The Allen-Bradley 1791-24A8 simplifies machine-side I/O integration. This block I/O module operates at 120VAC and handles both discrete input and output signals. It comes with three groups of 8 input channels, giving you 24 inputs total. The output side has one group of 8 channels. Therefore, you get balanced local control without extra chassis or rack mounting.
This module supports an input on-state voltage range from 79VAC to 132VAC. The on-state current reaches 12.3mA at 132V max. Additionally, the off-state voltage stays below 35V max, with off-state current at 2.3mA min. For outputs, the voltage range is 20-123VAC. The external power supply requires 85-132VAC at 150mA.
The 1791-24A8 consumes a maximum of 18.76 watts. Its thermal dissipation goes up to 63.97 BTU/hr. For communication, it supports standard Remote I/O baud rates: 57.6K, 115.2K, and 230.4K. The cable distance depends on the baud rate. Specifically, you can run 10,000 feet at 57.6K, 5,000 feet at 115.2K, or 2,500 feet at 230.4K. Use Belden 9463 cable (blue hose) for reliable connections.
You can mount the 1791-24A8 vertically or horizontally. This flexibility saves panel space. The module weighs only 1.88 lbs (0.85 kg). Its compact shape fits well inside crowded cabinets. Moreover, the built-in RIO adapter mode eliminates external communication adapters. As a result, installation becomes faster and simpler.
First, connect the 1791-24A8 to your PLC using Belden 9463 cable. This three-wire cable includes blue, clear, and shield wires. Next, set the baud rate switch on the module to match your scanner. Then, power the unit with 85-132VAC at 150mA. For daisy-chain topology, install a terminating resistor on the last node. Use either an 82 ohm or 150 ohm resistor. Without this resistor, you may face intermittent communication errors.
Furthermore, keep input and output wiring separate from communication cables. This practice reduces electrical noise. Also, ensure the on-state voltage stays between 79-132VAC for proper input detection. Finally, ground the shield wire at one end only.
The 1791-24A8 works with several Allen-Bradley controllers. For example, connect it to PLC-2, PLC-3, or PLC-5 families via their embedded RIO scanner. When using an SLC 500, add the 1747-SN Remote I/O scanner module. This setup turns the SLC 500 into a master scanner. Meanwhile, the 1791-24A8 acts as an adapter. Consequently, you can expand I/O points without changing the main controller.
This module reduces wiring labor significantly. Instead of running hundreds of individual wires to a central chassis, you place the 1791-24A8 near the field devices. Thus, cable costs drop. Troubleshooting also becomes easier because each group of 8 inputs has separate status indicators. Additionally, the embedded power supply means no external 24VDC converter is needed for the logic side.
1791-24A8 block I/O module (1 unit)
Built-in power supply (factory installed)
Embedded RIO interface adapter
Terminal block connectors (included)
User manual quick start guide
Note: No certificate or warranty document is included.
All orders ship via FedEx, UPS, or DHL. Tracking numbers are provided within 24 hours.
1. Can I use the 1791-24A8 with a ControlLogix controller?
Yes, but you need a 1756-RIO scanner module in the ControlLogix chassis. The 1791-24A8 then communicates as a remote adapter.
2. What happens if I exceed the off-state voltage of 35VAC?
The input may false-trigger. Therefore, always keep the off-state voltage below 35VAC to prevent random on signals.
3. How many 1791-24A8 modules can I connect to one scanner?
It depends on the scanner’s memory and RIO baud rate. Typically, you can connect up to 32 blocks at 57.6K baud on a single RIO link.
4. Does the module support hot swapping?
No. Disconnect power before removing or inserting the 1791-24A8. Otherwise, you may damage the internal circuits.
5. Which terminating resistor should I use for long cables?
Use 150 ohms for cable runs shorter than 500 feet. Use 82 ohms for longer runs up to 10,000 feet. Check your network length first.
| Brand Name | Allen-Bradley |
| Model No. | 1791-24A8 |
| Product Type | Block I/O Module |
| hasMerchantReturnPolicy | Adjust according to reality |
| Shippingdetails | Adjust according to reality |
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