Product Overview
The Allen-Bradley 1791-8BC is a 16-point combination discrete input and output module from the Block I/O series. It provides eight (8) 24 VDC discrete inputs and eight (8) 24 VDC discrete outputs, making it a compact solution for distributed I/O architectures. The module features a rackless design and supports DIN rail mounting, simplifying installation in space-constrained panels.
Technical Specifications Breakdown
Key parameters for the 1791-8BC are listed below for quick reference:
| Feature | Specification |
|---|---|
| Product Type | Combination Discrete Input and Output Module |
| Inputs per Block | 8 |
| Outputs per Block | 8 |
| Input On-state Voltage Range | 10-30V dc |
| Input On-state Current | 11mA @ 30V max. |
| Output Voltage Range | 10-30V dc |
| Output Current (Vertical Mounting) | 500mA @ 60oC, 1A @ 30oC |
| Output Current (Horizontal Mounting) | 250mA @ 60oC, 500mA @ 30oC |
| External Power Supply | 19.2-30V dc |
| Current Draw | 300 mA, maximum |
| Communication Protocol | Remote I/O (RIO) |
| Weight | 6.00 lbs (2.72 kg) |
Industry Applications
This module is well-suited for discrete manufacturing lines where distributed I/O reduces wiring costs. Its 24 VDC inputs and outputs interface directly with sensors and actuators. The RIO protocol enables integration with legacy PLC-2, PLC-3, PLC-5, and SLC 500 systems, as well as modern Logix platforms via a communication module. For cost-sensitive upgrades, the 1791-8BC offers a straightforward replacement path without requiring a complete control system overhaul.
Installation & Setup Guidance
Mount the module vertically or horizontally on a DIN rail. Ensure the external power supply delivers 19.2-30V dc with at least 300 mA capacity. For RIO communication, connect the 1771-CD or Belden 9463 cable to terminals 6 (BLU), 7 (SHD), and 8 (CLR). Terminate the first and last nodes with a 150-ohm resistor across terminals 6 and 8 for 57.6K or 115.2K baud, or 82 ohms for 230.4K baud. Verify input wiring: apply 10-30V dc to input terminals; on-state current is 2.5 mA minimum at 10V and 11.0 mA maximum at 30V.
Maintenance & Reliability
Periodically inspect the module for dust accumulation and ensure adequate ventilation. The module dissipates low power, but verify ambient temperature stays within operating limits. Check LED indicators—individual channel LEDs and an OK LED—to confirm status. No internal user-serviceable parts exist; replace the module if faults persist.
Compatibility & Integration
The 1791-8BC communicates via Remote I/O (RIO) protocol at 57.6K, 115.2K, or 230.4K baud. It is compatible with Allen-Bradley PLC-2, PLC-3, 1785 PLC-5, 1747 SLC 500, and Logix platforms (CompactLogix, ControlLogix) when paired with a RIO communication module. The module uses 14 gauge stranded wire maximum with 3/64 inch maximum insulation.
Selection Guide & Purchasing Advice
This product is discontinued/obsolete. AutoDCSTech offers verified, ready-to-ship 1791-8BC modules for replacement or expansion. Contact our team to confirm availability and lifecycle status before ordering. We stock legacy Allen-Bradley parts to support your installed base.
Troubleshooting & Common Issues
If outputs do not energize, check the external power supply voltage (19.2-30V dc) and output current limits based on mounting orientation. If inputs fail to register, verify input voltage is within 10-30V dc and on-state current meets minimum thresholds. For RIO communication errors, confirm proper termination resistors and cable connections. Replace the module if LED indicators show no activity after power verification.
Availability & Condition at AutoDCSTech
| Part Number | 1791-8BC |
|---|---|
| Availability | In Stock |
| Condition | New |
| Warranty | One Year |
| Origin | US |
