Honeywell 621-9928 Serial Link Selector Module

The Honeywell 621-9928 Serial Link Selector Module is a high-performance communication switching module designed for use in Honeywell’s 620/621 Distributed Control System (DCS) environments. This module enables the selection and routing of serial communication links between controllers, operator stations, engineering workstations, or external systems. It is a critical component when building fault-tolerant and multi-channel communication topologies for process automation environments that demand high reliability and data integrity.

With the ability to switch between redundant or alternate communication paths, the 621-9928 ensures continuity in serial data transmission even in the event of a link failure. It supports serial protocols such as RS-232 and RS-422 (depending on configuration), and is engineered for integration into legacy TPS systems or modern hybrid architectures.

Product Specifications

Product Applications

The Honeywell 621-9928 is widely utilized in industries where uninterrupted serial communication is crucial for plant safety, control, and real-time monitoring. Key application areas include:

• Redundant Operator Interfaces – Automatically switches to a backup serial path if the primary fails.

• PLC-to-DCS Communication – Routes multiple PLCs through different serial channels for distributed data access.

• Third-party Integration – Selects between serial interfaces for non-Honeywell SCADA or MES systems.

• Remote Monitoring Systems – Provides backup communication paths for remote telemetry units (RTUs).

• Industrial Batch Control – Ensures communication continuity with serially connected field controllers.

• Process Plant Gateways – Interfaces DCS with external historians, analyzers, or SCADA via serial ports.

Product Advantages

• Automatic or Manual Switching: Offers flexibility with configurable control logic to suit different system designs.

• Redundant Serial Communication: Enables communication failover to maintain continuous system operation.

• Flexible Protocol Support: Compatible with both RS-232 and RS-422 serial standards.

• Integrated Status LEDs: Provides immediate visual diagnostics of active paths and failure conditions.

• System Compatibility: Seamlessly integrates with Honeywell 620/621 series systems and legacy architectures.

• Electrical Isolation: Minimizes the risk of ground loop interference or voltage spikes on serial lines.

• Long-Term Reliability: Built to industrial standards for continuous 24/7 operation in harsh environments.

FAQ

1. What types of serial communication does the 621-9928 support?

   The module supports both RS-232 and RS-422 protocols, depending on configuration.

2. Can the link switching be controlled programmatically from the host system?

   Yes, switching can be controlled via remote command from a supervisory controller or configured to switch automatically.

3. Does the module support bidirectional serial data flow?

   Yes, it handles full-duplex communication for both RS-232 and RS-422 interfaces.

4. How many communication paths can it manage simultaneously?

   The module supports 2 selectable input serial paths with 1 output path to the destination device.

5. Is link switching instantaneous?

   Switching is near-instantaneous (< 10 ms), minimizing communication disruption.

6. Does it provide any feedback or status to the controller?

   Yes, it has front-panel LEDs and can send fault or switch status back to the host system.

7. Is galvanic isolation implemented between channels?

   Yes, the serial lines are optically isolated to prevent electrical faults from propagating.

8. Can this module be used with non-Honeywell systems?

   Yes, it can switch between serial sources regardless of the manufacturer, as long as electrical standards are respected.

9. Is the baud rate configurable or fixed?

   It supports a wide range of baud rates from 300 bps to 115.2 kbps; compatible with standard serial configurations.

10. Can multiple 621-9928 modules be used in the same rack?

    Yes, multiple modules can be installed for multi-channel serial switching scenarios.