The short answer
CompactLogix
CompactLogix is the small and medium member of the Rockwell Automation Logix family, with the same programming software, tag-based memory, and task model as ControlLogix in a controller that mounts on a DIN rail with its I/O modules beside it rather than in a chassis. It has been the standard controller for lift stations, well sites, and small treatment plants in that ecosystem for two decades, across several hardware generations with different local I/O families and Ethernet arrangements. Recent controllers carry two Ethernet ports that can form a ring, a linear chain, or two separate networks, which decides how the site connects to its drives, its SCADA radio, and its engineering access. It has no controller redundancy; a site that needs it is a ControlLogix application. Its administrative burden is the same version discipline as its larger sibling.
Key points
- Same software, tags, tasks, add-on instructions, and messaging as ControlLogix; logic moves between them.
- DIN-rail controller with local I/O modules beside it; the I/O family depends on the controller generation.
- Recent controllers have two Ethernet ports: ring or linear on one network, or two separate networks with different addresses.
- No controller redundancy; the spare on the shelf and the program backup are the redundancy.
- Firmware major revision and project version must match, as on every Logix controller.
Families
| Generation | Local I/O | Notes |
|---|---|---|
| Older CompactLogix with a single Ethernet port | The Compact I/O family on the rail beside the controller | Common on sites from the 2000s; one network, no ring; a serial port on many |
| CompactLogix with dual Ethernet ports | The same Compact I/O family | Ring or linear topology; the mainstay of many current lift stations |
| CompactLogix with embedded I/O | Built-in discrete points plus expansion | Small sites where a dozen points and a couple of analogs are enough |
| Newer CompactLogix generation | A newer high-density I/O family on the rail | Faster, more memory, dual ports that can be two networks; the current choice for new sites |
The two ports
A controller with two Ethernet ports can use them in more than one way, and the choice is a design decision recorded on the network drawing. In the ring or linear mode, both ports are on one network with one address, and the controller can sit in a ring with its drives and remote I/O or in a chain along a pipe gallery. In the dual-network mode, available on the newer generation, each port has its own address on its own network, so that the site network with the drives and instruments is separate from the network that carries SCADA and engineering through the radio or the fiber. The second arrangement is the one that makes segmentation at a remote site possible without an extra device.
What it shares with ControlLogix
- Studio 5000 Logix Designer, the tag model, user-defined types, add-on instructions, and the task model.
- EtherNet/IP for I/O, drives, messaging, produced and consumed tags, and SCADA.
- The same alarm, PID, and motion instructions within the capacity of the controller.
- The same firmware and software version rules.
What it lacks
- Controller redundancy.
- The chassis: local I/O count is limited to what the rail supports, and beyond that the I/O is remote.
- The legacy network modules; a CompactLogix reaches older networks through gateways, not chassis modules.
- Capacity: memory and connection limits are lower, and a site that grows into a plant can outgrow it.
Typical site
A duplex lift station: the controller on the rail with a discrete input module for floats, run feedbacks, and seal and temperature switches, a discrete output module for the starters, an analog input module for the level transmitter and the current transducers, and an analog output for a drive reference. The drives on Ethernet if they are Ethernet drives, or on hardwired control if not. A small touchscreen on the door on the same network. The SCADA radio on the second port or on the site switch. The program built from the site library: a level control routine, a lead and lag alternation with the standard pump add-on instruction, alarms in the controller, and a watchdog word for SCADA.
On a site visit
- 1
Status indicators
Run, fault, I/O, and the port link indicators on the front; the fault record in the software.
- 2
Local I/O
Each module status indicator; a module the project expects and does not find faults the I/O tree.
- 3
Network
The controller web page for port status, address, connections, and errors; the ring status if a ring is used.
- 4
Retentive data and the battery or capacitor
Older controllers hold memory with a battery; newer ones with a capacitor and non-volatile storage. Know which, and the replacement schedule.
- 5
Backup
Upload with the matching software version; store the project; record the firmware revision on the site record.
Frequently asked questions
- Can I replace an old single-port CompactLogix with a new one and keep the I/O?
- Within a generation that uses the same local I/O family, often yes with a project conversion; across generations the I/O family changes and the wiring is converted. Check the compatibility of the specific controller and modules and plan the conversion of the project to the new firmware.
- Which port goes to the radio?
- In dual-network mode, the port on the network designed for SCADA and engineering, separate from the drives and instruments. In ring or linear mode, both ports are the same network and the radio connects to the site switch on that network.
- How much I/O before I should be on ControlLogix?
- The local I/O limit of the controller generation is one threshold; the connection and memory limits are another; redundancy and legacy networks are the third. Many small plants run comfortably on a CompactLogix with remote I/O; a plant with hundreds of points, several drives on Ethernet, and a historian usually belongs on the larger controller.
- The controller lost its program after a long power outage.
- An older controller with a dead or missing battery loses its memory when power is off; a newer one stores the project in non-volatile memory or on a removable card and restores it on power-up if configured to. Check the battery schedule, the card, and the restore setting, and keep the project backup current.
Related topics
- ControlLogixField notes on the Rockwell Automation ControlLogix, the chassis-based controller of larger water plants: chassis and modules, tag-based memory and tasks, EtherNet/IP and older networks, controller redundancy, version matching, and what to check on site.
- Studio 5000Field notes on Studio 5000, the Rockwell Automation software for ControlLogix and CompactLogix: the version-to-firmware rule, side-by-side installs, project files, add-on instructions, online editing, documentation in the controller, source protection.
- Lift Station Backup ControlHow a lift station keeps pumping when the transmitter, the PLC, or the SCADA link is gone: float backup logic, the control transfer relay, redundant-off protection, and how to test it with the controller actually dead.
- Segmenting a Remote SiteA lift station, a well, or a tank site as its own zone: the site firewall and what it permits, the modem as transport with no public address, cameras and extras off the control segment, engineering access through the plant, and local control without the link.
- Retentive MemoryWhat survives a power cycle in a controller and what does not, how retention is backed up by battery, capacitor, or nonvolatile storage, what should be retentive, and how retained data gets lost.
- How to Program Lead/Lag Pump ControlBuild duplex pump control with alternation, failure detection, minimum run and off timers, and an operator override that survives a restart.
Direct contact
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