How to read an Allen-Bradley catalog number (and what “Ser B” means)

When a module fails at 2 a.m., the label on the dead unit is the most useful document in the building. Allen-Bradley (Rockwell Automation) labels pack a lot into a few lines: a catalog number, a series letter, often a firmware revision, and sometimes a hardware revision within the series. Reading them correctly is the difference between a part that drops in and a part that sits on the bench while the line stays down.

The anatomy of an Allen-Bradley catalog number

Most Allen-Bradley catalog numbers follow the same pattern: a bulletin number, a hyphen, and a designation for the specific product, sometimes followed by option letters. Take 1756-IB16:

• 1756 is the bulletin: the ControlLogix chassis-based platform.
• IB16 describes the module: an input module (I) for DC voltage (B) with 16 points.

The bulletin is the fastest way to know what family you are holding. A few you will see constantly in plants:

Module codes on I/O

On Logix and SLC I/O, the letters after the hyphen follow a readable convention. The first letter is usually the direction – I for input, O for output – and the second letter narrows it down: A for AC voltage, B for DC, W for relay contacts, F for analog. The number is the point or channel count. So 1746-OW16 is a 16-point relay output for SLC 500, and 1756-IF8 is an 8-channel analog input for ControlLogix. There are exceptions and variations within each family (sinking versus sourcing, isolated versus non-isolated, special-purpose modules), so treat the convention as a guide and confirm with the product's documentation.

Controllers and configured products

Controllers use designations such as 1756-L61 or 1756-L73, where the digits identify the controller generation and memory size. Drives and some other products use configured catalog numbers in which each position encodes an option. In a PowerFlex 525 number such as 25B-D010N104, 25B identifies the drive family, the D denotes the 480 V class and 010 points to the output-current rating; the remaining characters describe enclosure, filtering and other options. Decode a configured number with the manufacturer's selection guide rather than by guesswork – one different character can mean a different voltage.

Suffix letters

Watch for letters after the main designation. On many Allen-Bradley products a trailing K denotes a conformally coated version (for example 1756-L73K), and XT denotes an extreme-environment version. They do the same job as the base product but are built for harsher conditions. A coated unit can generally stand in for an uncoated one; the reverse may not be acceptable if the environment called for the coating.

Series letters: what “Ser B” means

Next to the catalog number, most labels show SER (or “Series”) followed by a letter. The series is a hardware revision: when Rockwell Automation changes a product's design enough to track it – a new component, a board redesign, an added feature – it moves the product to the next letter and keeps the catalog number. A 1756-ENBT Series A and a later series of the same catalog number are the same product built to a different hardware design. Letters run in order, so a later letter means a later design.

Where to find the series on the label

• Modules and controllers: on the side label, usually on or just below the catalog-number line – you may have to pull the module to read it. The original box label normally shows it too.
• Drives and motor-control devices: on the nameplate, often as “Ser.” followed by the letter.
• Next to the series: look for a hardware REV (a smaller revision within the series) and the firmware revision, often printed as FRN followed by a number.

Series, revision and firmware are different things

Two units of the same series can carry different firmware, and the series can limit which firmware a unit accepts: a newer series may not take an older firmware revision, and an older series may not support the newest one.

When the series matters for compatibility

In most cases a later series is intended to replace an earlier one: it fits the same slot, uses the same wiring and does the same job. The differences that bite are usually these:

• Minimum firmware. A newer series of a controller or communication module may ship with, and require, a firmware revision newer than the one your project uses.
• Software support. Newer hardware may need a newer programming-software version or device profile.
• Feature differences. A later series may add features, change defaults or behave differently in a case documented in the release notes.
• Going backwards. Replacing a later series with an earlier one is riskier: the earlier hardware may lack something your system relies on.

For simple digital I/O, pushbuttons and pilot lights, series differences rarely matter in practice. For controllers, communication and motion modules, safety products and drives, check the release notes and Rockwell Automation's compatibility information before you commit – the same applies when a drive family changes altogether, as in a PowerFlex 40 to 525 replacement.

Firmware: the part the label can't guarantee

A Logix controller must run a firmware revision that matches the major revision of the project you download to it. If a replacement arrives with a different revision, flash it with the matching firmware (Rockwell's ControlFLASH tool, with the right software and licenses) or update the project. Firmware on a surplus or used module reflects wherever it came from, which is why we state that firmware and series are not guaranteed on surplus items unless the listing says otherwise.

A checklist before you order a replacement

1. Photograph the label of the failed unit – catalog number, series, revision and firmware all in frame.
2. Write down the full catalog number, including suffix letters such as K or XT.
3. Note what the unit talks to: chassis, network, controller and firmware in use, and the software version you program with.
4. Decide what you can accept: same series only, same or later series, and which firmware.
5. Search by the base catalog number (for example 1756-IB16) and read each unit's photos for the series and firmware.
6. Back up the program and configuration before you swap anything.

Buying Allen-Bradley parts as surplus

Surplus is often the fastest source for discontinued and hard-to-find Allen-Bradley parts, and for current parts with long factory lead times. Because every unit we stock is photographed, you can read the series and, where it is printed, the firmware on the label before you buy. Unused Allen-Bradley items are listed as “New Surplus” rather than “New”, and no manufacturer warranty applies – see our condition definitions. If the exact series you need isn't on the shelf, ask us to source it.