Access to ConveyLinx ERSC modules is also available utilizing the Logix 5000 MSG instruction. The MSG instruction utilizes CIP Explicit Messaging. This means that the connection is not maintained as an implicit connection. Generic Ethernet Module and EDS connections are implicit and thus must be maintained at all times or there will be a communication fault. Explicit Messaging opens the connection, reads/writes data, and then closes the connection thus freeing up communications resources for the PLC.
When to use MSG Instructions
Because the MSG instruction is executed asynchronous to program scan and is not subject to implicit messaging RPI restrictions; the response time between requesting data and receiving data is not deterministic and can vary between separate requests for the same data from the same device. Therefore, we recommend that MSG instructions should not be used for dedicated “real time” control of equipment. For ConveyLinx ERSC modules, MSG instructions are intended to gather “low priority” status information and/or to send infrequent parameter changes. Please note that this is only a recommendation. Your particular application’s specifics, PLC’s capacity, available network bandwidth, etc. may allow you to get expected results with “real time” control utilizing MSG instructions to interface with ERSC modules.
Module VS. Assembly Addresses with MSG Instruction
Module Addresses are the 512 “actual” registers in the module and and Assemblies are “virtual” registers grouped based upon function. There are certain restrictions on what you can do with a MSG Instruction:
- You can us a single MSG instruction to read one and up to 30 consecutive Module registers
- You can use a single MSG instruction to write to one (an only one) of the Module registers
- You can use a single MSG instruction to read any of the available Input Assembly registers in their entirety
- You CANNOT use any MSG instruction to write to any Output Assembly registers.