Product Overview

The Allen-Bradley 1756-A17​ is the largest of the standard ControlLogix chassis, Bulletin 1756, and it exists for one reason: to hold a complete local control node in a single backplane. Seventeen module positions is not a cosmetic number — it is the difference between a controller, two communication modules, a block of analog I/O, a safety segment and still having room for the next network or the next transmitter bank without moving to a second remote rack. The 1756-A17​ is a passive backplane structure with no active electronics of its own, which is precisely why it is so dependable: the mechanical frame, the card guides, the P1 power connector and the backplane traces are the whole product, and there is nothing in them to firmware-update or burn out.What the backplane does is twofold. Electrically, it distributes four regulated rails from the left-end power supply to every slot: 1.2V DC at 1.5 A chassis maximum, 3.3V DC at 4 A, 5.1V DC at 15 A chassis / 6 A per slot, and 24V DC at 2.8 A chassis / 2.8 A per slot. Communicatively, it is the producer/consumer fabric across which a 1756-L85​ or 1756-L83​ controller reaches an 1756-EN3T​ port, an 1756-IF16​ analog input block, a 1756-IB16S​ safety card and a 1756-DHRIO​ legacy bridge in the same chassis, with backplane message routing moving data between modules without leaving the rack. Any module fits any slot, so a 1756-A17​ ordered for a 12-module system today can absorb five more modules over the next decade without a chassis change, a cable re-run or a re-approval of the panel layout.Physically the 1756-A17​ is a horizontal-only, back-panel-mount frame: 738 mm wide, 169 mm high and 145 mm deep, roughly 2.2 kg empty, demanding a minimum enclosure of 762 × 914 × 203 mm (30 × 36 × 8 in.). That footprint is honest about the air it needs. Convection cooling relies on the chimney formed by the module heat sinks, which is why empty slots must be closed with 1756-N2​ filler strips — an open slot breaks the airflow lane and quietly derates the ambient capability of the modules on either side. The chassis is currently an Active, Preferred product, with Series C the current build and Series B still widespread in the field; both accept the same module set, and the Series C frame added a second ground screw and tougher mounting-tab geometry for high-vibration cells. For a plant standardising on ControlLogix, keeping a bare 1756-A17​ on the spare shelf is the cheapest form of insurance against a rack-expansion outage.

Technical Specifications