The Traffic Signal Network

TS2-1998 MMU Compatibility

by Arthur J. Dock

The NEMA TS2-1998 specification made some very important changes to the terminals and facilities (TF) wiring, the controller unit (CU) and the malfunction management unit (MMU). The net result of these changes is an improvement in the way a cabinet exits local flash.

When the cabinet flash switch is returned to the "Auto" position from the "Flashing" position how the cabinet starts up is important. Restarting the controller in preparation for the transition from flashing operation to automatic can be accomplished in various ways in pre 1998 specification cabinets.

Some cabinets are setup to provide a brief pulse to the CU external start input as the switch is put back to the "Auto" position. This generally works acceptably in a TS1 cabinet as long as the reset pulse circuitry remains functional and the controller resets itself in a timely manner. However in a TS2 cabinet the inputs are read through the BIUs at 100 ms intervals which makes it possible for events to happen such that the controller is going through its reset while the MMU is watching the outputs... if the CU does something unacceptable (e.g. jump directly from side-street green to red) the MMU will latch the cabinet into flash and require a site visit by a technician.

Another (better) way of accomplishing this involves resetting the controller when the switch is first put into the "Flash" position. In this type of operation the flash switch places an input to both external start and stop time on the controller. This means that the controller will be in its startup phases ready to go when the switch is moved back to the "Auto" position. The down side of this being that the diagnostic information displayed on the controller is lost immediately when the swich is placed into the "Flash" position.

TS2-1998 uses a third and even better way of accomplishing exit from local flash. A TS2-1998 MMU has a local flash input pin that is active whenever the local flash switch is in the "Flash" position. The MMU is also now always in control of whether the cabinet is in flash because its relays are wired in series with any cabinet switches. The final piece of the puzzle involves the addition of a status bit to response frame 129 which is sent from the MMU to the CU. Going to and from flash using the switch in a TS2-1998 cabinet works as follows:

Going into flash:

Flash switch drops power to the flash transfer relays and inputs local flash to both the CU and MMU
The MMU drops its relays to make sure the cabinet stays in flash

Coming out of flash:

The switch drops the local flash input to the CU and MMU but cannot reenergize the relays because the MMU is holding its relays open
The MMU sets a bit in response frame 129 for 500 ms (5 frames)
The CU upon seeing the bit for 2 consecutive frames (200 ms) does a startup
At the end of 500 ms the MMU closes its relays and the cabinet is brought out of flash

The new operation is clean, simple, and reliable but it raises a few compatibility questions and potential issues. Firstly, if a new MMU is placed in an old cabinet there shouldn't be any problems. However, a 1992 MMU and/or CU in a 1998 wired cabinet can be a problem. For example, since a 1992 MMU doesn't look for local flash (it doesn't have that input pin), in a 1998 cabinet there would be no reset performed when the Auto/Flash switch is placed back in the "Auto" position - the cabinet would drop right into whatever phases the CU happened to be in at the time. Similarly if the CU is not looking for the reset bit in response frame 129 it will not reset and the cabinet will, again, drop directly to whatever phases the CU is in (just 500 ms later than in the first example).

The TS2-1998 operation offers a clean alternative to the less than ideal methods of the past. However, it requires that all of the necessary components (TF, CU, and MMU) be ready to perform as required.

One final word, it is always a good idea to test your MMUs on a regular basis and keep good records.

The Traffic Signal Network

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