CLX - Cache Connections in MSG configuration

Hello everybody,

I am working with a ControlLogix 1756 L63 controller and RIO, with a DHRIO module.

I have a doubt about the Cache Connections checkbox in the Message configuration window (under Communication tab). I know (because I tested it) I have to check that option, otherwise the message is not executed successfully (it generates errors). I have multiple Block Transfer Read and Write blocks, used to configure analog modules over RIO, and to read from and write to. What I really want to know is if there are any problem setting Cache Connections in every Message. Is there a limit?

Thank you in advance

Greetings

Fernando

Actually you can use un-cached MSG instructions with the 1756-DHRIO and Block Transfers, but you cannot mix cached and un-cached messages to the same Block Transfer Module.

In addition, there is an important limit to the number of cached connections that can be supported by the 1756-DHRIO.

As I understand it, there is a limit of 16 cached messages per channel of the 1756-DHRIO module as well as a limit of 32 cached messages on DH+ or RIO per ControlLogix CPU.

The 32-message limit is a per-controller limit.

This limits the number of Block Transfer devices that can be successfully used with a ControlLogix and the 1756-DHRIO scanner module.

The 1756-RIO is a special module that was released about 2 years ago and has an unlimited RIO Block Transfer capacity. I have used it successfully to convert large PLC-5 and PLC-3 systems to ControlLogix.

I recommend two excellent RA TechConnect documents that discuss the capacity of the 1756DHRIO in detail:

https://rockwellautomation.custhelp.com/app/answers/detail/a_id/37099

https://rockwellautomation.custhelp.com/app/answers/detail/a_id/41853

Thank you very much Ken! Your explanation was very clear! I was sending an uncached message to an analog input module, in order to configure it, and I was trying to read values by a cached message. That is not valid, as you said, you can't send cached and uncached messages to the same module, they all will fail.

So I will have to choose which modules will have cached connections and which not.

Thank you again!

Best regards

Fernando