Product Overview

The Allen Bradley 1790-0W8X CompactBlock LDX Relay Output Expansion Module is an 8-point relay output expansion unit used to add discrete switching channels to a CompactBlock LDX distributed I/O system. Unlike transistor-based output modules, which use sourcing or sinking electronic switching, the 1790-0W8X uses normally open electromechanical relay contacts to control external electrical circuits.

This difference makes the module useful in applications where the control system needs to switch different types of electrical loads rather than being limited to one fixed DC output arrangement. Its relay contacts support resistive loads operating at specified DC and AC voltages, allowing the module to interface with compatible indicator circuits, control relays, and auxiliary switching equipment.

The 1790-0W8X provides eight independent relay output channels. Each channel has a normally open Form A contact arrangement, also known as single-pole, single-throw (SPST). When the PLC activates an output point, the corresponding relay operates and closes its contact, completing the external circuit. When the output is deactivated, the contact returns to its normally open state.

For example, consider a material-handling machine with several conveyor sections, pneumatic control circuits, and auxiliary warning indicators. The controller may need to switch different electrical loads according to production conditions. The 1790-0W8X can provide eight separate switching points, allowing the PLC to control these circuits through individual relay contacts.

Another useful characteristic is the separation between the module’s control circuitry and the external relay contact circuits. The module uses a 24 V DC field supply for its internal relay operation, while the output contacts switch the connected external loads within their specified ratings. This arrangement is different from a sourcing or sinking output, where the module directly supplies or switches the field load current through semiconductor output circuitry.

Each relay output has its own common connection, giving the module an eight-point, eight-common arrangement. This allows individual switching circuits to be wired separately rather than requiring all outputs to share one common terminal.

The module also includes eight green output status indicators. During commissioning, these indicators help technicians identify which output points have been commanded on and simplify troubleshooting when a connected device does not respond as expected.

The 1790-0W8X operates as an expansion unit rather than an independent DeviceNet node. It connects to a compatible CompactBlock LDX base block through the expansion interface, while the base block handles communication with the PLC or other programmable controller.

For existing CompactBlock LDX installations that need additional relay switching points, the 1790-0W8X offers a practical way to extend the output arrangement without changing the entire distributed I/O architecture.

Technical Specifications

Product Features

• Eight Relay Output Points: Provides eight independent relay switching channels for expanding a CompactBlock LDX assembly.
• Normally Open Contacts: Uses Form A SPST relay contacts that close when the corresponding output is activated.
• AC and DC Load Switching: Supports specified resistive loads at 5–24 V DC and 30 V AC.
• 2.0 A Contact Rating: Handles resistive loads up to 2.0 A within the specified voltage limits.
• Individual Common Connections: Provides one common connection for each relay output channel.
• Photocoupler Isolation: Separates the I/O control circuitry from the internal logic circuitry with a specified isolation voltage of 1250 V AC RMS.
• Green Output Indicators: Provides eight individual status LEDs for checking output conditions.
• Independent Relay Switching: Allows each output contact to operate according to its assigned PLC control command.
• Ribbon Cable Expansion: Connects to a compatible CompactBlock LDX base block through the expansion interface.
• Compact Installation: Adds relay switching capacity within a small distributed I/O assembly.

Working Principle

The 1790-0W8X operates by converting digital output commands received from the CompactBlock LDX base block into physical relay contact switching actions.

Its basic control arrangement is:

PLC Controller → DeviceNet Scanner → CompactBlock LDX Base Block → Expansion Interface → 1790-0W8X → External Electrical Loads

Output Command Processing

The process begins when the PLC program activates one of the eight output addresses assigned to the expansion module.

The DeviceNet-connected base block receives the output data from the controller and transfers the corresponding information through the expansion interface. The 1790-0W8X then operates the relay associated with the selected output point.

For example, when the PLC activates Output 2 to control an auxiliary warning circuit, the corresponding relay changes state and closes its normally open contact. The external circuit is completed, allowing the connected device to operate.

Normally Open Relay Switching

Each output uses a Form A relay contact arrangement. Under normal conditions, the contact remains open, preventing current from flowing through the external circuit.

When the output is activated, the relay closes the contact and establishes electrical continuity between the output terminal and its corresponding common connection.

When the PLC removes the output command, the relay returns to its normally open condition.

This switching arrangement is particularly useful when the control system needs to operate external circuits without using a conventional sourcing or sinking transistor output.

Individual Contact Circuits

The eight outputs have separate common connections, with one point assigned to each COM terminal.

This configuration allows the output channels to control separate electrical circuits. For example, one output may operate an indicator circuit, another may control an interface relay, and additional channels may switch other auxiliary equipment.

The external load supply must be wired through the appropriate relay contacts and remain within the specified contact voltage and current ratings.

AC and DC Load Switching

The relay contacts support specified resistive loads at both DC and AC voltages.

For DC applications, the documented rating is 2.0 A at 5–24 V DC. For AC applications, the documented rating is 2.0 A at 30 V AC.

These ratings are load-dependent and should not be interpreted as unrestricted switching capacity. Inductive loads such as contactor coils and solenoids can produce switching transients that affect contact life. Appropriate suppression and load-protection arrangements should be considered when connecting such devices.

Relay Switching Response

The module has a maximum output delay of 10 ms for both OFF-to-ON and ON-to-OFF transitions.

This switching characteristic makes it suitable for conventional discrete control operations, including auxiliary equipment switching, machine-status circuits, and other applications that do not require high-speed semiconductor output switching.

Output Diagnostics

Eight green status LEDs correspond to the individual output channels. When an output is activated, its associated indicator provides a visual reference for checking the commanded output condition.

During troubleshooting, technicians can compare the PLC output command with the module’s status indication and the actual state of the external relay circuit.

By combining eight independent relay contacts with DeviceNet-based distributed I/O expansion, the 1790-0W8X allows additional switching functions to be incorporated into an existing CompactBlock LDX installation.

Role in the CompactBlock LDX Control Architecture

The Allen Bradley 1790-0W8X serves as a relay-based output expansion component within the CompactBlock LDX distributed I/O architecture. It increases the available discrete switching capacity while relying on the base block for network communication and output data management.

A typical system arrangement follows this structure:

PLC Controller → DeviceNet Scanner → CompactBlock LDX Base Block → 1790-0W8X → External Control Circuits

In a packaging machine, for example, the PLC may use the main CompactBlock LDX assembly to control conveyor movement, pneumatic equipment, and machine-status functions. The 1790-0W8X adds relay contacts for auxiliary circuits that require contact-based switching.

The controller sends output information through the DeviceNet scanner to the base block. The base block then transfers the assigned output data to the expansion module.

This arrangement keeps the communication function separate from the physical switching function. The base block manages the network connection, while the 1790-0W8X handles the eight external relay circuits.

The module is particularly useful when an existing distributed I/O installation requires additional relay outputs rather than more sourcing or sinking transistor channels.

Industrial Applications

Installation and Maintenance

1. Verify the Module Model: Confirm that Allen Bradley 1790-0W8X is the required eight-point relay 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, 37-pin D-shell connector, 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 external wiring and maintenance access.
5. Isolate the Power Supply: Disconnect the relevant field and control power supplies before installing the module or modifying the relay wiring.
6. Mount the Expansion Module: Secure the module using the specified mounting arrangement and check that it is mechanically stable.
7. Connect the Expansion Interface: Connect the ribbon cable to the compatible CompactBlock LDX base block and verify that the connection is correctly seated.
8. Wire the Relay Outputs: Connect the eight output channels and their corresponding common terminals according to the module’s 37-pin D-shell wiring diagram.
9. Verify Load Compatibility: Check that each connected load falls within the specified AC or DC contact voltage and current ratings.
10. Inspect Contact Circuit Wiring: Verify conductor connections, insulation condition, terminal assignments, and appropriate protection for the external switching circuits.
11. Test Each Relay Channel: Restore power and activate the output points individually through the PLC. Check the corresponding green LEDs and verify the actual switching operation of each external circuit.
12. Record the Output Configuration: Document the relay assignments, common connections, external load ratings, and wiring arrangement for future troubleshooting and maintenance.

Compatible System Components

Recommended Related Models

Key Advantages

• Provides eight independent relay output channels.
• Uses normally open Form A SPST relay contacts.
• Supports specified AC and DC resistive load switching.
• Offers a maximum resistive contact rating of 2.0 A within the specified voltage range.
• Provides one common connection for each output point.
• Includes eight green output status indicators.
• Separates external relay switching circuits from internal control logic through the relay and photocoupler arrangement.
• Adds relay output capacity to an existing CompactBlock LDX distributed I/O assembly.

Technical FAQs

What is the difference between the 1790-0W8X and the 1790-0B16X?

The two modules use different output technologies and provide different numbers of output points. The 1790-0W8X has eight electromechanical relay outputs with normally open Form A contacts, while the 1790-0B16X has 16 DC sourcing transistor outputs. The relay version is suitable for contact-based AC and DC switching within its specified ratings, whereas the sourcing version is intended for compatible DC field loads.

Can the 1790-0W8X switch both AC and DC loads?

Yes. Its relay contacts support specified resistive loads at 5–24 V DC and 30 V AC, with a maximum current rating of 2.0 A under the corresponding rated conditions. These ratings should not be extended to other voltages or load types without checking the electrical requirements. Inductive loads also require consideration of switching transients and contact protection.

What does Form A normally open mean for the relay outputs?

Form A refers to a normally open single-pole, single-throw contact arrangement. When the output is inactive, the contact remains open and the external circuit is interrupted. When the corresponding PLC output command is activated, the relay closes the contact and allows current to flow through the connected circuit.

Can the 1790-0W8X operate as an independent DeviceNet node?

No. The 1790-0W8X is an expansion module and relies on a compatible CompactBlock LDX base block for DeviceNet communication. The base block receives commands from the PLC and transfers the output information through the expansion connection. The relay module itself provides the eight additional switching channels rather than an independent network interface.