Allen-Bradley 1756-A4 ControlLogix 4-Slot Chassis

A compact four-slot ControlLogix backplane

The Allen-Bradley 1756-A4 is the smallest standard ControlLogix chassis. It has four slots and carries the backplane that joins the controller, communication and I/O modules into one system. Rockwell Automation states that any module can be placed in any slot, and that the backplane gives a high-speed communication path between them while distributing power from the power supply mounted at the left side of the chassis.

At 0.75 kg it is the lightest of the standard chassis, with a stated power dissipation of 4 W, which suits small machines and remote panels where only a controller, a network card and a few I/O cards are needed.

Technical specifications

Mounting and grounding

The chassis is designed for horizontal back-panel mounting only. The installation instructions give a minimum cabinet size of 50.8 x 50.8 x 20.3 cm for the four-slot size, and a spacing section for the chassis mounting system. Two ground connections are required: a functional earth ground with 8.3 mm² (8 AWG) wire and a protective earth ground with 2.1 mm² (14 AWG) wire, each rated at 90 °C or greater.

The chassis does not include a power supply. Choose a ControlLogix supply such as the 1756-PA72 or 1756-PA75, and remember that the total of every module current draw must stay inside the chassis limits shown in the table.

Series, harsh-environment variants and related chassis

Rockwell lists the A4 in series B and series C. The operating range is 0 to 60 °C for series B and -25 to +60 °C for series C. The 1756-A4K is the conformal-coated version for harsh environments. The same documents cover the 7-slot 1756-A7, the 10-slot 1756-A10, the 13-slot 1756-A13 and the 17-slot 1756-A17, so the four-slot size is the right choice when you do not need to leave room for later expansion.

Planning a four-slot ControlLogix system

A four-slot chassis forces careful use of every slot. A typical small configuration places the controller in one slot, an EtherNet/IP communication module in another, and uses the remaining two slots for local I/O or a further communication module. Because ControlLogix allows any module in any slot, the layout can follow the wiring logic of the panel rather than a fixed slot rule.

When more I/O is needed than four slots can hold, the usual approach is to keep the 1756-A4 for the controller and communication modules and to place additional I/O in remote racks or distributed I/O over the network. This keeps the main panel compact, which is useful in skids, package units and small remote stations.

Checking the backplane power budget

The chassis itself only distributes power; the limits in the specification table apply to the sum of all installed modules. Before finalising the bill of materials:

• List the backplane current draw of every module at 1.2 V, 3.3 V, 5.1 V and 24 V DC from the module technical data.
• Check that each total stays below the chassis limit for that voltage, and that the per-slot limits are respected.
• Check that the selected power supply can deliver the totals, including its own derating at the expected cabinet temperature.
• Leave margin for future module changes rather than designing to the exact limit.

Key points of the chassis

• Four slots, the smallest standard ControlLogix chassis.
• Power supply mounts on the left side and is ordered separately.
• High-speed backplane communication between all modules.
• Series C extends the operating range down to -25 °C.
• Conformal-coated 1756-A4K available for corrosive or humid environments.
• Hazardous-location markings for Class I Division 2 and Zone 2 as printed on the product.

Use in Oman and GCC panels

ControlLogix systems are widely used in oil and gas, water and wastewater, power and process plants across the region. Control panels installed in field shelters or outdoor enclosures can run warm, so the cabinet design should keep the internal temperature within the 60 °C maximum operating limit, using shading, ventilation or air conditioning as needed. In coastal areas with salt-laden air, and in plants where hydrogen sulphide or other corrosive gases may be present, the conformal-coated 1756-A4K is worth considering. Note that the chassis is open-style equipment and must always be installed inside a suitable enclosure.

Installation checklist

• Mount horizontally on the back panel only, respecting the minimum cabinet size and the spacing given in the installation instructions.
• Connect both the functional earth and the protective earth with the specified conductor sizes.
• Fit slot fillers in any empty slots to protect the backplane from dust.
• Route power and signal wiring separately and keep heat-generating devices away from the chassis.

What to include in your RFQ

• Catalog number 1756-A4 or 1756-A4K and the required series (B or C).
• Quantity and whether a power supply and slot fillers are needed.
• The existing system details if the chassis is a replacement.
• Delivery location in Oman or the GCC and the required date.

Documents and quotation

The Rockwell Automation ControlLogix chassis installation instructions are attached; the full technical data sheet, publication 1756-TD006, is on the Rockwell Automation literature library. See our PLC and I/O cards catalogue for compatible controllers and I/O. Confirm the exact catalog number, series letter and firmware or hardware revision on the product label, and the compatibility with your existing system, before ordering. Stock and lead time are confirmed at quotation.