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