Product Overview

The Allen Bradley 1790-T0A8X CompactBlock LDX Digital Output Expansion Module is an eight-point AC output expansion block used to switch external AC loads in CompactBlock LDX distributed I/O systems. It is intended for applications where a PLC-controlled machine needs additional AC switching points without replacing its existing I/O base block.

Unlike the 24 V DC transistor output modules in the CompactBlock LDX family, the 1790-T0A8X uses AC output switching circuitry. It supports an AC load voltage range of 15–132 V RMS and is commonly associated with 110 V AC field circuits operating at 60 Hz. This makes it suitable for controlling compatible AC-operated equipment and auxiliary control devices.

A typical application can be found in industrial machinery where a controller needs to switch several AC loads independently. For example, a packaging machine may use separate output points to operate AC indicator circuits, control relays, and other auxiliary electrical equipment. The PLC determines when each device should operate, while the 1790-T0A8X switches the corresponding field circuit.

The module provides eight non-isolated output points, with eight individually assigned common connections. Each output supports a maximum load current of 0.5 A RMS and a minimum load current of 10 mA RMS. When selecting connected loads, both the current limits and the AC voltage range need to be considered.

One important characteristic of this module is its switching behavior. The maximum operating time is 1 ms, while the maximum release time is one-half AC cycle plus 1 ms. Since the output switches AC loads, its operation is different from that of a conventional 24 V DC transistor output module. It should not be selected as a direct replacement for a DC output block simply because both products have eight output channels.

The 1790-T0A8X uses a screw-clamp terminal block with M3.0 terminals for field connections. The wiring arrangement includes VAC and COM connections, with the VAC terminals internally connected and the COM terminals internally connected. External AC power and load wiring must follow the designated terminal arrangement.

Within the CompactBlock LDX architecture, the 1790-T0A8X operates as an expansion block connected to a compatible base block. The base block handles the DeviceNet network connection, while the expansion module adds eight AC output points to the existing I/O assembly.

For existing automation equipment that requires additional 110 V AC switching capacity, the 1790-T0A8X offers a dedicated AC output expansion option within the CompactBlock LDX system.

Technical Specifications

Product Features

• Eight AC Output Points: Adds eight independently controlled AC output channels to a compatible CompactBlock LDX assembly.
• 110 V AC Field Operation: Supports a nominal 110 V RMS, 60 Hz external AC supply.
• Wide Load Voltage Range: Accommodates compatible AC loads operating between 15 and 132 V RMS.
• 0.5 A Output Capacity: Supports a maximum load current of 0.5 A RMS per output point.
• Individual Common Connections: Uses an eight-point, eight-COM wiring arrangement for field load connections.
• Fast Switching Response: Provides a maximum operating time of 1 ms.
• Photocoupler Isolation: Separates the I/O circuitry from the internal logic circuitry.
• Screw-Clamp Terminals: Uses M3.0 terminal connections for field wiring.
• Compact Expansion Design: Adds AC output capacity without requiring another independent network adapter.
• Multiple Mounting Options: Supports DIN rail and panel mounting within industrial control enclosures.

Working Principle

The 1790-T0A8X operates as an AC switching module within the CompactBlock LDX distributed I/O system. It receives output commands through the expansion arrangement and uses its internal output switching devices to control connected AC loads.

The basic control arrangement is:

PLC Controller → DeviceNet Scanner → CompactBlock LDX Base Block → 1790-T0A8X → AC Field Loads

Receiving Output Commands

The PLC determines when an external AC device should be switched on or off. Its output data is transferred through the DeviceNet network to the compatible CompactBlock LDX base block.

The base block manages the communication with the expansion assembly and makes the corresponding output command available to the 1790-T0A8X.

Each of the eight output points corresponds to an individual bit in the output data. When the associated output bit is set, the corresponding AC output channel is activated.

For example, a machine controller may activate Output 2 when a production station reaches a particular operating condition. The module switches the connected AC load circuit, allowing the assigned device to operate.

AC Load Switching

The 1790-T0A8X uses AC output switching circuitry rather than the DC transistor arrangement found in modules such as the 1790-0B16X and 1790-0V16X.

The module supports an AC load voltage range of 15–132 V RMS. Its external AC supply is specified at 110 V RMS, 60 Hz.

Each output can handle up to 0.5 A RMS, with a minimum specified load current of 10 mA RMS. These values are important when selecting connected equipment, particularly when several AC loads operate simultaneously.

The maximum ON-state voltage drop is 1.3 V RMS at maximum load, which should also be considered when evaluating the electrical requirements of the connected devices.

Output Circuit Configuration

The module provides eight output points with eight common connections. The VAC terminals are internally connected, and all COM terminals are internally connected.

The field power connection is made through the designated VAC and COM terminals. The external AC supply must be connected according to the module’s wiring diagram, and each load must be connected to its assigned output point.

This arrangement allows the eight output channels to operate separate loads while sharing the specified AC field supply connections.

Switching Response

The module has a maximum operating time of 1 ms. Its maximum release time is one-half AC cycle plus 1 ms.

The operating time represents the delay between an output command and activation of the output circuit. The release time accounts for the AC switching characteristics when the output command is removed.

This behavior is relevant when the module controls equipment that follows a programmed sequence. For example, several AC auxiliary devices may be switched at different stages of a production cycle.

Electrical Isolation

The 1790-T0A8X uses photocoupler isolation between its I/O circuitry and internal logic. Its specified isolation voltage is 1250 V AC RMS.

The input-to-output insulation resistance is specified at a minimum of 1,000 MΩ, with a dielectric strength of 2500 V RMS for one minute.

The field power circuit itself is non-isolated. This distinction is important when planning the control panel’s electrical grounding and field wiring arrangements.

DeviceNet Expansion Communication

The 1790-T0A8X does not operate as an independent DeviceNet network adapter. It connects to a compatible CompactBlock LDX base block through the expansion connection.

The base block manages the network communication, while the 1790-T0A8X handles the additional AC output switching functions.

This arrangement allows the PLC to control additional AC loads through the existing distributed I/O system without requiring a separate network connection for the expansion block.

Role in the CompactBlock LDX Control Architecture

The Allen Bradley 1790-T0A8X functions as an AC output expansion component within the CompactBlock LDX distributed I/O architecture. It adds eight AC switching points to a compatible base block, allowing the controller to operate additional field devices through the existing DeviceNet connection.

A typical system arrangement follows this structure:

PLC Controller → DeviceNet Scanner → CompactBlock LDX Base Block → 1790-T0A8X → AC Field Loads

In a typical production machine, the PLC controls several AC-operated auxiliary devices through the 1790-T0A8X. Each output point is assigned to a particular machine function, allowing different loads to operate independently.

For example, an automated production line may use the module to switch AC indicator circuits and auxiliary control relays at different stages of the process. The controller determines the switching sequence, while the base block and expansion module handle communication and field-level output operation.

The module is particularly relevant when the existing CompactBlock LDX installation uses AC output devices rather than 24 V DC transistor loads.

Industrial Applications

Installation and Maintenance

1. Verify the Module Model: Confirm that Allen Bradley 1790-T0A8X is the required eight-point AC digital output expansion module.
2. Check Base Block Compatibility: Ensure that the selected CompactBlock LDX base block supports digital output expansion modules.
3. Inspect the Module: Examine the housing, terminal connections, expansion interface, and mounting points for physical damage.
4. Prepare the Mounting Location: Select a suitable DIN rail or panel mounting position with sufficient space for the AC field wiring.
5. Isolate the Power Supply: Disconnect the external AC supply and relevant control power before installing the module or changing its wiring.
6. Mount the Expansion Block: Secure the module in its intended position and ensure that the mounting arrangement is mechanically stable.
7. Connect the Expansion Cable: Connect the ribbon cable between the base block and expansion module. Ensure that the connector is correctly seated and the red stripe faces downward.
8. Wire the AC Field Supply: Connect the nominal 110 V RMS, 60 Hz field supply to the designated VAC and COM terminals according to the module wiring diagram.
9. Connect the Output Loads: Wire each external AC load to its assigned output terminal and common connection. Check that the load voltage and current remain within the specified limits.
10. Check Terminal Connections: Inspect the M3.0 screw-clamp terminals and tighten them according to the specified maximum torque of 7 lb-in. Use suitable copper or copper-clad aluminum conductors.
11. Test Individual Output Channels: Restore power and activate each output through the PLC program. Verify that the corresponding AC load responds correctly and check for abnormal heating or wiring conditions.
12. Document the Output Configuration: Record the output assignments, connected loads, AC supply arrangement, and terminal wiring for future troubleshooting and maintenance.

Compatible System Components

Recommended Related Models

Key Advantages

• Adds eight AC digital output points to a compatible CompactBlock LDX assembly.
• Supports 110 V RMS, 60 Hz external AC field power.
• Accommodates AC loads within the 15–132 V RMS operating range.
• Handles up to 0.5 A RMS per output point.
• Provides eight common connections for the output wiring arrangement.
• Uses photocoupler isolation between the I/O circuits and internal logic.
• Offers a maximum output operating time of 1 ms.
• Uses screw-clamp terminals for direct field wiring.

Technical FAQs

Is the 1790-T0A8X a 24 V DC transistor output module?

No. The 1790-T0A8X is an AC digital output expansion module. It supports an AC load voltage range of 15–132 V RMS, with a nominal external supply of 110 V RMS at 60 Hz. It should not be confused with the 24 V DC transistor output modules in the CompactBlock LDX family.

What is the maximum load current for each output?

Each output supports a maximum load current of 0.5 A RMS, with a specified minimum load current of 10 mA RMS. The maximum OFF-state leakage current is 1.0 mA RMS at 100 V RMS, 60 Hz. Connected loads should remain within these electrical limits.

Can the 1790-T0A8X directly control an AC motor?

The module should not be treated as a general-purpose motor starter. Its maximum output current is 0.5 A RMS per point, and motor starting current can be considerably higher than normal running current. For motor applications, an appropriately rated contactor or other suitable power switching device should be used, with the module controlling the compatible interface circuit.

What is the difference between the 1790-T0A8X and the 1790-T0W8X?

Both modules provide eight output points, but their switching arrangements differ. The 1790-T0A8X is an AC output expansion block intended for 15–132 V RMS loads. The 1790-T0W8X uses relay outputs, allowing switching through relay contacts. The selection depends on the required load type, voltage, current, and switching characteristics.