Ken Roach
Lifetime Supporting Member + Moderator
Thank you for that follow-up ! I was away from my usual resources so I didn't have an easy way to show you the specific settings I was talking about.
In an application like this one, the 1756-DHRIO accepts an incoming DH+ message and examines it for the local/remote flag and for the command type.
In this case the command is simple: it's a "local" message, and the command code is for a "PLC-5 Typed Read". So the DHRIO executes a "PCCC Read" across the backplane to the ControlLogix in the designated Controller Slot, and returns the response to the PLC-5 on DH+.
There are other more complex services the DHRIO can offer, to access multiple ControlLogix CPUs in the chassis, or to access PLC/SLC and ControlLogix that are connected to other ControlNet, Ethernet, or DH+ networks. Those can be very complicated and begin with the late-1970's "data highway remote link" addressing, a sort of super-primitive inter-network routing protocol.
I am surprised that the error message was misleading, but I'm not sure it's worth the effort to reproduce it and figure out why.
In an application like this one, the 1756-DHRIO accepts an incoming DH+ message and examines it for the local/remote flag and for the command type.
In this case the command is simple: it's a "local" message, and the command code is for a "PLC-5 Typed Read". So the DHRIO executes a "PCCC Read" across the backplane to the ControlLogix in the designated Controller Slot, and returns the response to the PLC-5 on DH+.
There are other more complex services the DHRIO can offer, to access multiple ControlLogix CPUs in the chassis, or to access PLC/SLC and ControlLogix that are connected to other ControlNet, Ethernet, or DH+ networks. Those can be very complicated and begin with the late-1970's "data highway remote link" addressing, a sort of super-primitive inter-network routing protocol.
I am surprised that the error message was misleading, but I'm not sure it's worth the effort to reproduce it and figure out why.
