Product Overview

The Allen Bradley 1790-8BV8VX CompactBlock LDX I/O Expansion Block is a mixed digital I/O expansion module combining eight universal DC inputs with eight sinking transistor outputs. It belongs to the CompactBlock LDX family and is used to extend the signal acquisition and output control capacity of DeviceNet-based automation systems.

The module is particularly suitable for machine installations where additional sensor inputs and DC output points are needed in the same equipment section. Instead of allocating separate expansion blocks for input and output functions, the 1790-8BV8VX combines both within one assembly.

Its eight input channels accept sinking or sourcing DC signals, allowing compatible NPN and PNP sensors to be connected through the appropriate common-terminal configuration. The eight output channels use sinking transistor circuitry and switch compatible DC loads by providing a current path toward field power negative (GND).

A typical application can be found in automated material handling equipment. Proximity sensors and photoelectric switches monitor product positions, while the output channels control compatible DC solenoid valves, indicator circuits, and other auxiliary loads. The PLC processes the incoming sensor information and determines when the output channels should operate.

The distinction between the input and output configurations is important when integrating this module into an existing control panel. Although the input section supports both sinking and sourcing sensor arrangements, the output section is specifically intended for sinking operation. Selecting compatible field devices and connecting the output common terminals correctly are essential for proper operation.

The module operates with a nominal 24 V DC field supply. Its input channels recognize ON-state signals within a range of 9.6–28.8 V DC, while the output section supports a 10–28.8 V DC operating range. Each output supports a maximum current of 0.5 A, with a maximum combined current of 4.0 A per common connection.

The 1790-8BV8VX has eight green input status indicators and eight green output status indicators. These indicators are useful during machine commissioning because technicians can distinguish between missing field signals and output channels that are not responding to controller commands.

The module uses a 37-pin D-shell connector for field wiring and connects to a compatible CompactBlock LDX base block through its expansion interface. It does not function as an independent DeviceNet network adapter. Instead, the base block manages network communication and incorporates the additional I/O information into the existing distributed control system.

For equipment that needs additional sensor monitoring together with sinking-type DC output control, the 1790-8BV8VX offers a combined I/O arrangement without requiring separate expansion blocks for these two functions.

Technical Specifications

Product Features

• Combined Digital I/O: Integrates eight universal digital inputs and eight sinking transistor outputs in one expansion block.
• Universal Input Compatibility: Accepts both sinking and sourcing DC input signals for compatible NPN and PNP sensors.
• Sinking Output Configuration: Provides eight transistor output channels for controlling compatible DC field loads.
• 24 V DC Operation: Supports common industrial low-voltage sensor and actuator circuits.
• Individual Channel Indicators: Includes eight green input LEDs and eight green output LEDs for checking signal conditions.
• 0.5 A Output Capacity: Supports up to 0.5 A per output, with a maximum combined current of 4.0 A per common connection.
• Fast Switching Response: Offers maximum output 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 Interface: Provides the field wiring connection for the input and output channels.
• Compact Expansion Format: Adds mixed I/O capacity to compatible CompactBlock LDX assemblies without requiring another independent network adapter.

Working Principle

The 1790-8BV8VX combines discrete signal acquisition and DC load switching. Its input section receives electrical signals from field sensors, while its output section controls external loads according to commands from the PLC.

The basic operating arrangement is:

Field Sensors → 1790-8BV8VX Input Channels → CompactBlock LDX Base Block → DeviceNet → PLC Controller → Output Commands → 1790-8BV8VX Output Channels → Field Loads

Digital Input Signal Acquisition

The eight input channels monitor the electrical states of connected field devices. When a sensor detects an object or reaches a switching position, its output circuit changes state. The corresponding input channel recognizes the voltage change and updates the module’s input information.

For example, a photoelectric sensor installed at a conveyor transfer point detects a passing package. Its signal reaches one of the module’s input channels, allowing the PLC to determine whether the package has reached the expected position.

The input section supports both sinking and sourcing configurations. This allows compatible NPN and PNP sensors to be used in the same expansion block, provided the common connections are arranged correctly.

Sinking and Sourcing Input Configuration

The input section has eight non-isolated universal DC input points with eight common connections.

For sinking input operation, the common connection is wired to field power negative (GND).

For sourcing input operation, the common connection is wired to the positive 24 V DC field supply.

The common connections are internally arranged according to the module’s input wiring configuration. The D-shell connector pin assignments must be followed when preparing the field wiring.

This arrangement is useful in equipment where different sensor types are already installed and replacing all field devices with one output configuration would be unnecessary.

Sinking Digital Output Operation

The eight output channels use sinking transistor circuitry. When the PLC activates an output point, the corresponding transistor switches the output circuit toward field power negative (GND), allowing current to flow through a correctly connected external load.

For example, a PLC command can activate Output 2 to operate a compatible DC solenoid valve. The load receives current through the field supply and the active sinking output circuit.

The output section supports a maximum of 0.5 A per channel and 4.0 A per common connection. Both limits must be considered when several loads are energized simultaneously.

The output wiring must follow the sinking arrangement. COM0 is connected to field power negative (GND), while the corresponding load supply connections are arranged according to the module’s field wiring diagram.

Unlike a sourcing output module, the 1790-8BV8VX does not switch positive voltage directly to the load. This distinction is particularly important when selecting replacement modules for an existing machine.

Input and Output Status Monitoring

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

When a field sensor activates an input channel, the corresponding input LED provides a visual indication of the signal condition. The output LEDs show the operating status of individual output channels.

During troubleshooting, technicians can compare the input and output indications with the actual field-device conditions. For example, if a proximity sensor activates but the expected solenoid valve does not operate, the technician can check the PLC command, output indication, field supply, and load wiring separately.

DeviceNet Expansion Communication

The 1790-8BV8VX is an expansion block rather than an independent DeviceNet communication adapter.

It connects to a compatible CompactBlock LDX base block through the expansion interface. The base block manages the network connection with the DeviceNet scanner and transfers the associated I/O information between the expansion assembly and PLC.

The eight input states are made available to the controller, while output commands from the PLC are transferred to the corresponding sinking transistor channels.

Coordinated Input and Output Operation

In an automated sorting machine, the input channels may monitor product-detection sensors and conveyor-position switches. After processing these signals, the PLC activates the output channels to operate compatible DC actuators or pneumatic control valves.

The module handles the field-level signal acquisition and switching, while the PLC determines the sequence and timing of machine operations.

This arrangement allows sensor monitoring and actuator control to share one expansion block within the existing CompactBlock LDX architecture.

Role in the CompactBlock LDX Control Architecture

The Allen Bradley 1790-8BV8VX functions as a mixed digital I/O expansion component in the CompactBlock LDX distributed control architecture. It adds eight universal input channels and eight sinking output channels to a compatible base block.

A typical system arrangement follows this structure:

PLC Controller → DeviceNet Scanner → CompactBlock LDX Base Block → 1790-8BV8VX → Field Sensors and Actuators

In a typical production installation, the base block communicates with the PLC through DeviceNet. The 1790-8BV8VX adds field connections for sensors and DC loads located around a machine station.

For example, a conveyor system may use the input channels to detect incoming products and confirm transfer positions. The PLC processes these signals and activates the sinking outputs to operate compatible solenoid valves and auxiliary DC devices.

The mixed I/O configuration is useful when both signal acquisition and output control are required at the same location. The base block continues to manage the network connection, while the expansion module handles the additional field I/O points.

Industrial Applications

Installation and Maintenance

1. Verify the Module Model: Confirm that Allen Bradley 1790-8BV8VX is the required eight-input and eight-sinking-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, 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 clearance for field wiring and maintenance access.
5. Isolate the Power Supply: Disconnect the relevant system and field power supplies before installing the expansion module or changing its connections.
6. Mount the Expansion Block: Secure the module in the intended position and check 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 cable is correctly aligned and that its red stripe faces 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: Connect the input commons according to the selected sinking or sourcing sensor arrangement. Verify the sensor polarity and connector pin assignments.
10. Connect the Sinking Outputs: Wire the output common connections to field power negative (GND) and connect each load according to the module’s sinking output wiring diagram. Check the current requirements of every load.
11. Test Input and Output Channels: Restore power and activate the field sensors and PLC output commands individually. Check the corresponding green indicators and verify the actual field-device response.
12. Document the I/O Configuration: Record the sensor assignments, output loads, common-terminal wiring, field power arrangement, and expansion module position for future troubleshooting.

Compatible System Components

Recommended Related Models

Key Advantages

• Combines eight universal digital inputs and eight sinking transistor outputs.
• Accepts both NPN and PNP sensor signals through the appropriate input wiring configuration.
• Supports 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.
• Uses photocoupler isolation between the I/O circuits and internal logic.
• Supports compatible DC solenoid valves and other sinking-output loads.
• Adds mixed I/O capacity to an existing CompactBlock LDX installation.

Technical FAQs

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

Both modules combine eight universal digital inputs with eight transistor outputs. Their main difference is the output configuration. The 1790-8BV8VX uses sinking outputs, while the 1790-8BV8BX uses sourcing outputs. The correct selection depends on the electrical arrangement of the connected field loads and the existing control circuit.

Can the 1790-8BV8VX accept both NPN and PNP sensor signals?

Yes. Its eight input channels support both sinking and sourcing DC input configurations. The input common connections must be wired according to the selected sensor type. The connector pin assignments and field wiring diagram should be checked before connecting the sensors.

What is the maximum current capacity of the sinking outputs?

Each output supports a maximum current of 0.5 A, with a maximum combined current of 4.0 A per common connection. The output circuit also has a maximum voltage drop of 0.5 V DC and an OFF-state leakage current of 0.5 mA. The connected loads must remain within these electrical limits.

Can the 1790-8BV8VX operate as an independent DeviceNet I/O node?

No. The 1790-8BV8VX is an expansion block and does not contain an independent DeviceNet network adapter. It must connect to a compatible CompactBlock LDX base block through the expansion interface. The base block manages communication with the DeviceNet scanner and PLC controller.