South Downs Railway

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Detecting Trains

Sensor board numbers used on the railway

LocoNet supports up to 4096 track sensors. Sensors are generally numbered on the basis of 256 sensor boards each of which can have 16 sensors: 256*16=4096. This convention comes from the BDL168 detector having 16 inputs, and having to be assigned a sensor board number when set up.

On the South Downs Railway we have sensor messages generated by DAC20, BDL168, DTM30 and SIGM20 devices. To keep a record, here is the list of sensor numbers in use:

Board type Board number Sensor number(s) Purpose
SIGM20 serial 65 20 1-8 Track occupancy sensors, from BD4
BDL168 21 1-16 Track occupancy sensors
BDL168 22 1-16 Track occupancy sensors
BDL168 23 1-16 Track occupancy sensors
BDL168 24 1-16 Track occupancy sensors
BDL168 25 1-16 Track occupancy sensors
BDL168 26 1-16 Track occupancy sensors
BDL168 27 1-16 Track occupancy sensors
BDL168 28 1-16 Track occupancy sensors
BDL168 29 1-16 Track occupancy sensors in Portsmouth goods yard
BDL168 30 1-16 Track occupancy sensors in Portsmouth goods yard
DAC20 serial 2 161 1 microswitch on Eastleigh lift-up section. ON when flap in normal position.
DAC20 serial 2 161 2-5 Track occupancy sensors, from BD4
DAC20 serial 4 158 1-4 Track occupancy sensors, from BD4
DAC20 serial 684 159 1 microswitch on Portsmouth lift-up section. ON when flap in normal position.
DTM30 160 1-4 interlocks with DTM30 at Eastleigh
DTM30 160 5 button press to signal traincontroller to start oil train schedule
DTM30 160 6 A "toggle" button to signal traincontroller that track 1 is/isn't available
DTM30 160 8 A "toggle" button to signal traincontroller that track 2 is/isn't available
DAC20 serial no. 6 162 1-4 Track occupancy sensors, from BD4
DTM30 ser 457 163 1-16 DTM30 special functions triggered by button presses
DTM30 ser 457 164 1-4 DTM30 interlocks
DTM30 ser 457 164 5-8 DTM30 special functions triggered by button press
DTM30 clanfield 165 1-4 DTM30 interlocks
DTM30 clanfield 165 5-7 & 9,10

special functions: platform 3-1 access & schedule requests

TXC1 240 1 reports occupied while turntable is turning

BDL168 Settings

The BDL168 settings are set as follows. There are more option switches than are published in the BDL168 manual; Stefan Trachsler's excellent "LocoNet-Checker " program finds them all and can edit them.

sw1=t direct home wiring.
sw3=c reverse railsync polarity for transponding detect (not important with no transponding)
sw5=t transponding NOT enabled
sw9=c Do NOT show "occupied" when track power is off
sw10=t detector 16=normal detection
sw11=c do not allow to become LocoNet master
sw12=c do not allow to terminate LocoNet
sw13=t 5s power up delay
sw19=t normal sense threshold
sw25=t 16 LEDs show occupancy (we don't use them though)
sw26=t LEDs would be driven by DCC (not relevant)
sw33=t OPS read detect is enabled (not relevant)
sw36=c ignore GPON message
sw37=t standard OFF timing
sw38=c Extra long release delay for sensors (prevents "chattering")
sw39=t verbose mode not enabled (I don't know what this is)
sw40=t don't reset factory defaults
sw42=c respond to first "interrogate" request
sw43=t don't disable filter for transponding (not relevant to us)
sw44=t don't select extra transponding filter (not relevant to us)
sw45=t send transponding messages at GPOFF (not relevant to us)

 When viewed in the LocoNet-Checker Module configuration window, a check box is ticked for that option being CLOSED. It is unticked for THROWN. 

 

Train Detection

For a manually operated railway, train detection isn't important. However when automatic control is needed, it is necessary to know "where the trains are" so that a computer can decide if it is safe to move trains.

The fundamental need is to be able to split the track into blocks, and monitor the presence of trains in each block. Each block must be longer than the largest train; movement from block to block sets the basis for automatic control. If the block ahead is free, a train can proceed; if it is not, a train must stop.

It is logical for each block to have a signal, which tells the train driver if it is safe to proceed; on this railway, those signals are automatically set. That requires an ability to detect trains.

We need to be clear at this point that the computer control software packages need to know that something is occupying the track. They don't need to know what is occupying the track. Consequently Digitrax's "Transponding" is not required.

Detection Methods

There are two quite different detector technologies:
  • Occupancy detectors determine if a train is present in the track ahead. They do this by monitoring the current fed into each track section. This requires that the track is fed with power in short sections, and that each feed has a current detector. These, nowadays, are sensitive enough to detect a loco that is present but not moving.
  • Position detectorsidentify the presence of a train at a particular location. This might be an optical detector (either using reflected light, or an interrupted light beam), a microswitch, or a reed relay operated by magnets. These can reliably indicate that a train is here right now. However they provide no reliable indication that a train is between one location and the next. Some manufacturers have provided ways to join them together, so that a train passing the next detectors cancels the indication from the previous one: but it is essential to make sure that such a scheme can't be fooled by human hands near the rail etc.

On my railway there is a need for both kinds of detector. Occupancy detectors are great for identifying that there is a train ahead, but not exactly knowing where. This is good to detect if a signal is red or not. Position detectors would be good at identifying the exact time a train reaches a location. This would be good for stopping at a specific position - e.g. in a station or at a signal.

In practice, I've selected occupancy detectors for both roles. A "main" block detector covers the majority of the path to the next signal; a short section covers the piece of track immediately in front of the signal to tell a train to stop if it is not safe to proceed. Some research into stopping distances identified that the train would typically stop within 40-100mm of a new occupancy detector region, depending on speed. In practice the speed would be low by then, with a smaller margin of uncertainty.

I've mostly used Digitrax BDL168 block detectors. A single board provides 16 separate detect channels, split into groups of 4 (each group may be in a different power zone, but all 4 within a group are connected to a single power zone feed). Some of the predecessors of this product had issues with some kinds of loco decoder; but the newer ones are pretty reliable and I've seen them in use with no problems on other computer controlled railways. In the fiddle yard, a late design change resulted in a need for a few extra detectors. The Digitrax BD-4 has been used to provide 4 detection channels, and connected to LocoNet by SIGM20 units.

Allocation of Block Detectors

The block detection sections were numbered on the track plan. Each also belongs to a lettered power zone. A large spreadsheet was used to work out how best to allocate these to the BDL168 block detector outputs; each BDL168 provides 4 groups of 4 detectors, but each group  can only be powered by one power zone.

Track feed numberPower ZoneRev?Track LocationSensor numberWire Label
1 S   Green line Fiddle yard track 5 22,13 S1
2 P   Green line Fiddle yard track 1, exit end 21,1 P2
3 P   Green line Fiddle yard track 2, exit end 21,2 P3
4 P   Green line Fiddle yard track 3 21,3 P4
5 P   Green line Fiddle yard track 4 21,4 P5
6 R   Orange line Fiddle yard track 1, exit end 21,9 R6
7 R   Orange line Fiddle yard track 2, exit end 21,10 R7
8 R   Orange line Fiddle yard track 3 21,11 R8
9 R   Orange line Fiddle yard track 4 21,12 R9
10 S   Orange line Fiddle yard track 5 22,14 S10
11 N Y Reverse loop near Petersfield 22,9 N11
12 L   Green line exit from fiddle yard 23,1 L12
13 M   Orange line entry to fiddle yard 24,1 M13
14 L   Green line Clanfield junction area 23,3 L14
15 L   Green line after Clanfield junction 23,4 L15
16 M   Orange line approach to Clanfield junction 24,3 M16
17 K   Approach road into Clanfield 22,1 K17
18 K   Exit road from Clanfield 22,2 K18
19 K   Clanfield platform 3 22,3 K19
20 K   Clanfield platform 2 22,4 K20
21 K   Clanfield platform 1 22,5 K21
22 L   Green line exit after junction from Petersfield loop 23,6 L22
23 G   Orange line storage road 1 25,9 G23
24 G   Orange line storage road 2 25,10 G24
25 C   Green line storage road 1 25,5 C25
26 C   Green line storage road 2 25,6 C26
27 C   Green line exit from storage road 26,15 C27
28 C   Green line at foot of Butser Hill 26,16 C28
29 G   Orange line at foot of Butser Hill 26,1 G29
30 G   Approach to Portsmouth Loop 26,9 G30
31 G   Orange line running under Butser Hill 26,3 G31
32 G   Exit from Portsmouth Lop 26,4 G32
33 D Y Portsmouth loop line 1 27,13 D33
34 F Y Portsmouth loop line 2 27,9 F34
35 E Y Portsmouth loop line 3 27,5 E35
36 C   Portsmouth line junction to Green line 25,7 C36
37 G   Portsmouth line junction to Orange line 26,7 G37
38 C   Green line to junction for Portsmouth 25,8 C38
39 G   Orange line from junction for Portsmouth 26,6 G39
40 M   Approach road into Eastleigh 24,12 M40
41 M   Orange line by junction for Eastleigh 24,11 M41
42 L   Green line approach to Petersfield 23,9 L42
43 L   Petersfield Platform 1 23,10 L43
44 L   Petersfield Platform 2 23,11 L44
45 L   Petersfield Platform 3 23,12 L45
46 M   Petersfield Platform 4 24,5 M46
47 M   Petersfield Platform 5 24,6 M47
48 M   Orange line exit from fiddle yard 24,7 M48
49 L   Green line entry to fiddle yard 23,16 L49
50 B   Eastleigh depot stand 1 25,1 B50
51 B   Eastleigh depot stand 2 25,2 B51
52 A   Approach road into Portsmouth 28,1 A52
53 A   Exit road from Portsmouth 28,2 A53
54 A   Portsmouth platforms to throat, incoming 28,3 A54
55 A   Portsmouth platforms to throat, outgoing 28,4 A55
56 B   Portsmouth ladder junction to goods area 27,1 B56
57 A   Portsmouth Platform 1 28,5 A57
58 A   Portsmouth Platform 2 28,6 A58
59 A   Portsmouth Platform 3 28,7 A59
60 A   Portsmouth Platform 4 28,8 A60
61 A   Portsmouth Platform 1 START 28,13 A61
62 A   Portsmouth Platform 2 START 28,14 A62
63 B   Eastleigh depot stand 1 STOP 25,3 B63
64 B   Eastleigh depot stand 2 STOP 25,4 B64
65 A   Portsmouth Platform 1 STOP 28,9 A65
66 A   Portsmouth Platform 2 STOP 28,10 A66
67 A   Portsmouth Platform 3 STOP 28,11 A67
68 A   Portsmouth Platform 4 STOP 28,12 A68
69 A   Portsmouth Platform 3 START 28,15 A69
70 A   Portsmouth Platform 4 START 28,16 A70
71 C   Green line at foot of Butser Hill STOP 26,14 C71
72 G   Orange line running under Butser Hill STOP 26,2 G72
73 C   Green line storage road 1 STOP 25,13 C73
74 C   Green line storage road 2 STOP 25,14 C74
75 G   Orange line storage road 1 STOP 25,11 G75
76 G   Orange line storage road 2 STOP 25,12 G76
77 L   Green line after Clanfield junction STOP 23,5 L77
79 M   Orange line approach to Clanfield junction STOP 24,2 M79
80 L   Green line exit from fiddle yard STOP 23,2 L80
81 K   Clanfield platform 3 STOP 22,6 K81
82 K   Clanfield platform 2 STOP 22,7 K82
83 K   Clanfield platform 1 STOP 22,8 K83
84 P   Green line Fiddle yard track 1 STOP, exit end 21,5 P84
85 P   Green line Fiddle yard track 2 STOP, exit end 21,6 P85
86 P   Green line Fiddle yard track 3 STOP 21,7 P86
87 P   Green line Fiddle yard track 4 STOP 21,8 P87
88 N Y Reverse loop near Petersfield block 1 STOP 22,10 N88
89 S   Orange line Fiddle yard track 5 STOP 22,15 S89
90 R   Orange line Fiddle yard track 1 STOP, exit end 21,13 R90
91 R   Orange line Fiddle yard track 2 STOP, exit end 21,14 R91
92 R   Orange line Fiddle yard track 3 STOP 21,15 R92
93 R   Orange line Fiddle yard track 4 STOP 21,16 R93
94 L   Petersfield Platform 1 STOP 23,13 L94
95 L   Petersfield Platform 2 STOP 23,14 L95
96 L   Petersfield Platform 3 STOP 23,15 L96
97 M   Petersfield Platform 4 STOP 24,9 M97
98 M   Petersfield Platform 5 STOP 24,10 M98
99 C   Portsmouth line junction to Green line STOP 25,15 C99
100 C   Green line to junction for Portsmouth STOP 25,16 C100
101 G   Portsmouth line approach STOP 26,8 G101
102 D Y Portsmouth loop line 1 STOP 27,14 D102
103 F Y Portsmouth loop line 2 STOP 27,10 F103
104 E Y Portsmouth loop line 3 STOP 27,6 E104
105 G   Orange line from junction for Portsmouth STOP 26,5 G105
106 L   Reverse loop near Petersfield block 2 23,8 L106
107 L   Green line approach to Petersfield 23,7 L107
108 M   Orange line by junction for Eastleigh 24,4 M108
109 C   Green line behind Butser hill 26,13 C109
110 G   Orange line behind Butser hill 26,10 G110
111 M   Orange line approach to junction for Petersfield loop 24,8 M111
112 S   Green line Fiddle yard track 5 STOP 22,16 S112
113 R   Orange line Fiddle yard track 1, approach end 20,1 R113
114 R   Orange line Fiddle yard track 1 STOP, approach end 20,2 R114
115 R   Orange line Fiddle yard track 2, approach end 20,3 R115
116 R   Orange line Fiddle yard track 2 STOP, approach end 20,4 R116
117 P   Green line Fiddle yard track 1, approach end 20,5 P117
118 P   Green line Fiddle yard track 1 STOP, approach end 20,6 P118
119 P   Green line Fiddle yard track 2, approach end 20,7 P119
120 P   Green line Fiddle yard track 2 STOP, approach end 20,8 P120
121 B   Eastleigh depot far end headshunt 161,2 B121
122 B   Eastleigh depot run past loop 161,3 B122
123 B   Eastleigh depot near end headshunt 161,4 B123
124 B   Eastleigh depot sidings 161,5 B124
125 B   Eastleigh depot stand 1 START 27,2 B125
126 B   Eastleigh depot stand 2 START 27,3 B126
127 B   Portsmouth station loco spur 27,4 B127
128 B   Main throat area for Portsmouth contained yard 29,1 B128
129 B   Portsmouth yard head shunt START 29,2 B129
130 B   Access road to turntable at Portsmouth engine shed 29,3 B130
131 B   Turntable bed in Portsmouth engine shed 29,4 B131
132 B   Portsmouth turntable road 1 29,5 B132
133 B   Portsmouth turntable road 2 29,6 B133
134 B   Portsmouth container yard road 1 START 29,7 B134
135 B   Portsmouth container yard road 2 START 29,8 B135
136 B   Portsmouth container yard road 1 29,9 B136
137 B   Portsmouth container yard road 2 29,10 B137
138 B   Portsmouth container yard road 1 STOP 29,11 B138
139 B   Portsmouth container yard road 2 STOP 29,12 B139
140 B   Portsmouth container yard runaround 29,13 B140
141 B   Portsmouth container hard shunt at far end 29,14 B141
142 B   Portsmouth carriage siding road 1 START 29,15 B142
143 B   Portsmouth carriage siding road 2 START 29,16 B143
144 B   Portsmouth carriage siding road 1 30,1 B144
145 B   Portsmouth carriage siding road 2 30,2 B145
146 B   Portsmouth contained yard runaround START 30,4 B146
147 B   Portsmouth contained yard runaround STOP 30,3 B147
148 B   Portsmouth yard headshunt middle 30,5 B148
149 B   Portsmouth yard headshunt STOP 30,6 B149
150 K   Clanfield platform 1 START 162,1 K150
151 B   Portsmouth carriage siding road 1 STOP 30,8 B151
152 B   Portsmouth carriage siding road 2 STOP 30,9 B152
153 B   Portsmouth engine shed road 1 30,10 B153
154 B   Portsmouth engine shed road 2 30,11 B154
155 K   Clanfield factory 162,2 K155
156 K   Clanfield Factory STOP 162,3 K156
157 K   Chanfield Shunter end 162,4 K157
158 G   Eastleigh junction to Portsmouth approach 26,11 G158
159 G   Orange 1 STOP 26,12 G159
160 C   Green 4 STOP 158,1 C160

Sensors 113-120 are connected to 2xBD4, wired to SIGM20 #6

Sensors 121-124 are connected to a BD4, wired to DAC20 #7, inputs 2-5

sensors 150, 155-157 are connected to a BD4, wired to DAC20 #6. 

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