Product Overview

The Allen Bradley 1788-FBJB4R is a 4-way redundant intelligent fieldbus junction box designed for FOUNDATION Fieldbus H1 and PROFIBUS-PA networks. It belongs to the Allen-Bradley Bulletin 1788 family and provides a fault-tolerant connection between field instruments and the main fieldbus trunk.

The junction box provides four field device drop-line connections and supports redundant trunk arrangements. Its internal monitoring circuits continuously supervise the connected drops and trunk conditions, helping prevent a fault on one connection from interrupting communication across the entire fieldbus segment.

One of its main functions is automatic fault isolation. When an individual drop develops an overcurrent condition or short circuit, the unit disconnects the affected branch while allowing the remaining field devices to continue operating.

The 1788-FBJB4R also monitors redundant trunk connections. If a trunk cable fault occurs, the junction box can reroute communication through the alternate path, improving fieldbus network availability.

Status LEDs provide information about fieldbus power and fault conditions, allowing maintenance personnel to identify wiring problems during commissioning and routine operation.

The unit is suitable for process automation systems that require protected field instrument connections and redundant fieldbus communication paths.

Technical Specifications

Product Features

• Four Field Device Connections: Provides four individual drop-line ports for connecting FOUNDATION Fieldbus or PROFIBUS-PA instruments.
• Redundant Trunk Architecture: Supports redundant fieldbus trunk connections to maintain communication when one cable path develops a fault.
• Automatic Drop Protection: Monitors individual branch currents and disconnects faulty drops to protect the remaining fieldbus segment.
• Trunk Fault Monitoring: Detects trunk open-circuit and short-circuit conditions in redundant configurations.
• Automatic Trunk Rerouting: Allows communication to use an alternate trunk path when a cable fault is detected.
• Overcurrent Protection: Uses a trunk trip mechanism to isolate an affected trunk under overcurrent conditions.
• LED Diagnostics: Provides visual status information for fieldbus connections and fault conditions.
• Fieldbus Compatibility: Supports both FOUNDATION Fieldbus H1 and PROFIBUS-PA installations.
• Industrial Wiring Connections: Accepts copper conductors within the specified wire-size range.

Product Working Principle

The 1788-FBJB4R operates as an intelligent fieldbus junction box. It distributes the main fieldbus connection to individual field instrument drops while continuously monitoring the electrical condition of the network.

Its operation involves several stages.

1. Fieldbus Trunk Connection

The main FOUNDATION Fieldbus H1 or PROFIBUS-PA trunk connects to the junction box. In a redundant configuration, two trunk paths provide alternative communication routes.

2. Field Device Distribution

The junction box distributes the fieldbus connection to four individual drop-line ports. Each port provides a connection point for a field instrument such as a transmitter or valve positioner.

3. Drop-Line Current Monitoring

The internal protection circuits monitor the current drawn by each field device connection. If a drop develops an overcurrent condition, the affected branch is disconnected to prevent the fault from affecting the other instruments.

4. Trunk Monitoring

The device monitors trunk connections for open-circuit and short-circuit conditions. In redundant installations, the alternate trunk provides another communication path when a cable fault occurs.

5. Trunk Trip Protection

When the trunk current exceeds the specified trip threshold, the protection circuit disconnects the affected trunk. The fault condition is indicated through the red LED.

6. Fault Reset

After the electrical fault has been corrected, the trunk trip can be reset using the reset button or the automatic reset configuration through DIP switches 3 and 4.

A simplified network arrangement is:

Fieldbus Trunk A / Trunk B → 1788-FBJB4R → Four Protected Drop Connections → Field Instruments

The junction box provides network distribution and electrical fault protection rather than performing controller logic or field instrument measurement.

Role in FOUNDATION Fieldbus and PROFIBUS-PA Control Architecture

Within a process automation system, the 1788-FBJB4R is installed between the main fieldbus trunk and the individual field instruments.

It provides protected branch connections and supports redundant trunk arrangements, making it particularly useful in installations where a short circuit or wiring fault on one field device should not interrupt the entire communication segment.

Industrial Applications

The 1788-FBJB4R is used in process installations where fieldbus connections require individual branch protection and redundant communication paths.

Installation and Maintenance

1. Verify the Model Number: Confirm that 1788-FBJB4R is the required four-way redundant fieldbus junction box.
2. Check Network Compatibility: Verify whether the installation uses FOUNDATION Fieldbus H1 or PROFIBUS-PA and confirm that the network design supports the junction box.
3. Inspect the Device: Check the housing, wiring terminals, status LEDs, and reset button for visible damage.
4. Prepare the Installation Location: Select a suitable mounting position within the control or field junction enclosure.
5. Mount the Junction Box: Secure the unit according to the installation instructions and ensure that the wiring terminals remain accessible.
6. Check the Power Supply: Verify that the external DC supply operates within the specified 12–32V DC input range.
7. Connect the Fieldbus Trunk: Wire the primary and redundant trunk connections according to the network wiring diagram.
8. Connect Field Device Drops: Connect the field instruments to the four designated drop-line ports and verify the wiring polarity.
9. Tighten the Terminals: Apply the specified terminal torque of 0.5–0.6 N·m to prevent loose electrical connections.
10. Verify Network Redundancy: Check that both trunk paths are correctly wired and that the alternate communication route is available.
11. Check Status Indicators: Energize the network and inspect the LEDs for normal operation and any trunk or drop fault indications.
12. Perform Routine Inspection: Periodically inspect the fieldbus wiring, power connections, branch protection status, and trunk connections for signs of deterioration.

Compatible System Components

Recommended Related Models

Key Advantages

• Provides four protected field device drop connections.
• Supports FOUNDATION Fieldbus H1 and PROFIBUS-PA.
• Supports redundant fieldbus trunk arrangements.
• Automatically isolates faulty drop connections.
• Monitors trunk open-circuit and short-circuit conditions.
• Provides alternate trunk communication paths.
• Includes LED-based fault indication.
• Helps maintain fieldbus communication during individual branch faults.

Technical FAQs

1. How many field devices can the 1788-FBJB4R connect?

The 1788-FBJB4R provides four individual drop-line ports. Each port can connect a compatible FOUNDATION Fieldbus H1 or PROFIBUS-PA field instrument, subject to the electrical and network loading limits.

2. What happens when one field device drop develops a short circuit?

The junction box monitors the individual drop connections and isolates the faulty branch when an overcurrent condition is detected. The remaining fieldbus connections can continue operating without being affected by that individual drop fault.

3. How does the 1788-FBJB4R handle a redundant trunk fault?

The device monitors the redundant trunk connections. When an open-circuit or short-circuit condition is detected, it can reroute trunk communication through the alternate path, helping maintain fieldbus network availability.

4. How can a trunk trip condition be reset?

After the cause of the overcurrent condition has been corrected, the trunk trip can be reset using the dedicated reset button. Alternatively, DIP switches 3 and 4 can be configured for automatic reset.