Product Overview

The Allen Bradley 1790-8BV8BX CompactBlock LDX I/O Expansion Block is a 16-point mixed digital I/O expansion module combining eight universal DC inputs with eight sourcing DC outputs. It belongs to the CompactBlock LDX distributed I/O family and operates as an expansion unit in DeviceNet-based industrial automation systems.

The combination of input and output channels makes this module particularly useful in machines where sensor monitoring and actuator control are required within the same equipment section. Instead of installing separate input and output expansion blocks, the 1790-8BV8BX provides both functions in one compact assembly.

The eight input channels accept either sinking or sourcing signals, making them compatible with appropriately wired NPN and PNP DC sensors. The eight output channels use a sourcing transistor configuration and are intended to switch compatible 24 V DC field loads.

Consider an automated packaging machine with several photoelectric sensors monitoring product movement and a group of pneumatic valves controlling package positioning. The input channels can collect the sensor signals, while the output channels operate the compatible DC control devices. The PLC processes the input information and updates the output commands according to the machine sequence.

One important feature is that the input and output sections have different electrical configurations. The input side supports both sinking and sourcing arrangements, whereas the output side is specifically designed for sourcing operation. This distinction matters when preparing field wiring, particularly when the machine contains a mixture of PNP and NPN sensors alongside DC actuators.

The input channels have a nominal operating voltage of 24 V DC, with an ON-state voltage range of 9.6–28.8 V DC. The output section also uses a nominal 24 V DC supply and supports a specified ON-state voltage range of 10–28.8 V DC. Each output can handle up to 0.5 A, subject to a maximum combined current of 4.0 A per common connection.

The module uses a 37-pin D-shell connector for field wiring. Eight green input indicators and eight green output indicators provide individual channel status information. During commissioning, technicians can compare sensor activation with input indications and check whether the corresponding output channels respond to PLC commands.

The 1790-8BV8BX is an expansion block rather than an independent DeviceNet network adapter. It connects to a compatible CompactBlock LDX base block through the expansion interface. The base block manages network communication, while the expansion module adds the additional input and output capacity.

For machine retrofits, compact production equipment, and distributed control panels, the 1790-8BV8BX offers a practical combination of sensor acquisition and DC output control without requiring separate expansion modules for each function.

Technical Specifications

Product Features

• Combined Input and Output Channels: Integrates eight digital inputs and eight digital outputs in one expansion block.
• Universal Input Configuration: Accepts both sinking and sourcing DC input signals for compatible NPN and PNP sensors.
• Sourcing Transistor Outputs: Provides eight DC sourcing output channels for controlling compatible field devices.
• 24 V DC Operation: Uses a nominal 24 V DC field supply for industrial sensor and actuator circuits.
• Individual Channel Monitoring: Includes eight green input indicators and eight green output indicators.
• 0.5 A Output Rating: Supports up to 0.5 A per output, with a maximum of 4.0 A per common connection.
• Fast Output Response: Provides maximum switching delays of 0.5 ms OFF to ON and 1.0 ms ON to OFF.
• Photocoupler Isolation: Provides 1250 V AC RMS isolation between the I/O circuits and internal logic.
• 37-Pin D-Shell Connection: Provides a common field wiring interface for the input and output channels.
• Compact Expansion Format: Adds mixed I/O capacity to a compatible CompactBlock LDX assembly.

Working Principle

The 1790-8BV8BX combines two different functions: receiving discrete signals from field devices and switching external DC loads according to PLC commands.

Its basic operating arrangement is:

Field Sensors → 1790-8BV8BX Input Channels → CompactBlock LDX Base Block → DeviceNet → PLC Controller → Output Commands → 1790-8BV8BX Output Channels → Field Loads

Digital Input Signal Acquisition

The eight input channels monitor the electrical states of connected sensors and switches. When a field device changes its output state, the corresponding input circuit detects the voltage change and updates the module’s input information.

For example, a proximity sensor installed near a conveyor detects a metal component. Its output signal reaches one of the module’s input channels, allowing the PLC to identify the component’s presence and continue the programmed production sequence.

The input section supports both sinking and sourcing configurations. This means the module can accommodate compatible NPN and PNP sensor arrangements, provided the common connections are wired correctly.

Sinking and Sourcing Input Configuration

The input section uses a non-isolated universal input arrangement with eight points and eight common connections.

For sinking input operation, COM1 is connected to the positive 24 V DC field supply, while COM0 is connected to field power negative (GND).

For sourcing input operation, COM1 is connected to field power negative (GND), while COM0 is connected to the positive 24 V DC supply.

All COM1 connections are internally connected. The input wiring must follow the correct terminal arrangement for the selected sensor type.

This configuration is useful in machines where different field devices use different output conventions. However, the input wiring should be checked against the connector pin assignments rather than assuming that all sensor connections share the same polarity arrangement.

Sourcing Digital Output Operation

The eight output channels use sourcing transistor circuitry. When the PLC activates an output point, the corresponding output circuit switches the positive DC supply to the connected load.

For example, the PLC may activate Output 3 to operate a compatible pneumatic valve control circuit. The module switches the DC output, allowing the connected load to operate.

The output section has a maximum current rating of 0.5 A per channel and 4.0 A per common connection. These limits must be considered when several loads operate simultaneously.

The output common connections are arranged separately from the input commons. COM2 is connected to field power positive (+24 V DC), while COM3 is connected to field power negative (GND). All COM3 connections are internally connected.

Input and Output Status Monitoring

The module includes eight green input status indicators and eight green output status indicators.

When a sensor activates an input channel, the corresponding input indicator provides a visual reference for checking the signal condition. Similarly, the output indicators help technicians identify which output channels are operating.

This arrangement simplifies commissioning because the input and output sides can be checked separately. If a sensor signal is present but the expected actuator does not respond, technicians can examine the PLC command, output indication, field supply, and external wiring in sequence.

DeviceNet Expansion Communication

The 1790-8BV8BX does not operate as an independent DeviceNet node. Instead, it connects to a compatible CompactBlock LDX base block through the expansion interface.

The base block receives communication data from the DeviceNet scanner and manages the I/O information associated with the expansion assembly.

The module contributes eight input points and eight output points to the system. The input states are transferred through the expansion arrangement to the controller, while output commands are delivered to the corresponding transistor channels.

Coordinated Machine Operation

In a packaging application, the eight inputs may monitor product presence, conveyor position, and machine limit switches. The eight outputs may control compatible DC solenoids, indicator circuits, and auxiliary actuators.

The PLC processes the incoming signals and determines when each output should operate. The 1790-8BV8BX handles the actual field signal acquisition and switching operations.

This combination allows one expansion block to manage both sides of a small machine-control sequence while remaining integrated into the existing CompactBlock LDX architecture.

Role in the CompactBlock LDX Control Architecture

The Allen Bradley 1790-8BV8BX functions as a mixed digital I/O expansion component within the CompactBlock LDX distributed control architecture. It adds eight input channels and eight sourcing output channels to a compatible base block.

A typical system arrangement follows this structure:

PLC Controller → DeviceNet Scanner → CompactBlock LDX Base Block → 1790-8BV8BX → Field Sensors and Actuators

In a typical production line, the input section collects signals from different machine stations, while the output section operates the corresponding control devices.

For example, a conveyor system may use the input channels to detect incoming products and monitor transfer positions. The PLC processes these signals and activates the output channels to control compatible pneumatic equipment and auxiliary DC loads.

The mixed I/O arrangement reduces the need to allocate separate expansion blocks for basic input and output functions. The existing DeviceNet communication structure remains managed by the base block, while the 1790-8BV8BX handles the additional field connections.

Industrial Applications

Installation and Maintenance

1. Verify the Module Model: Confirm that Allen Bradley 1790-8BV8BX is the required eight-input and eight-sourcing-output expansion block.
2. Check Base Block Compatibility: Ensure that the selected CompactBlock LDX base block supports digital I/O expansion modules.
3. Inspect the Module: Examine the housing, 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 enough clearance for the module and field wiring.
5. Isolate the Power Supply: Disconnect the relevant control and field power supplies before installing the module or modifying its connections.
6. Mount the Expansion Block: Secure the module in the intended position and verify that its mounting arrangement is mechanically stable.
7. Connect the Expansion Cable: Connect the ribbon cable to the compatible base block and check that the connector is correctly seated. Follow the specified cable orientation, with the red stripe facing downward.
8. Wire the Digital Inputs: Connect the eight field sensors to their assigned input terminals through the 37-pin D-shell connector.
9. Configure Input Common Connections: Wire COM1 and COM0 according to the selected sinking or sourcing sensor arrangement. Verify the electrical polarity before applying power.
10. Connect the Sourcing Outputs: Wire COM2 to field power positive and COM3 to field power negative. Check that each external load is compatible with the module’s output ratings.
11. Test Input and Output Channels: Restore power and activate the sensors and output commands individually. Check the corresponding green indicators and confirm the actual field-device response.
12. Record the I/O Configuration: Document the input assignments, output loads, common connections, field supply arrangement, and expansion module position for future maintenance.

Compatible System Components

Recommended Related Models

Key Advantages

• Combines eight universal digital inputs and eight sourcing digital outputs.
• Supports both sinking and sourcing input configurations.
• Uses a nominal 24 V DC field supply.
• Handles up to 0.5 A per output and 4.0 A per common connection.
• Provides individual green indicators for all 16 I/O points.
• Supports photocoupler isolation between I/O circuits and internal logic.
• Uses a 37-pin D-shell field wiring interface.
• Expands existing CompactBlock LDX installations without requiring a separate network adapter for the expansion block.

Technical FAQs

What is the difference between the 1790-8BV8BX and the 1790-8BV8VX?

Both modules provide eight universal DC input channels and eight digital output channels. The main difference is the output configuration. The 1790-8BV8BX uses sourcing transistor outputs, while the 1790-8BV8VX uses sinking transistor outputs. The appropriate model depends on the electrical arrangement of the connected field loads and power supply.

Can the input channels accept both NPN and PNP sensors?

Yes. The eight input channels support both sinking and sourcing DC signals. The input common connections must be wired according to the selected sensor configuration. For sinking input operation, COM1 connects to +24 V DC and COM0 connects to GND. For sourcing input operation, COM1 connects to GND and COM0 connects to +24 V DC.

What is the maximum load current supported by each output?

Each sourcing output supports a maximum current of 0.5 A, with a maximum combined current of 4.0 A per common connection. These limits must both be observed when connecting multiple loads. The field supply must also remain within the specified 10–28.8 V DC operating range.

Can the 1790-8BV8BX communicate directly with a PLC through DeviceNet?

No. The 1790-8BV8BX is an expansion block without an independent DeviceNet network adapter. It connects to a compatible CompactBlock LDX base block through the expansion interface. The base block manages DeviceNet communication and transfers input and output information between the expansion assembly and the PLC.