I utilize a 1769-L16ER for code testing and recently discovered that MSG tags can only be created at the controller scope, not the local program scope. Initially, I attributed this limitation to firmware, but given that both my L71 and L16ER are operating on v32, it seems to be a hardware constraint.
It has always been present since the inception.
I presume that is the case. It just seems unusual for it to be a restriction related to hardware.
It's fascinating to discover that certain hardware restrictions exist, such as the inability to assign string literals on L7x controllers compared to L8xE controllers. While I haven't experimented much with compactlogix like I have with controllogix, I was surprised to find that message tags only function in controller tags and not program tags on the L16ER. Have you tried updating the firmware to see if this limitation was resolved in a newer version on that hardware? It's worth investigating if Rockwell eventually added support for it on the L16ER. I can't recall which firmware update included support for message instructions in program tags - perhaps it has always been exclusive to L8xE controllers. Now I'm curious if the L7x controllers also allow for this capability.
I believe the decision to prioritize revenue over technology often drives product development. For example, if you desire a specific feature, you may need to upgrade your device. The focus tends to be on generating income through new product releases, rather than updating firmware.
Interesting find! I've also run into similar limitations and it strikes me that system architecture, rather than firmware, could be the major determinant. Considering the smaller size and reduced capacity of the CompactLogix L16ER relative to the larger ControlLogix L71, there might be built-in hardware restrictions to optimize performance given its tighter resource envelope. It would be intriguing to know if Rockwell provides detailed documentation about such hardware-specific constraints.
You're absolutely correct, and this is a common confusion. Contrary to what some may instinctively think, the scope of creating Message (MSG) tags in Rockwell Automation Controllers is not actually tied to the firmware version, but is indeed a hardware-based limitation. This follows the rule that Session-based comms (like MSGs) and I/O need global scope, whereas routine scoped tags can only be accessed locally. It’s something to bear in mind when programming and setting up communication with these controllers.
Interesting point you've highlighted about the 1769-L16ER. I've also encountered this scope limitation with MSG tags and it actually seems to be a feature of CompactLogix PLCs. These controllers store the MSG instruction's data at the controller scope to ensure the data remains accurate and consistent, irrespective of the specific routine being executed. It's less about hardware constraints and more about Rockwell's design decision to optimize data integrity in their micro-controller line. So, even if you upgrade your firmware, the controller-level scope for MSG tags will likely persist.
You're correct in your understanding, it's less of a firmware issue and more of a design constraint pertaining to the hardware platform. The ControlLogix platform, which includes the L71, has greater flexibility and resources compared to the CompactLogix (like your L16ER). Despite running on the same firmware version, the CompactLogix series has more limitations when it comes to tag scope. You might find this document helpful, it discusses PLC scopes in detail and explains why certain features only work at controller level.
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Answer: 1. Q: Can MSG tags be created at the local program scope on a 1769-L16ER controller? No, MSG tags can only be created at the controller scope on a 1769-L16ER controller due to a hardware constraint.
Answer: No, the limitation of creating MSG tags at the controller scope is not related to firmware versions, as it seems to be a hardware constraint on the 1769-L16ER controller.
Answer: It appears that the hardware constraint of creating MSG tags at the controller scope is specific to the 1769-L16ER controller, as other controllers like the L71 do not have the same limitation.
Answer: Unfortunately, due to the hardware constraint, creating MSG tags at the local program scope on the 1769-L16ER controller is not possible, and there is no known workaround for this limitation.
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