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T400 Controller Facilities Manual

667/EB/20200/000 Page 9-30 Issue 9

9.4.6 Use of Priority Mode for Control in Light Rail Transit Systems

Shown in Figure 9.2 is a block diagram of the additional features available in Priority

mode which may be used when controlling Light Rail Vehicles.

9.4.6(a) Serving Priority Demands in Order of Receipt

In general the controller will serve the Priority Demands cyclically. On receipt of one

or more Priority Demands the controller will move to the next stage cyclically which

serves a demanded Priority Phase. If another stage further round the cycle can serve

this Priority Phase and another Priority Phase, then the controller will move there

instead. This is the same movement strategy as used in VA.

However, when using Priority Mode with a LRT System, in deciding which Priority

Demand the controller will serve next, the controller needs to take into account that

one LRV approach may be used by 2 LRV’s which turn in different directions at the

intersection.

When a signal is received indicating that the LRV Request for Unit 0 is active, a

decision has to be made as to whether this Unit is one of a pair requesting ROW on the

same approach, which must therefore be served in order of receipt relative to each

other or are not the same approach and can therefore be served cyclically. The

information specifying which LRV units have to be treated as pairs is entered at

configuration “associated priority unit” see Figure 21.7 “Associated Priority Unit”

column. Figure 9.2 assumes that Unit 1 is associated with Unit 0.

When the LRV Request for Unit 0 goes active, the state of the LRV request for Unit 1

is tested and if this is not active, then Unit 0 will be enabled, such that when its First

Priority Delay timer has expired, it will set a flag readable by Special Conditioning.

This flag can be used to inhibit moves other than to specified stages if required during

the 2nd delay period. i.e. “During Second Delay Prevent Except Stage” on the

configuration sheets. (This is to enable immediate servicing of the Priority Demand

after the Second Priority Delay by avoiding the controller being caught in a stage

change.)

After expiry of this second delay timer, the request gets passed to special conditioning

(‘B’) provided that Unit 0 request is enabled by the “first come first served logic”, i.e.

provided that a request for Unit 0 is not already present. If a request for Unit 1 is

already present, then the request for Unit 0 is stopped until the request for Unit 1

clears. If the request for Unit 0 clears before 1 clears, then the request for Unit 0 is

ignored.

The Priority Inputs at ‘B’ are inputs to the special Conditioning. The outputs from the

Special Conditioning are shown at point ‘C’. The Special Conditioning outputs ‘C’

connect to the inputs of the Vehicle Priority Software. If the Priority signal does not

require conditioning, then point ‘B’ can effectively be connected directly to point ‘C’

without passing through Special Conditioning.

(For points B and C refer to Figure 9.2.)

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Siemens EH.75M.11. Especificaciones Pagina 90