Specifications

Engineering Standard –NSW
Signalling SMS 02
Cerberus Level Crossing Monitor Equipment
March 2005 Version 1.2 © Australian Rail Track Corporation Page 38 of 118
This document is uncontrolled when printed.
Refer to drawing M08-861 for details of the cable between the main box
and the input optoisolator. Note that the resistor values have been
changed for 12-15V DC volt operation.
3.5.2 Function
The 12 volt Input Opto Board allows connection for up to 32 signalling
inputs. The inputs can be from 9 VDC to 20 VDC.
3.5.3 Operation
Inputs 1 to 16 from the signalling side (green 8-way connector side) are
connected (via the optoisolator circuits) to output pins 1 to 16 on the D25
connector labelled 'O/P 1'. A cable plugs into this D25 connector to
connect 16 inputs into the level crossing monitor on Input 1. Similarly,
inputs 17 to 32 are connected to output pins 1 to 16 of the D25 connector,
labelled 'O/P 2'. A cable plugs into this D25 connector to connect 16 inputs
into the level crossing monitor on Input 2.
All 32 circuits are identical and have consecutive component numbering.
Referring to Drawing M08-782, the circuit operation is as follows:
When a voltage of between 9 - 20 VDC is applied to input 1 on the 8-way
connector, a current flows across pins 1 & 2 of the 4N35 optoisolator (U1)
and illuminates the internal LED which turns on the internal
phototransistor. This causes the LED (LD1) to illuminate and pin 1 of
'O/P1' to be high (+5 VDC).
Similarly, when the voltage at the input is less than 3 VDC, no current
flows across pins 1 & 2 of the 4N35 optoisolator (U1), the internal LED is
extinguished and hence the internal phototransistor is turned off. This
causes the LED (LD1) to turn off and the voltage at pin 1 of 'O/P1' will be
low (0 VDC).
A +5 VDC voltage is fed to the Optoisolator Board from the level crossing
monitor via the Cables. The Cable that is connected to 'O/P1' connector
feeds +5 VDC to circuits 1 to 16 only. The Cable that is connected to
'O/P2' connector feeds +5 VDC to circuits 17 to 32. Test points next to
each O/P connector are used to verify that +5 VDC is present for each
group of 16 circuits.
Wires from the signalling circuits are terminated on the 8-way (green)
connectors. A positive voltage is switched onto a terminal via a relay
contact, push-button or switch. The terminal on the Input Optoisolator
Board can be likened to the positive terminal of a non-vital relay coil. The
return path for the signalling voltage is via the bottom 2-way (green)
connector (labelled 'N24'). This is a common return for all 32 inputs.
The 2-way (green) connector at the top of the board (labelled 'B24') is a
test point for fault finding. It does not perform any function in the