756-DH485 module makes
these connections when it receives DH-485 message traffic. The 1756-DH485
module recovers connections if they are not being used.
Because of the various paths involved, the 1756-DH485 module responds to
‘out of connections’ in one of the following ways:
•
The 1756-DH485 module may generate a Routing Error on DH-485 for
DH-485 message requests if there are no connections available.
•
The Message Originator may generate an application timeout if a remote
1756-DH485 module has no connections available for a DH-485
Message response.
For more information on application timeouts, refer to Application
Timeout on page 3-12.
•
The Message Originator may receive an ‘out of connections’ error if the
path from the originator to the 1756-DH485 module is a ControlLogix
chassis, a ControlNet network, or an EtherNet/IP network.