Allen-Bradley 96657704 Fiber Optic Converter

The Allen-Bradley 96657704 (also cataloged as 1771-AF) is a fiber optic media converter designed for use with PLC-5 remote I/O (RIO) networks. It converts the standard 1771 RIO coaxial or twisted-pair electrical signals into fiber optic light signals, allowing extended communication distances up to 3 km and providing immunity to electrical noise. This device is essential for installations where the PLC chassis and remote I/O racks are separated by long distances or harsh electromagnetic environments.

Technical Specifications

Input interface: 1771 RIO (Belden 9463 twinaxial or shielded twisted pair) with standard 57.6 kbps or 115.2 kbps data rate. Output interface: duplex fiber optic (ST connectors) for 820 nm multimode fiber (62.5/125 µm or 50/125 µm). Optical power budget: typical 8 dB, supporting distances up to 3 km. Power supply: 24V DC (via external source) or derived from RIO cable (optional). The unit has a built-in optical receiver and transmitter with automatic gain control. LEDs indicate power, RIO activity, fiber link status, and fault. The converter is enclosed in a metal case for DIN rail mounting. Operating temperature: 0°C to 60°C. Dimensions: 120 mm x 75 mm x 45 mm. Weight approx. 0.5 kg.

Key Features

• Converts 1771 RIO to multimode fiber optic

• Supports 57.6 kbps and 115.2 kbps data rates

• Extends RIO distance up to 3 km (fiber)

• ST type fiber connectors (duplex)

• Electrical noise immunity (ground loop elimination)

• LED diagnostics for power, link, and activity

• DIN rail or panel mountable

• Compatible with 1771-ASB and 1771-AS series adapter modules

Functional Description

The 96657704 acts as a transparent repeater for the Allen-Bradley Remote I/O (RIO) protocol. It accepts the Manchester-encoded electrical signal from a PLC-5 processor or scanner module (e.g., 1771-SN) and converts it to light pulses. At the remote end, a second converter re-translates the optical signal back to RIO electrical format for the adapter module (e.g., 1771-ASB). The pair of converters replaces the RIO cable with a fiber optic cable, eliminating ground loops and reducing susceptibility to radiated EMI. Up to 31 remote racks can be connected in a daisy-chain topology using multiple converters.

Installation and Wiring

Mount the converter on a 35 mm DIN rail. Connect the 1771 RIO cable (Belden 9463) to the screw terminal block marked “RIO A, RIO B, Shield”. Connect the fiber optic cables: TX port to the RX port of the remote converter, and vice versa (cross-over). Power the unit with 24V DC (3-pin terminal: +24V, COM, GND). The maximum current consumption is 200 mA. If powered from the RIO cable (optional), no external supply is needed, but the cable length becomes limited. For distances > 300 m, external power is recommended. The fiber cable should be multimode (62.5/125 µm), with ST connectors and a maximum attenuation of 1 dB/km.

Applications

Typical applications include connecting PLC-5 controllers to remote I/O racks in large factories, power plants, or outdoor installations where copper RIO cables are impractical due to lightning risk or noise. The converter is also used in retrofits where legacy RIO networks are extended to new buildings. It is compatible with any device that uses the 1771 RIO protocol, such as some older variable frequency drives and operator interfaces.

Troubleshooting and Diagnostics

The converter has three LEDs: PWR (green) indicates external power present; LNK (yellow) indicates fiber link established (both converters communicating); RIO (green flashing) indicates data activity on the electrical RIO port. If the LNK LED is off, check the fiber cables for continuity and correct orientation (TX to RX). If RIO LED is off but LNK is on, check the RIO cable terminations and termination resistors (82 ohms) at both ends of the RIO segment. The converter can be used in a redundant pair configuration; consult the installation manual for high-availability setups.

Standards and Compliance

The 96657704 complies with CE (EMC directive), UL 508, and CSA. It is RoHS compliant (lead-free). The fiber optic interface is compliant with IEC 60825-1 class 1 for laser safety. The unit should be installed in a dry, pollution degree 2 environment.

Conclusion

The Allen-Bradley 96657704 fiber optic converter extends the life and reach of PLC-5 remote I/O systems. By replacing copper with fiber, it improves noise immunity and allows communication over long distances, making it a valuable component for maintaining legacy automation infrastructure.