When physically working with Rockwell Automation’s older Allen-Bradley PLC-2 or PLC-5 system on the 1771-series chassis, the situation is more complicated. The 1771 series has a much larger footprint than the replacement 1756 series: ControlLogix. Both are chassis-based, but that is where the similarities stop.
The process of pulling all the I/O module connectors off and then completely removing the old chassis is quite time-consuming. The new chassis needs new mounting holes, and then each I/O point must be transferred from the old module to the new one.
There are plenty of chances to make mistakes when moving the wires, and, if old enough, the moving and bending of wires might result in breaking off the insulation or ending up with a wire that is too short to re-purpose.
Luckily, there is an alternative for this very involved task. The manufacturer provides a migration kit that converts the 1771 chassis-based I/O into the current 1756 chassis-based I/O system. A transition kit starts with a mounting plate that matches the dimensions of the original I/O chassis and even uses the same mounting holes.
The technician starts by removing the wiring arms off the original I/O modules and leaves them hanging down in the panel. The original I/O chassis is removed, and the new transition plate is mounted in place of it. Once in place, the original wiring arms are plugged into the base of the new transition plate and then a cover plate mounts to the transition plate with holes that match the mounting holes for the new chassis.
The new chassis mounts directly to the plate, and then pre-molded interface cables are added to connect the older wiring arm, on the base transition plate, to the new I/O modules in the new chassis. The great thing about this whole process is individual wires do not need to be de-termed from the old module and then re-termed to the new module.
All this sounds like a win-win situation, but care must be taken when it comes to the legacy network protocols that might be present on the original control system. Most newer logic controllers utilize the Ethernet/IP protocol, or an equivalent, as a standard method of communication, and that simplifies the migration process. However, depending on the size of the legacy system, the project might be a multi-phase event to step through the process and still be able to run the existing system for operations if it can’t be down for long periods of time.
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The phases of execution might be presented as follows:
1. Program conversion. This phase will identify the actual I/O configuration of the system and what network protocols might be present in the original application. There are manufacturers’ software tools that can help with the migration of the code and give the designer some flags to code that might need some changes to maintain the functionality of the original code. Most of the new code can be tested without being plugged into the operation original system.
2. Replace the controller. This phase would change out the programmable logic controller (PLC) system based on the migration plan outlined previously. Any additional chassis-based I/O would remain in the original network protocol—RIO for Allen-Bradley—and communicate through a gateway module in the new PLC rack. This phase should have a low impact on the need to run the operational system as the code was already tested in the previous phase.
3. Replace any operator stations. This phase can be done concurrently with operating the actual process, as the new human-machine interfaces (HMIs) can be tested alongside the originals to complete most, if not all, of the validation.
4. Replace any remote I/O left operating at Phase 2. By following the first three phases, the operation of the system with the new processor is already validated, as is the interaction with the operators via the HMI upgrades in Phase 3. This remaining step is rather anticlimactic, as it mirrors the “replace the controller” steps; but, instead of a processor in the slot, a remote communications module, likely Ethernet/IP, occupies the space and talks to the main processor by that means.
Migrating vintage controllers over to current technology might not be such a feared task if it is well-planned and executed. The clear benefit is to eliminate obsolete equipment, and the result is a control system with a faster controller, more memory and greater flexibility when it comes to connecting to the incredible technology that’s available.
