ABS Signaling with
Semaphores controlled by the
SMC4
You can easily implement a 4
block control system with 2 DBD22s (block detectors) and
a SMC4. By setting the SMC4 to Mode 4, you can use the
inputs connected to DBD22s to move the servos to three
positions. This provides an easy way to implement ABS
signal control with semaphores. The primary input
connector provides a 5 volt source, so DBD22s can be
powered.
In mode 4, the SMC4 inputs are used exclusively to
control four servos to three positions. Outputs 5 - 8 are
still available to control stall motors for turnout
control.
Here are the connections of the DBD22s to a SMC4 for west
bound travel. S1-S4 represent the semaphores and servos
linked to the respective semaphore. Servos S1-S4 are
connected to SMC4 outputs 1-4 respectively. B1- B4
indicate blocks. Also shown are stall motors connected to
the SMC4.
The semaphore backlights are shown wired in series power
from the TV (rectified track voltage) terminal. Lamps
wired in series has a disadvantage in that if one burns
out it is hard to determine which one. However if the
current limit resistor value is chosen so that the lamps
are running below their current rating, they will last a
long time. Also, series wiring results in less current
draw from the SMC4 and reduces the amount of wiring.
This is the same diagram as above with a stall motor
contact and a turnout (SW1) added. The outputs of the
DBD22 are open collector which means that the outputs can
be wired together (and with other devices) for OR type
logic. Here the stall motor contact is wired to the same
SMC4 input as a DBD22 output. In this case the contact
closing will cause the same SMC4 response as B1 becoming
occupied. So semaphore 1 will show red or stop if either
block 1 becomes occupied or the turnout 1 is thrown.
