Finding the Right Allen-Bradley 1756 Chassis and Power Supply

How 1756 Chassis and Power Supplies Work Together

A ControlLogix system is built around a chassis with a backplane. The chassis is the physical enclosure with a fixed number of module slots. Every module installed in the chassis draws its operating power from the chassis backplane, and the power supply is what feeds that backplane. A few practical points:

• The power supply installs in a dedicated position at the leftmost end of the chassis. It does not take up one of the numbered module slots. A 1756-A7 chassis has seven module slots plus a separate power-supply position.
• Two power supplies can run in a redundant pair for high-availability applications. The 1756-PA75R, PB75R, and the associated 1756-PAR2 / PBR2 adapter modules let two supplies share the load, with one taking over if the other fails.
• The power supply’s output current is the constraint that matters most. A 5A power supply will fault out if the modules in the chassis collectively draw more than 5A, regardless of slot count. This is what catches expansion buyers off guard: a new module pushes total draw past the supply’s rating, and the supply trips. Section 4 covers how to size your supply for your modules.

With that in place, the next sections cover the chassis and power supply catalogs.

Companion ControlLogix Sourcing Guides

Building or modifying a 1756 configuration requires coordination between your backbone enclosures, controller capabilities, and field terminations. Review our companion hardware guides to map your complete hardware profile:

• Processor Sizing: Read our 1756 ControlLogix Processor Replacement Guide to evaluate memory capacities and CPU generations across backplanes.
• Chassis Networking Bridges: Visit our 1756 ControlLogix Communication Module Guide to coordinate EtherNet/IP distributed routing architectures and ControlNet loops.
• Field Wiring & Signal Cards: Visit our 1756 ControlLogix I/O Module Guide to total up power bus current constraints across discrete and analog signal racks.

The 1756 ControlLogix Chassis Catalog

The 1756 chassis lineup is straightforward: five standard chassis sizes from 4 to 17 slots, with conformal-coated and extended-temperature variants for harsher environments. Slot count drives the selection decision.

Standard chassis

All five standard chassis share the same electrical backplane and module compatibility; the only difference is physical size and slot count. A 1756-IB16 digital input module (detailed thoroughly in our ControlLogix I/O Module Guide) that works in a 1756-A4 works identically in a 1756-A17.

Choosing the right slot count

The practical advice is simple: size up for future expansion. Most plants that buy a chassis are still using it 10-15 years later, and the modules in that chassis tend to grow over time as I/O is added, communications are upgraded, or specialty modules get installed. A chassis that fits today exactly may not fit in three years. TCI Supply’s suggestion:

• Count the modules you need today (processor, comms, I/O, specialty).
• Add 30-50% headroom for future expansion.
• Pick the chassis size that accommodates the total.

For a system that needs 7 modules today, the 1756-A10 is usually the right call (10 slots, with 3 free for growth) rather than the 1756-A7 (7 slots, which leaves no room). Going from A7 to A10 is a small cost increase relative to the future hassle of replacing the chassis when you run out of slots. TCI Supply normally stock the slot fillers. One important reminder: the power supply installs in a separate position at the leftmost end of the chassis and does not count against your module slot count.

For installations exposed to corrosive atmospheres, conductive dust, or extreme temperatures, Rockwell offers two specialty chassis families:

Conformal-coated chassis (K variants). Standard chassis with a protective conformal coating on the backplane to resist corrosive and high-humidity environments. Available across the standard sizes: 1756-A4K, A7K, A10K, A13K, A17K.

Extended-temperature chassis (XT variants). Designed for the ControlLogix-XT product line, rated for operation from -25°C to +70°C (-13°F to +158°F) compared to the standard 0°C to +60°C (32°F to 140°F) range. Common XT catalog numbers include 1756-A5XT, A7XT, A10XT, A7ZXT, and A10ZXT. The XT chassis are conformally coated as well, so they cover both harsh atmosphere and temperature extremes.

If your installation environment is standard industrial (controlled temperature, clean atmosphere), the standard chassis covers it. The K and XT variants exist for the harsh areas where they’re genuinely needed.

The 1756 ControlLogix Power Supply Catalog

The 1756 power supply lineup covers six standard models, each defined by input voltage and output current capacity, plus redundant variants for high-availability applications. Selection depends on what voltage is available at the installation, how much current the modules in your chassis will collectively draw, and whether your application requires redundancy.

Standard power supplies

Redundant power supplies

Choosing between AC and DC

The first selection question is which voltage is available at the installation:

• 120/240V AC is by far the most common in North American industrial plants. The 1756-PA72 (5A) and 1756-PA75 (13A) cover virtually all AC applications.
• 24V DC is used in many process plants, control panels with existing 24V power buses, and applications where AC is not readily available at the chassis. The 1756-PB72 and PB75 are the DC equivalents.
• 48V DC and 125V DC are less common and tied to specific installation environments. The PC75 and PH75 exist for those applications but are not standard for most industrial installations.

Choosing between 5A and 13A output

The second selection question is how much current your modules will draw. This is the question section 4 walks through in detail. As a quick rule of thumb:

• A 1756-PA72 or 1756-PB72 (5A) is sufficient for small systems: a processor, a comms module, and a handful of I/O modules in a 4- or 7-slot chassis.
• A 1756-PA75 or 1756-PB75 (13A) is the default for medium and larger systems (10+ modules), multiple comms modules, or specialty I/O like motion or HART modules that draw more current.

When in doubt, the 13A supply is the safer choice. The cost difference between a 5A and 13A supply is modest, and oversizing prevents the most common power supply problem: tripping out when modules are added later. View our complete 1756 power supply catalog.

Conformal-coated, slim, and extended-temperature variants

For harsh environments and specialty installations, Rockwell offers several variant families: K variants (conformal coated), slim profile (1756-PA50, PB50), and XT variants (extended temperature, including 1756-PAXT, PBXT, PA30XT, PB30XT, and the redundant 1756-PAXTR, PBXTR). For most standard industrial applications, the six standard supplies in the table above cover the selection. The variants exist for the harsh and specialty applications where they’re genuinely needed.

Sizing Your Power Supply: Matching Capacity to Your Modules

The power supply is the single most common point of failure in an expanding ControlLogix system, not because the supply itself fails, but because the system grows past what the original supply was sized for. A line that ran fine for years starts faulting out after a few I/O modules get added during a project, and the technician troubleshooting it finds the power supply tripping at startup or under load. The fix is to size the supply correctly upfront, with headroom for expansion. Here’s how.

How power supply sizing works

Every 1756 module that installs in a chassis drives current from the chassis backplane at one or more voltage rails. The power supply provides current at four voltage rails to the backplane: 1.2V DC, 3.3V DC, 5.1V DC, and 24V DC. Each module’s current draw at each rail is published in Rockwell’s technical data publications.

To size the supply correctly, you add up the current draws of every module that will be installed in the chassis, then pick a supply rated for at least that much current at each voltage rail. The supply’s total output power is also limited (75 watts on most standard supplies); the combined power draw across all rails can’t exceed that ceiling.

In practice, the 5.1V rail is almost always the binding constraint. Most modules draw the bulk of their current at 5.1V, and that’s where supplies are most likely to run out of capacity first. The 1756-PA72 and 1756-PB72 (5A supplies) provide roughly 5A at 5.1V. The 1756-PA75 and 1756-PB75 (13A supplies) provide roughly 13A. Sizing for 5.1V is usually sufficient for the rest.

A worked example

Imagine a typical 10-slot ControlLogix system:

• 1 processor (1756-L73) → roughly 1.7 A at 5.1V
• 1 EtherNet/IP communication module (1756-EN2TR) → roughly 1.0 A at 5.1V
• 4 digital input modules (1756-IB16) → roughly 0.5 A each at 5.1V = 2.0 A total
• 2 digital output modules (1756-OB16E) → roughly 0.5 A each at 5.1V = 1.0 A total
• 1 analog input module (1756-IF8H) → roughly 0.6 A at 5.1V
• 1 analog output module (1756-OF8) → roughly 0.5 A at 5.1V

Total draw at 5.1V: approximately 6.8 A.

A 1756-PA72 (5A capacity) cannot support this system. It would trip at startup or shortly after. A 1756-PA75 (13A capacity) handles it with substantial headroom for future expansion, which is exactly why the 13A supply is the better default for most systems.

A practical rule of thumb

If you don’t want to do the math module by module, use this simplified approach:

1. Count the modules in your chassis (every processor, comms module, and I/O module).
2. Assume an average of roughly 0.6 A at 5.1V per module as a conservative estimate.
3. Multiply to get the approximate total draw.
4. Add 30-50% headroom for future expansion.
5. Pick the supply that meets or exceeds that requirement.

Common sizing mistakes

• Underspeccing for the original install. A 1756-PA72 chosen for a 4-slot system that grew into a 10-slot system over five years. The fix is to step up to a 1756-PA75. The chassis stays; only the supply changes.
• Adding HART or motion modules without recalculating. HART-enabled analog modules and motion modules draw substantially more current than standard I/O. A system that ran fine with 8 standard I/O modules can trip when one of those is swapped for a 1756-IF8H or a 1756-M02AE.
• Adding a redundant comms module. Going from one 1756-EN2T to a redundant pair effectively doubles that line item’s current draw. Worth checking the supply headroom before committing.

Redundant Chassis and Power Supply Configurations

For applications where unplanned downtime is unacceptable (process plants, batch operations, pharmaceutical manufacturing, certain power generation and water treatment systems), ControlLogix supports a redundancy architecture that lets two complete controller systems run in parallel, with one taking over automatically if the other fails. The chassis and power supply choices for redundant systems differ from standard configurations in a few specific ways worth knowing.

The redundant chassis: 1756-A4

In a ControlLogix Redundancy system, each redundant pair uses a small 1756-A4 chassis dedicated to the redundant controller and its required modules (a redundancy module like the 1756-RM2 or 1756-RM2XT, the processor, and an EtherNet/IP comms module). Two of these chassis pair together, one primary and one secondary, sharing state through the redundancy modules and the network.

The A4 is the chassis of choice for redundancy because the redundant pair needs only the minimum modules and a small footprint, and using a larger chassis would just leave empty slots. Standard ControlLogix Redundancy systems run on two 1756-A4 chassis at the controller level, with separate (often larger) chassis for distributed I/O networked over EtherNet/IP.

Redundant power supplies

Redundant power supply configurations let two power supplies feed a single chassis, with one taking over if the other fails. The architecture requires three parts:

You order two of the redundant supply (e.g., two 1756-PA75R units), one 1756-PSCA2 adapter, and the power cables. The two supplies share the load under normal operation; if one fails, the other continues alone without interruption.

Redundant systems usually involve more parts than standard ones, and getting all the pieces right (two supplies of the same type and series, the right adapter, the right cables, compatible chassis) matters. If you’re planning a redundant configuration or sourcing replacements for an existing redundant system, send us the system details and we’ll spec the full set.

Power supply catalog numbers

The pattern is 1756-P followed by the input voltage letter, the capacity code, and any suffix:

Example – 1756-PB75R → 24V DC input, 13A output, redundant pair. DC redundant supply.

Source Your 1756 Chassis and Power Supplies From TCI Supply

Whether you’re building a new ControlLogix system, replacing a failed chassis or power supply in a running plant, or expanding an existing system and need to size up your power supply, the starting point is the same: knowing what you have, what you need, and confirming what’s actually available.

The 1756 chassis and power supply lineup is one of the most lifecycle-stable in the Allen-Bradley catalog. Most catalog numbers covered in this guide are in current production and available new, including the workhorses (1756-A7, 1756-A10, 1756-PA75, 1756-PB75) that show up in most US industrial installations. For the few legacy and discontinued variants, the secondary market remains a reliable source.

Send us your chassis size, your module list, or your existing catalog numbers, and we’ll confirm what’s available, identify the right combination, and get you a quote. Whether you need a 1756-A10 with a 1756-PA75 for a new build, a replacement 1756-PSCA2 redundancy adapter, or help sizing the right supply for a system you’re expanding, contact TCI Supply or call us at 1-800-755-2883.

TCI Supply is an unauthorized Allen-Bradley distributor.

Companion ControlLogix Sourcing Guides

Building or modifying a 1756 configuration requires coordination between your backbone enclosures, controller capabilities, and field terminations. Review our companion hardware guides to map your complete hardware profile:

• Processor Sizing: Read our 1756 ControlLogix Processor Replacement Guide to evaluate memory capacities and CPU generations across backplanes.
• Chassis Networking Bridges: Visit our 1756 ControlLogix Communication Module Guide to coordinate EtherNet/IP distributed routing architectures and ControlNet loops.
• Field Wiring & Signal Cards: Visit our 1756 ControlLogix I/O Module Guide to total up power bus current constraints across discrete and analog signal racks.

Frequently Asked Questions