3.2 MMU Programming, Bench Testing, and Annual Certification
Key Takeaways
- The permissive program card defines which channel pairs may display conflicting signals simultaneously; a jumper or diode present means the pair is permitted, and its absence means the pair is a conflict.
- Removing or seating the program card ajar produces a Diagnostic / Program Card fault, which trips the cabinet to flash independently of any field wiring problem.
- The Red Enable input must be active for red monitoring and minimum yellow change plus red clearance monitoring to operate; when it is inactive, those functions are disabled.
- Manufacturers and agencies recommend that malfunction management units be tested at least annually against factory specifications, using a monitor tester that exercises conflict, red fail, dual indication, clearance, and voltage functions.
- A monitor swap is only valid if the replacement unit is the same type (Type 12 versus Type 16) and receives the correct program card for that specific intersection.
3.2 MMU Programming, Bench Testing, and Annual Certification
The malfunction management unit is the only device in the cabinet whose job is to distrust the controller. Its authority comes from a permissive program card that a technician builds, and from periodic verification that the unit still meets its published response times. Both tasks belong to the field technician, and both are examinable.
The Permissive Program Card
A program card is a passive circuit board that tells the monitor which pairs of channels are allowed to be active simultaneously.
Reading the Logic
The card is organized as a matrix of channel pairs — 1-2, 1-3, 1-4, and so on. For a 16-channel Type 16 monitor the card carries every combination of the 16 channels.
- Jumper or diode installed at a channel pair = the two channels are permissive (compatible). The monitor will accept both being active at once.
- No jumper at a channel pair = the two channels are conflicting. Simultaneous active greens on that pair trip the monitor.
The logic is deliberately fail-restrictive: a blank card makes every pair a conflict, so a missing or mis-built card causes nuisance flash rather than an unsafe permission. The dangerous error is the opposite — installing a jumper that permits a pair which physically conflicts in the field. That jumper silently disables the intersection's only conflict protection for that movement.
Building the Card from the Phasing
The card is derived from the intersection's phase diagram, not copied from another cabinet. In a standard NEMA dual-ring layout, channels 1 through 8 map to vehicle phases 1 through 8. Phases 1 and 6, 2 and 5, 2 and 6, 3 and 8, 4 and 7, and 4 and 8 are compatible pairs in a conventional eight-phase arrangement; pairs that cross the barrier (for example 2 and 4, or 1 and 4) are not. Overlaps and pedestrian channels add rows that must be worked out from the actual movements each channel drives.
A flashing yellow arrow left-turn head consumes two monitored channels rather than one, and the permissive relationship between the FYA channel and the opposing through channel must be entered deliberately. FYA program cards are the most common source of mis-built matrices.
Program Card Faults
Modern monitors detect the card itself. Removing the card, or seating it so that the edge connector is partially engaged, produces a Diagnostic / Program Card fault and drops the cabinet to flash. When a cabinet trips immediately after someone worked in it, the card being ajar is the first thing to check — earlier than any field wiring theory.
Configuration Inputs That Change Monitor Behavior
Beyond the card, several hardwired inputs reconfigure the monitor. A technician who does not know these will chase phantom faults.
| Input | Effect |
|---|---|
| Type Select | Selects Type 16 operation (16 channels, three inputs each: Red/Don't Walk, Yellow, Green/Walk) or Type 12 operation (12 channels, four inputs each: Red, Yellow, Green, Walk) for backward compatibility with TS 1 assemblies. The input is read only at power-up or reset. |
| Red Enable | Enables red monitoring and minimum yellow change plus red clearance monitoring. With Red Enable inactive, those functions do not operate — which is why an intersection in flash does not generate continuous red-fail faults. |
| +24 V Monitor Inhibit | Suppresses the 24 V DC monitor faults during a known power event. |
| Port 1 Disable | Takes the monitor off the SDLC serial link so it operates on discrete inputs only. |
| External Reset | Clears a latched fault remotely; equivalent to the front-panel RESET button. |
| External Watchdog | Monitors a changing logic-level output from the controller. On a typical unit, no state change for 1,500 ms (±100 ms) trips the CVM/Watchdog fault. |
What a Monitor Tester Exercises
Annual certification is performed with a conflict monitor / MMU tester — a purpose-built instrument that plugs into the monitor's MS-A and MS-B connectors and drives the channel inputs. Manufacturers state plainly that the unit should be tested at least annually to ensure proper operation and compliance with factory specifications, and most agencies write that interval into their preventive maintenance program.
The tester walks the unit through each protected function and confirms both that the monitor does trip when it should and that it does not trip when it should not. The published response windows for a representative NEMA TS 2 unit are the numbers to know:
| Function | No Fault (must not trip) | Fault (must trip) | Typical |
|---|---|---|---|
| Conflict | less than 200 ms | greater than 450 ms | 350 ms |
| Red Fail | less than 700 ms | greater than 1000 ms | 850 ms |
| Dual Indication | less than 200 ms | greater than 500 ms | 400 ms |
| Dual Indication (pedestrian channels) | less than 700 ms | greater than 1000 ms | 850 ms |
| Controller Voltage Monitor (CVM) | less than 125 ms | greater than 175 ms | 150 ms |
| +24 V DC Monitor | less than 125 ms | greater than 175 ms | 150 ms |
| Clearance Fail (Yellow + Red) | greater than 2.8 s | less than 2.6 s | less than 2.7 s |
| Port 1 Fail | — | — | 300 ms |
| MMU Power Fail | less than 450 ms | greater than 500 ms | 475 ms |
The no fault column is as important as the fault column. A monitor that trips on a 150 ms transient will put an intersection into flash every time a load switch fires near a voltage sag, generating nuisance calls that erode confidence in the whole system.
The tester also verifies the voltage thresholds: signal inputs must not detect below roughly 15 Vrms and must detect above roughly 25 Vrms in standard mode, and the AC line power-fail monitor must drop out below 89 Vrms and restore above 98 Vrms.
Replacing a Monitor in the Field
Swapping a monitor is a routine repair with three hard rules.
- Match the type. A Type 16 unit dropped into a TS 1 assembly wired for Type 12 will misinterpret the input map. Confirm the Type Select wiring, and remember the input is only read at power-up or reset.
- Move the program card. The card belongs to the intersection, not to the monitor. A replacement monitor with the previous intersection's card is worse than no monitor at all.
- Verify after installation. Confirm the unit powers up, passes its self-diagnostics, shows the expected channel status during a full cycle, and that the intersection completes several cycles without a latched fault. Log the removed unit's serial number, the installed unit's serial number, and the reason for the swap.
LED Signal Heads and Monitor Thresholds
LED signal modules draw a small fraction of the current an incandescent lamp drew. A load switch's off-state leakage current that was harmless with incandescent lamps can partially illuminate an LED module or, more insidiously, can present enough voltage that the monitor reads a dark or failed circuit as active. Manufacturers address this with elevated-threshold modes (marketed under names such as LEDguard) that raise the detect threshold to roughly 70 Vrms so that a leakage-driven LED is correctly classified as inactive. When an agency converts an intersection to LEDs, the monitor's thresholds have to be reviewed as part of the conversion, not afterward.
On a malfunction management unit permissive program card, what does the absence of a jumper between two channels mean?
A technician finishes work in a cabinet and the intersection immediately drops to flash with the Diagnostic / Program Card indicator illuminated. What should be checked first?
During annual certification, a monitor trips to the fault state when the tester applies a 150-millisecond simultaneous green on a conflicting channel pair. How should this result be interpreted?
An intersection is converted from incandescent lamps to LED signal modules and begins generating intermittent Red Fail trips. What is the most likely mechanism?