Stop Bar Control Module

A Stop Bar Control Module is an electronic control unit that manages the operation of stop bar lights, receiving commands from the ATC or ALCMS and switching the stop bar circuit on/off, often with intensity control and interlock features to ensure safe sequencing with other AGL systems. The module ensures that stop bars are correctly illuminated only when the runway is occupied, preventing runway incursions. It typically interfaces with the constant current regulator (CCR), the ALCMS, and the stop bar monitoring unit to provide a complete stop bar control and supervision solution.

The stop bar control module exists to make sure a stop bar's on/off state is never left to chance or a fixture-level fault — it's the component that translates an actual ATC clearance decision into the physical illumination state of the barrier a pilot sees. Because a stop bar's entire safety function depends on it accurately reflecting whether the runway is actually clear, this module's interlocking logic is what prevents the bar from being switched off, and an aircraft cleared to cross, while a genuine conflict still exists.

Its coordination with other AGL systems is just as important as the switching itself: sequencing stop bar state correctly relative to runway lighting and other holding indicators is what stops mixed signals from ever reaching a pilot, which is exactly the kind of ambiguity that runway incursion defences are designed to eliminate entirely.

Stop bar control modules are installed at runway holding positions on taxiways, particularly where precise control and sequencing are required. They are used on:

• Precision approach runways (CAT II and III) where stop bar control is critical
• Busy airports with high traffic volumes where automated control is beneficial
• Low visibility operations where reliable stop bar operation is essential
• Complex taxiway intersections with multiple stop bars requiring coordination
• Airports with ALCMS where remote and automated control is implemented
• Military airfields requiring robust and secure control systems

The module is installed in an equipment shelter, CCR room, or dedicated control cabinet close to the stop bar circuit it serves, wired into the constant current regulator, the ALCMS, and the stop bar monitoring unit so switching commands, brightness settings, and status feedback can all pass through a single, centrally located point. Mounting is typically on an equipment rack or panel alongside related AGL control equipment, with clear labelling of the specific holding position or stop bar circuit each module serves, since a control room may house several modules covering different taxiway intersections. Control and monitoring cabling runs from the module out to the field circuit and to the stop bar monitoring unit over dedicated, clearly identified cable routes, kept separate from unrelated control circuits to avoid cross-connection errors during installation or later maintenance. Because the module's interlock logic is safety-critical, its wiring and configuration are verified against the site's specific stop bar sequencing requirements during commissioning, rather than installed to a generic default configuration.

Stop bar control modules operate as intelligent switching and control devices:

• Command Reception: The module receives on/off commands from the ATC via a control panel or from the ALCMS via digital communication (e.g., Modbus, Ethernet).
• Switching: It uses relays or solid-state switches to energize or de-energize the stop bar circuit, turning the lights on or off.
• Intensity Control: Some modules can adjust the intensity of the stop bar lights by controlling the CCR current step, matching the prevailing visibility conditions.
• Interlocks: The module may include interlock logic to prevent conflicting commands, such as ensuring that a stop bar is not turned on while a clearance bar is active, or that the stop bar is deactivated before the runway entrance light is turned on.
• Status Feedback: The module reports its status (on/off, intensity step, fault) to the ALCMS, providing real-time visibility for the controller.

The control module is the layer that turns a stop bar from a passive series-circuit fixture into an addressable safety device. Above it sits the ALCMS and, ultimately, the tower controller who issues the clearance decision; below it sits the CCR and the physical stop bar circuit. What makes the module's logic safety-critical rather than a simple relay is the interlocking it enforces between adjacent controlled elements: a stop bar cannot typically be switched off while the corresponding runway entrance lights or a downstream clearance bar are commanded on, and at CAT II/III-protected intersections the module may additionally require confirmation that the associated taxiway centreline lead-on lights are correctly sequenced before releasing the stop bar. At larger or more complex airports, the module is one node in a wider Advanced Surface Movement Guidance and Control System (A-SMGCS), where "follow-the-greens" routing software commands stop bars in and out of service automatically as part of a planned taxi route rather than through a single ATC button press — the module still performs the same switching and interlock function, just on commands originating from route logic instead of a human operator.

The module is built as an electronic control unit rather than an electromechanical one: a DIN-rail or panel-mounted PCB assembly carries the relay contacts or solid-state switches (rated up to 10A) that actually break and make the stop bar circuit, alongside the logic that interprets ALCMS commands and interlock inputs. Housing ranges from a simple IP20 enclosure for equipment-room installation up to an IP66 weatherproof housing where the module sits closer to the field, and status LEDs on the front panel — power, circuit status, communication, fault — give a technician an at-a-glance read without opening a cover or connecting test equipment. Terminal blocks bring in supply power (120/230V AC or 24V DC), interlock inputs from adjacent systems such as the clearance bar or runway entrance, and the communication bus (Modbus RTU or Ethernet) linking the module back to the ALCMS, keeping field wiring, logic, and switching contained in a single serviceable unit.