The programmable logic controller, or PLC, has been around forever by high tech standards. The original PLCs came out in the late 1960s, and the venerable PLC 5 has been doing admirable service since its introduction in 1988. Time and technology have caught up with the PLC 5, however, and the product is no longer manufactured. Indeed, Rockwell/Allen-Bradley now recommends replacing your aging PLC 5 hardware with ControlLogix (CLX) control systems.
It’s a good idea. And I would strongly recommend that you not try to keep your PLC 5s going by cannibalizing old controllers you find out on e-Bay; since Allen-Bradley no longer produces PLC5s, ( see here ) the stock of replacement parts will soon start to dwindle significantly. The last thing you want is to be caught in an emergency and have reliable replacement parts unavailable.
There are also some very good reasons to go ahead with an upgrade:
– With CLX controllers using a tag based system rather than data tables you can match the tags in the PLC to the tag used on the Process and Instrumentation diagrams. For instance, when troubleshooting an issue with Temperature Transmitter TIT007 as displayed on the HMI you can search the program for that tag name.
– The ramp-up time for people who are not familiar with either system is often much quicker with ControlLogix than with PLC5. Since CLX tags are in English instead of PLC5’s data table addresses, the tags are far easier to understand and remember.
– ControlLogix programs allow you to take advantage of user defined datatypes (UDTs). The UDTs facilitate standardization of like systems across processors or within a program. They do wonders for keeping your program organized.
– ControlLogix programs are ‘task-based’ rather than ‘continuous scan’. If you need a certain function to execute every x seconds, you can implement it very easily with ControlLogix hardware, where such a setting would be difficult or even impossible with PLC5.
Assuming, then, that you’ve decided to upgrade, what’s the best way to do that? Well, you can go with either simple or thorough:
The simple method is to use a conversion kit, as it’s very low risk and requires little downtime. You don’t need to do any rewiring, and your existing swing arms will fit directly onto the conversion modules. You can even prep and pre-assemble the conversion kit before going into the field. You typically will not have to do the IO checks or error checking you need to do with a complete rewiring.
The conversion kit includes conversion modules which the existing PLC5 swing arms snap directly into. The Pre-wired cables that connect the conversion modules to the new ControlLogix modules are available in both 25 and 37 pin. Then you can use the base plate to mount the new rack and the cover plate to protect the conversion modules. The pre-wired cables that connect the conversion modules to the new ControlLogix modules, available in 25 and 37 pin configurations.
If you can afford the downtime, however, you might want to seriously consider a complete rewiring. While it’s a more painstaking approach than using a conversion kit, rewiring allows for a clean and complete break from your old PLC5 hardware. The benefit of this approach is that it affords you greater flexibility for the future. If you rewire, you can expand your CLX hardware. If you use the conversion kit, that’s not possible.
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