The MSV21-BB is a "Micro Comm" series Position Indicator (PI) manufactured by C.E. Electronics. It is a 1-inch character, segmented LED display used to show floor position and car direction in elevator transoms, hall fixtures, or car operating panels (COP). The "BB" suffix specifically designates the visual configuration of the unit.

Core Specifications

Manufacturer: C.E. Electronics, Inc.
Part Number: MSV21-BB
Display Type: Segmented LED (1-inch character height)
Visual Config (BB): Blue Display / Blue Directional Arrows
Input Interface: 3-wire Micro Comm (Serial)
Power Requirement: Typically 24V DC (supplied via the Micro Comm driver)
Functionality: Alphanumeric floor designation and directional arrows

Elevator Logic

Micro Comm Architecture: This is not a standalone unit. It must be connected to a Micro Comm Interface/Driver board. The driver converts discrete signals (like binary, gray code, or per-floor inputs) into the 3-wire serial data that the MSV21-BB decodes.
Serial Communication: Uses a 3-wire hookup (Power, Ground, and Data). This significantly reduces the number of wires required in the traveling cable compared to traditional "bulb-per-floor" indicators.
Directional Logic: The "BB" configuration features integrated up/down arrows. The arrows are triggered by the serial data stream based on the controller's directional preference (Up/Down/Nudging).
Self-Testing: The unit features a self-test mode upon power-up, usually cycling through segments to verify that all blue LEDs are functional.
Error Detection: Includes internal error correction to prevent "ghost" floor readings if the serial data is momentarily corrupted by electrical noise in the hoistway.

Accessories

Lens Options: The MSV21 series is often paired with a contrast-enhanced lens. For the "BB" model, a clear or blue-tinted lens is standard to maximize the visibility of the blue LEDs.
Micro Comm Driver: Required for operation. Common drivers include the CIF (Central Interface) or SIF (Serial Interface) boards found in the main controller or car-top station.
Mounting Studs: Typically surface-mounted to the back of the fixture plate using integrated threaded studs.
Passing Chime: An optional accessory that can be requested to trigger an audible "gong" or chime through the display unit as the car passes a floor.

Maintenance & Code

B44 Visual Requirements: Under Canadian Elevator Code (B44), position indicators must be clearly legible and provide direction of travel. The high-contrast blue LED of the MSV21-BB is often chosen to meet modern aesthetic requirements while maintaining code-compliant visibility.
Wiring Integrity: Since it relies on a 3-wire serial bus, a loose "Data" wire will cause the display to freeze, go blank, or show erratic segments across multiple floors. Check for tight terminations at the Micro Comm driver.
Segment Failure: If a single blue segment is dark (e.g., an "8" looks like a "6"), the LED segment itself has failed. Because these are sealed electronic units, the entire MSV21-BB board must be replaced; it is not field-repairable at the component level.
Cross-Wire Protection: The unit is protected against accidental cross-wiring of the 3-wire bus, but applying 110V AC to the 24V DC inputs will result in immediate board failure.