Allen-Bradley 1791-16AC 120VAC Discrete I/O Block

Allen-Bradley 1791-16AC Block I/O with 16 Inputs and 16 Outputs

Product Introduction

The 1791-16AC from Allen-Bradley belongs to the 1791 Block I/O series of distributed I/O products. This complete I/O block combines a power supply, Remote I/O adapter, communication interface, and I/O circuitry into one housing. Specifically, it provides 16 discrete input channels and 16 discrete output channels, both operating at 120VAC. You can connect this module to a main controller via Remote I/O protocol using a daisy-chain topology. Therefore, the 1791-16AC eliminates the need for separate adapter modules and power supplies in remote applications.

Technical Specifications

Input Characteristics

• Number of inputs: Sixteen (16)

• Group configuration: 2 groups of 8

• Nominal input voltage: 120VAC

• Nominal input current: 11 mA

• On-state voltage range: 79-132VAC, 47-63 Hz

• On-state current: 12.3 mA maximum at 132V

• Off-state voltage maximum: 35VAC

• Off-state current minimum: 2.3 mA

• Input signal delay (Off to On): 1.0 ms

• Input signal delay (On to Off): 26 ms

Output Characteristics

• Number of outputs: Sixteen (16) 120VAC outputs

• Output voltage range: 20-132VAC

• Output signal delay (Off to On): 1.0 ms

• Output signal delay (On to Off): 8.3 ms @ 60 Hz or 10.0 ms @ 50 Hz

External Power Requirements

• Voltage range: 85-132VAC, 47-63 Hz

• Current draw: 150 mA @ 132VAC

Power & Thermal Dissipation

• Maximum power dissipation: 31.3 Watts

• Maximum thermal dissipation: 106.73 BTU/Hr

Physical & Environmental

• Mounting options: Vertical or Horizontal

• Weight: 2.00 lbs (0.91 kg)

• UPC: 612598124749

System Architecture & Remote I/O Integration

The 1791-16AC acts as a Remote I/O adapter. It connects to the main controller through an integrated interface configured as a Remote I/O scanner. Using the Remote I/O protocol, block I/O devices like the 1791-16AC connect to the communication interface via a daisy-chain topology.

Isolation Ratings

• Power supply to RIO: 500VAC

• I/O group to I/O group: 1250VAC

• I/O group to Logic: 1250VAC

These isolation ratings protect the 1791-16AC from voltage spikes and ground loops. Therefore, you can safely use this module in industrial environments with electrical noise.

Compatible Field Devices

Input Devices (16 channels)
The 1791-16AC supports various discrete input field devices, including:

• Proximity switches

• Limit switches

• Selector switches

• Float switches

• Pushbutton switches

• Auxiliary contacts

• Other binary devices

Output Devices (16 channels)
The 1791-16AC can drive the following output devices:

• Pilot lamps

• Relay coils

• Horns, buzzers, and sirens

• Solenoid actuated devices

• Other electrical devices within the voltage and current ratings

Mounting & Output Current Ratings

The 1791-16AC supports two mounting orientations. Your choice directly affects the output current rating. This happens because vertical mounting allows better air circulation and cooling through the block.

Vertical Mounting (Recommended for higher loads)

• At 30°C: 600 mA per output

• At 60°C: 300 mA per output

Horizontal Mounting (Space-constrained cabinets)

• At 30°C: 300 mA per output

• At 60°C: 150 mA per output

Installation Clearance Requirements

• Maintain adequate space around the 1791-16AC for proper ventilation.

• Keep at least 2 inches (58.0 cm) clearance on all sides.

• Avoid placing heat-generating devices directly next to this module.

Input & Output Timing Considerations

First, the input Off-to-On transition takes only 1.0 ms. This makes the 1791-16AC suitable for high-speed discrete sensing applications. Next, the input On-to-Off transition takes 26 ms, which filters out brief contact bounces. Meanwhile, the output Off-to-On transition is also 1.0 ms for fast response. However, the output On-to-Off transition depends on line frequency. At 60 Hz, you get 8.3 ms. At 50 Hz, you get 10.0 ms. Therefore, account for these delays when designing time-critical control logic.

Installation Guidelines

Step-by-Step Wiring Notes

1. Disconnect all power before handling the 1791-16AC.

2. Choose vertical mounting for higher output current capacity.

3. Secure the module with appropriate hardware for your chosen orientation.

4. Connect external power between 85-132VAC, 47-63 Hz.

5. Wire input devices to the 16 input channels (2 groups of 8).

6. Wire output loads to the 16 output channels within the voltage range of 20-132VAC.

7. Connect the Remote I/O communication link in a daisy-chain topology.

8. Verify that the continuous current of each output device does not exceed the rating for your mounting orientation and temperature.

Frequently Asked Questions (FAQ)

1. Can I use the 1791-16AC with ControlLogix or CompactLogix PLCs?
Yes, the 1791-16AC works with these PLCs through a Remote I/O scanner module. You need a 1756-RIO or similar scanner that supports the Remote I/O protocol. The module connects via daisy-chain topology to the scanner.

2. Why does the output current rating change between vertical and horizontal mounting?
Vertical mounting allows better air circulation around the 1791-16AC. This improved cooling lets the module handle higher output currents. Horizontal mounting restricts airflow, so the rating drops by 50%. Therefore, always check your mounting orientation before calculating load capacity.

3. What happens if I connect a 132VAC input signal to this module?
The 1791-16AC accepts input voltages up to 132VAC within its on-state range. This is safe and within specification. However, voltages above 132VAC may damage the input circuitry. Always verify your field device output voltage before connection.

4. Can I mix different output device types on the same 1791-16AC?
Yes, you can connect different output devices across the 16 output channels. Each channel operates independently within the 20-132VAC voltage range. However, ensure that each device's continuous current stays within the rating for your mounting orientation and ambient temperature.

5. What is the difference between 1791-16AC and 1791-16A0?
The 1791-16AC provides 16 inputs AND 16 outputs (mixed I/O block). The 1791-16A0 provides 16 inputs only (input-only block). Choose the 1791-16AC when you need both sensing and actuation at a remote location. Choose the 1791-16A0 for sensing-only applications.