Finding the Right Allen-Bradley 1756 ControlLogix I/O Module

Companion ControlLogix Sourcing Guides

Building or modifying a 1756 configuration requires coordination between your network cards, 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.
• Chassis & Power Sizing: Read our 1756 ControlLogix Chassis & Power Supply Selection Guide to plan backplane configurations and power requirements.
• Chassis Networking bridges: Visit our 1756 ControlLogix Communication Module Guide to select EtherNet/IP, ControlNet, and legacy routing adapters.

How 1756 I/O Modules Fit in a ControlLogix System

An I/O module in an Allen-Bradley 1756 chassis does one job: it gives the controller a connection to the physical world. The module installs in any available slot on the chassis, draws power from the chassis backplane, and exchanges data between the processor and whatever’s wired to the module’s front-panel terminals, sensors, switches, drives, valves, indicators, and so on.

The 1756 platform was designed around modularity: you build a system by choosing a chassis, a power supply, a processor, the communication modules you need, and the I/O modules that fit your application. There are no fixed I/O configurations; you pick exactly the modules your system needs and put them in any slot order. For a deep look into mapping out the backplane and power framework, consult our ControlLogix chassis and power supply selection guide. That flexibility is one of the main reasons ControlLogix has been the workhorse of large Allen-Bradley control systems for over two decades.

A few practical realities are worth knowing before you read the family tables below.

• Modules are hot-swappable in most cases. Removal and Insertion Under Power (RIUP) is supported on the standard 1756 chassis, meaning you can replace a failed module without shutting down the rest of the system. That’s an important detail when you’re working a live plant.
• Channel count and signal type drive most selection decisions. For digital modules, the big questions are AC vs. DC, sink vs. source, and how many channels. For analog modules, it’s voltage vs. current, isolation, and whether you need HART, RTD, or thermocouple support. The tables in the next sections lay these out per family.
• Most 1756 I/O modules are still active. Unlike the comms side, where modules like the 1756-ENBT and 1756-CNB have been discontinued, the I/O families have had remarkable lifecycle stability. Many modules introduced in the late 1990s remain in current production. That’s good news if you’re matching an existing system, the part you need is usually still available new, and obsolescence is less of a story here than it is for the processor and communication modules covered in our ControlLogix processor guide and communication module guide.

Digital Input Modules

Allen-Bradley digital input modules read on/off signals from the field, switches, pushbuttons, sensors, auxiliary contacts, anything that gives the controller a binary “is this circuit energized?” answer. The full lineup is larger than most people realize, and while you can evaluate the complete range of options in our dedicated 1756 ControlLogix I/O modules catalog, the catalog number tells you almost everything you need to know once you can read it.

How to read a 1756 digital input catalog number

The first letter after 1756-I identifies the voltage type:

• IA = 120V AC
• IM = 240V AC
• IN = 24V AC
• IB = 24V DC sink/source
• IV = 24V DC source (sinking inputs)
• IC = 48V DC
• IG = 5V DC (TTL)
• IH = 125V DC

The number that follows is the channel count (8, 16, or 32). And the trailing letters flag the functional variant:

• I = individually isolated channels (each channel can have its own common)
• D = diagnostic inputs (per-channel fault detection)
• IF = fast-response (high-speed inputs)
• ISOE = isolated with Sequence of Events time-stamping
• K = conformal coated (see decoder section)
• XT = extended temperature (see decoder section)

So a 1756-IB16I reads as ControlLogix DC sink/source digital input, 16 channels, individually isolated. A 1756-IA32 reads as 120V AC input, 32 channels, no isolation. Once you have the pattern, the table almost reads itself.

The 1756 digital input lineup

Digital Output Modules

Allen-Bradley digital output modules drive on/off devices in the field, solenoids, relay coils, lamps, motor starters, contactors, and similar binary loads. The 1756 output lineup is even broader than the input side, and the catalog-number logic follows the same pattern with a few extra suffixes worth understanding.

How to read a 1756 digital output catalog number

The first letter after 1756-O identifies the voltage type and switching technology:

• OA = 120V AC
• ON = 24V AC
• OB = 24V DC source (solid-state)
• OV = 24V DC sink (solid-state)
• OC = 30V DC
• OG = 5V DC (TTL)
• OH = 125V DC
• OW = relay contact (AC or DC)
• OX = isolated relay contact

The number that follows is the channel count (8, 16, or 32), and the trailing letters flag the functional variant:

• D = diagnostic outputs (per-channel fault detection)
• E = electronically fused (per-group fuse protection)
• I = individually isolated channels
• IEF = isolated, electronically fused, per-channel
• IEFS = isolated, electronically fused, per-channel, scheduled
• IS = isolated, scheduled outputs

So a 1756-OB16E reads as ControlLogix DC source output, 16 channels, electronically fused. A 1756-OW16I reads as 16 channels of individually isolated relay contacts. The pattern reveals most of the module on sight.

The 1756 digital output lineup

Analog Input Modules

Analog input modules read continuous signals from the field, voltage levels, current loops, temperature sensors, and pressure transmitters. This is where the 1756 selection logic gets the most interesting, because analog modules have more meaningful spec differences than digital ones, and the right module depends on what you’re measuring and how precise you need to be.

How to read a 1756 analog input catalog number

The first letters after 1756-I identify the sensor type:

• IF = voltage or current (general-purpose analog)
• IR = RTD (resistance temperature detector)
• IT = thermocouple or millivolt
• IRT = combination RTD and thermocouple

The number that follows is the channel count (4, 6, 8, 12, or 16). The trailing letters flag the functional variant:

• I = individually isolated channels
• H = HART-enabled
• IH = isolated and HART-enabled
• CIS = current source for two-wire transmitters
• FX…F = high-speed combination input/output module

So a 1756-IF16H reads as ControlLogix analog input, 16 channels, voltage/current, HART-enabled. A 1756-IT6I reads as a thermocouple input, 6 channels, individually isolated.

The 1756 analog input lineup

Choosing the right analog input module

A few selection points from this lineup:

• The 1756-IF8 and 1756-IF16 are the analog workhorses. Eight or sixteen channels of voltage or current input. Most pressure, flow, and level transmitters in a typical industrial plant connect through these two modules. When you don’t have a specific reason to need isolation or HART, these are the defaults.
• Isolation matters when channels are at different ground references. The “I” variants give each channel its own isolation barrier, preventing ground loops when transmitters are powered from different sources or grounded in different domains. For instrumentation spanning long distances or noisy environments, the I variants are worth it.
• HART support unlocks transmitter intelligence. A HART-enabled module reads not just the 4-20 mA process signal but the digital data superimposed on the loop. For process plants running smart Allen-Bradley-compatible instrumentation, HART modules eliminate the need for separate HART multiplexers and pay back in asset management.
• Current sourcing (CIS) powers two-wire transmitters. The 1756-IF6CIS provides loop power to two-wire transmitters without their own supply, simplifying wiring and reducing parts.
• Temperature inputs deserve dedicated modules. For RTDs (1756-IR6I, IR12) or thermocouples (1756-IT6I, IT6I2, IT16), use a dedicated module rather than feeding a converted signal into an IF module. The dedicated modules handle linearization, cold-junction compensation, and sensor-specific scaling automatically.

Official Documentation: ControlLogix I/O Technical Data

For explicit signal filtering indices, backplane current draws, conversion times, and exact wiring connection terminal pinouts across all analog and digital channels, download the complete factory manual: 1756 ControlLogix I/O Modules Specifications Technical Data (PDF).

Analog Output Modules (1756-OF Series)

Analog output modules drive continuous signals to the field, variable-frequency drives, valve positioners, control-loop final elements, and any device that takes a 4-20 mA or voltage command. The 1756 output lineup is smaller than the input side because the field of outputs is simpler: fewer signal types, no temperature outputs, no sensor variety.

How to read a 1756 analog output catalog number

All analog outputs start with 1756-OF. The number is the channel count (4, 6, or 8). The trailing letters flag the variant:

• CI = current output only, individually isolated
• VI = voltage output only, individually isolated
• I = individually isolated (current or voltage)
• H = HART-enabled
• IH = isolated and HART-enabled

A 1756-OF8H reads as 8-channel analog output, current or voltage, HART-enabled. A 1756-OF6CI reads as 6 channels, current only, isolated.

The 1756 analog output lineup

Choosing the right analog output module

• The 1756-OF8 is the analog output workhorse. Eight channels, voltage or current per channel, no isolation, no HART. The default for most drive control, valve positioning, and analog output applications.
• Isolation matters for the same reasons as on the input side. The OF8I, OF6CI, and OF6VI variants protect against ground loops when outputs drive equipment at different ground references. For most plant-floor 4-20 mA loops sharing a common return, the standard OF8 is fine.
• HART on the output side closes the loop. A HART-enabled output module not only commands the field device’s process variable but also reads back HART diagnostic data from smart positioners and final elements. Combined with HART-enabled input modules, it gives the controller bidirectional visibility into smart Allen-Bradley-compatible instrumentation.
• Current-only vs. voltage-only is a wiring decision. The 1756-OF6CI and 1756-OF6VI are specialized: CI does only current outputs (the more common industrial choice for long wire runs), VI does only voltage outputs (used in shorter runs and certain laboratory or specialty applications). Most general-purpose applications use the OF8 or OF4 in current mode and don’t need the specialized variants.

Specialty 1756 I/O Modules

Beyond the four main families, the 1756 platform includes a handful of specialty I/O modules for applications outside standard digital and analog control:

• 1756-HSC — high-speed counter, for encoders, flow meters, and other pulse inputs that exceed the speed of standard digital inputs.
• 1756-CFM — configurable flow meter input, for specific flow-measurement applications.
• 1756-LSC8XIB8I — low-speed counter / digital input combination module.
• 1756-PLS — programmable limit switch module, used in rotary-position applications.
• 1756-CMS series (CMS1B1, CMS1C1) — ControlLogix Compute modules, the engineered replacement for the discontinued 1756-EWEB web server module.

Motion control modules (1756-M-series) are a category of their own and beyond the scope of this guide. If you’re working on a coordinated motion application, the advice is the same as the rest of the article: send us the catalog number you have or the application you’re trying to solve, and we’ll confirm what fits.

Final Architecture and BOM Validation

Before finalizing your parts order or initiating a scheduled plant shutdown, ensure that your physical signal terminations match your routing hardware and compute environment. Review our sister deployment resources:

• Allen-Bradley 1756 ControlLogix Processor Replacement Guide — Choose processing components, compare memory configurations, and map CPU generations.
• Allen-Bradley 1756 ControlLogix Chassis & Power Supply Selection Guide — Size racks and specify power delivery to prevent backplane drops.
• Allen-Bradley 1756 ControlLogix Communication Module Guide — Coordinate EtherNet/IP distributed routing architectures and ControlNet loops.

Source Your 1756 I/O Modules From TCI Supply

Sourcing Allen-Bradley 1756 I/O modules is core to what we do. The vast majority of modules covered in this guide remain in current production and are available new. Whatever you’re after, you get one place to ask instead of a dozen, with parts checked for authenticity before they ship.

TCI Supply is not an authorized distributor.

Frequently Asked Questions