17.1 The destination signs are manufactured by Luminator, and for the most part, have been very reliable. The only problem have with the sign system is why do we have them to
begin with. With only two destinations in the basic system, (and four once the extensions are open) it seems to be overkill. Yea it's nice to be able to put notices like "ride light rail"
or "take light rail to the Orioles", but who besides people already riding the light rail pay any attention to the signs anyway? It seems to me that sometimes the MTA engineers are awed by all this
techno-gizmo stuff and just gotta have it. The only good thing is that the system has been very reliable, experiencing only a few failures here and there.
17.2 The sign system consists of three major parts: the sign control unit (SCU), operator display keyboard (ODK), and the displays. The SCU resides
under the console in the "A" cab. This unit houses the brains and the trainline interface. There is also an SCU in the "B" cab, but it only houses a power supply for the
ODK. Figure 17-1 is a block diagram of the destination sign system. Sheet 1321 in the wiring diagram manual shows in detail how the units are connected.
17.3 The "A" end SCU contains the brains for the car. It receives commands from either of the ODKs (you don't have to be keyed up for it to respond
to new commands). It sends commands to both the ODKs and the displays. It continuously interrogates all of the units for proper operation, and it listens on the trainline for commands.
All of the SCU's are slaves until one of the cabs is keyed up. Once that happens, it is considered the master, and its commands are not only sent to the units within the car, but also out
over the trainline to the other SCU's (if it is a 2 or 3 pack). If two cars are coupled together with different messages displayed, the car that is keyed up will automatically send its info to the
other "un-keyed up"car(s).
17.4 The ODK units contain a 2 line LCD display and a keypad. The operator enters a destination in by pushing either the DESTINATION A or B button, then punching in
the 2 or 3 digit code, and finally pressing ENTER. The new destination should appear within a few seconds. Similarly, a public relations message is entered by pressing the P/R button,
the number, and enter.
17.5 The displays come in two flavors, large and small, with the large ones on the ends and the small ones on the sides. There are three connections to eachdisplay. One is for
the 39VDC power. The other two are for the IEE-485 communications line, and are interchangeable with each other. The actual display elements are called "flip-dots", and are arranged in a single
line 7x90 matrix (3 sections-30 columns in each). One side of the flip-dots is black and the other is fluorescent yellow. These two colors cover a magnetic element, which is free to rotate in the flip-dot
housing. Under the flip-dot element, is located a "U" shaped magnetic core. The core has a small coil associated with it, which, when driven by the control electronics, will pulse either a "N"
or "S" magnetic field, and flip the dot correspondingly. Drive current thru the coil is around 350ma and the pulse lasts about 1ms. "Writing" to the flip-dot makes it yellow, "erasing"
it makes it black. The magnetic core retains a sufficient residual magnetic field to hold the flip-dot in position for months. Each display, in the drive electronics, contains an IEE-485 line
receiver and transmitter, a microprocessor, power supply, DC ballast, address select, and drive array.
17.6 Communications between the SCU, ODK and displays is done over an IEE-485 twisted pair cable.
17.7 Each component of the destination sign system must have its own address to prevent conflict on the IEE-485 bus. If two units have the same address, you will probably get an error code
on the ODK besides preventing proper operation because two units are trying to talk to the SCU at the same time. Addresses for the various units are:
1 "A" ODK
2 "A" end display
3 "A" side display
4 "B" ODK
5 "B" end display
6 "B" side display
17.8 Programming of the SCU is performed by plugging in a MTU to the front of the SCU. The MTU has an alpha-numeric display on it that
counts up to 100% as it is downloading information to the SCU. When finished, it will flash the message "finished". The MTU is programmed by an ordinary PC over a
3 wire RS-232 line.
17.9 As of early 2007, here are the message assignments:
1 (sign blank)
2 (test pattern)
3 MD DOT
4 MTA TEST TRAIN
17 NO STOPS
18 NOT IN SERVICE
21 HUNT VALLEY
22 BWI AIRPORT
23 PENN STATION
24 CAMDEN STATION
31 FALLS ROAD
32 COLD SPRING LANE
34 NORTH AVENUE
36 CHERRY HILL
38 NURSERY RD
44 TRAINING VEHICLE
50 EMERGENCY/CALL POLICE
100 (odk test/address check)
17.10 In the 2007 timeframe (or so), someone at the MTA got the bright idea to change the "out of service" sign to "finished service". I don't know about you, but that is really
dumb, especially at 5am when they are just coming out of the yard. Thankfully, someone saw the light, and they are now back to using "out of service".
"Finished Service" sign on an LRV on shop track 3S
17.11 Also, in each cab, is the block number sign. This is (supposedly) the only thing on the light rail cars that is made by the same manufacturer as they were on the
old PCC streetcars. The block number is also known as the train number, and if you're listening in on 161.01, this is the number you will hear light rail control calling the trains by.