Signal Controllers, Detection, and Communications
Platforms, phasing logic, and what the detector is really saying
Controller questions combine hardware familiarity with timing logic.
Hardware: NEMA TS-1 and TS-2 (Type 1 uses the SDLC serial bus; Type 2 uses Ethernet), ATC controllers, and legacy 170/2070 platforms with their respective rack and detector-rack architectures. Know which cabinet type pairs with which controller and monitor unit.
Logic: actuated versus pretimed operation, the ring-barrier structure, phase numbering, overlaps, detector channels, and the functions that shape a phase - minimum green, passage/gap, maximum green, recall (minimum, maximum, pedestrian), and coordination parameters (cycle length, offset, split, permissive period). Preemption (emergency vehicle, railroad) and transit signal priority override normal coordination.
Key concept: phases in the same ring may never be green together; a barrier separates conflicting phases. Most timing errors in exam questions come from crossing a barrier or from double-counting a pedestrian interval in a split.
Detection: inductive loops (geometry, lead-in, inductance and Q readings), video, radar/microwave, magnetometers, wireless pucks, and push buttons including accessible pedestrian signals. Know presence versus pulse operation and why lock vs non-lock matters at a stop line.
Communications: twisted pair, fiber (single-mode versus multimode), spread-spectrum radio, cellular, and NTCIP/SNMP management. Know that a failed communications link typically leaves the local controller running its local timing plan.
Worked example: cycle 90 s, offset 0, main-street split 50 s, side-street split 40 s. The side street starts green at 50 s into the cycle. If the offset is changed to 20 s, side street green starts at 70 s.
Trap: assuming detection failure always forces flash - usually it forces recall.