Allen-Bradley 1746-OB16 Replacement for SLC 500

Product Overview

Allen-Bradley 1746-OB16 Replacement for SLC 500: Seamless Transition & Legacy System Upgrade

The Allen-Bradley 1746-OB16 is a 16-point DC sourcing output module designed for the SLC 500 modular I/O platform. As Allen-Bradley progressively transitions its installed base from the SLC 500 family toward the ControlLogix and CompactLogix architectures, many facilities still rely on SLC 500 racks for proven, stable process control. TOPNLMS supplies the 1746-OB16 as a direct drop-in replacement to minimize downtime, preserve existing wiring infrastructure, and extend the operational life of your SLC 500 control system without requiring an immediate full-platform migration.

Whether you are sourcing a spare to cover a failed module, building a critical-spares inventory for a production line, or executing a phased modernization that keeps the SLC 500 chassis in service while upgrading peripheral systems, the 1746-OB16 fits directly into any 1746 modular chassis — including the 1746-A4, 1746-A7, 1746-A10, and 1746-A13 rack sizes — without mechanical modification or re-engineering of the backplane wiring.

Each output point sources 24 V DC at up to 1 A continuous, with a total module current rating of 8 A. The module interfaces directly with the SLC 500 processor — including the 1747-L20, 1747-L30, 1747-L40, and 1747-L553 — through the standard 1746 backplane, requiring no additional communication configuration. Output addressing maps directly to the existing ladder logic program, meaning no software changes are required when replacing a failed 1746-OB16 with a new unit of the same part number.

Compatibility Comparison Table

Seamless Upgrade Solutions

A successful SLC 500 output module replacement rarely involves the 1746-OB16 in isolation. In practice, field engineers sourcing this module are often simultaneously addressing adjacent components in the same control cabinet or rack. The removable terminal block — typically the 1746-RT25 — should be inspected and replaced if terminal screws show corrosion or if the connector body has been thermally stressed by a prior output fault. Retaining the original RTB allows the field wiring to remain undisturbed during the module swap, which is the primary advantage of the removable terminal block design.

If the SLC 500 rack also contains analog I/O — such as the 1746-NI4 analog input module or the 1746-NO4I analog output module — these should be verified for firmware compatibility with the installed processor revision. Analog modules in the same chassis share the backplane power budget, and adding or replacing digital output modules can affect the chassis power supply loading. The 1746-P4 power supply, rated at 10 A at 5 V DC backplane, is the most common supply paired with larger 1746-A10 and 1746-A13 chassis configurations and should be load-checked after any module change.

For facilities running DH+ networks alongside the SLC 500 system, the 1747-SDN DeviceNet Scanner or the 1747-KE DH-485 to RS-232C interface module may be present in the same rack. These communication modules are unaffected by a digital output module replacement but should be noted in the spare-parts plan, as they are equally subject to obsolescence risk. Similarly, if the system uses a PanelView 550 or PanelView 600 HMI communicating over DH-485, the HMI program and tag mapping remain valid after a like-for-like 1746-OB16 replacement.

In modernization projects where the SLC 500 is being partially migrated — for example, retaining the existing I/O wiring while upgrading the processor to a 1769-L30ER CompactLogix — the 1746-OB16 can remain in service through a 1747-AENTR EtherNet/IP adapter, which allows legacy 1746 I/O modules to be addressed by a ControlLogix or CompactLogix processor over EtherNet/IP. This approach eliminates the need to rewire field devices while enabling the control program to be migrated to Studio 5000 Logix Designer at a controlled pace.

Retrofit Guidance FAQ

Q: Can I replace a failed 1746-OB16 without modifying the RSLogix 500 program?
A: Yes. A like-for-like replacement of the 1746-OB16 with the same part number requires no program changes. The output file address (O:x.0 through O:x.1) remains identical, and the module will be recognized automatically by the SLC 500 processor on power-up.

Q: Is the wiring compatible if I replace a 1746-OB16 with a 1746-OB16E?
A: The 1746-OB16E uses the same 20-pin removable terminal block (1746-RT25) and the same output wiring scheme. The enhanced version adds output fault detection and status bits, which are backward-compatible with existing programs — unused status bits are simply ignored.

Q: What is the backplane slot requirement?
A: The 1746-OB16 occupies one slot in any 1746 modular chassis. It can be installed in any slot position; the processor assigns the I/O address based on slot number, which is automatically detected. No jumper or DIP switch configuration is required.

Q: Can this module be used in a ControlLogix or CompactLogix system directly?
A: No. The 1746-OB16 is a 1746 backplane module and is not directly compatible with 1756 ControlLogix or 1769 CompactLogix chassis. However, it can be integrated into a ControlLogix system via the 1747-AENTR EtherNet/IP Remote I/O Adapter, which bridges 1746 I/O to an EtherNet/IP network.

Q: What communication protocol does the 1746-OB16 use?
A: The module communicates exclusively over the SLC 500 proprietary backplane. It does not require or support DH+, EtherNet/IP, DeviceNet, or DF1 configuration. All communication is handled automatically by the SLC 500 processor through the chassis backplane.

Q: How do I verify the module is functioning correctly after installation?
A: Use RSLogix 500 online monitoring to force individual output bits and verify field device response. The module’s output LED indicators (one per point) provide real-time status. A successful power-up with no fault LED on the processor confirms the module has been recognized and is operating within normal parameters.

Warranty Assurance

Every 1746-OB16 shipped by TOPNLMS is covered by a 12-month warranty from the date of delivery. Prior to shipment, each unit undergoes functional verification including backplane interface continuity, output point actuation testing, and visual inspection for physical damage or counterfeit indicators. Units that do not pass inspection are not shipped.

In the event of a warranty claim, TOPNLMS provides a replacement unit or full refund based on the customer’s preference. Our standard response time for warranty inquiries is within one business day. Replacement units are dispatched with priority shipping to minimize production downtime for customers with urgent operational requirements.

TOPNLMS maintains stock of the 1746-OB16 and related SLC 500 components to support both emergency replacement orders and planned maintenance procurement. Orders placed before 3:00 PM CST on business days are processed for same-day dispatch. International orders are shipped via DHL Express, FedEx International Priority, or UPS Worldwide Expedited, with full tracking provided at the time of shipment.

For B2B customers requiring volume pricing, consignment stock arrangements, or vendor qualification documentation (including test reports, certificates of conformance, or country-of-origin declarations), please contact our sales team directly.