I was using rotary switches from my electronics scrap box. It
turned out they were make-before-break. That is, if the rotary
switch was set for the red aspect, when I turned it to yellow,
it made contact with yellow before the red was disconnected.
This momentary short of the red and yellow wires would
sometimes confuse the Dual 3-Way. The solution was simple, I
further rumaged around in my electronics scrap box and even-
tually found a break-before-make rotary switch. Now the sema-
phore blades moved properly. Well, almost...
The haunted semaphores
Two nights later I was shooting photos of the semaphore for
this column. What had worked perfectly before now exhibited
an amazing problem. When I put either semaphore in its yel-
low aspect, it promptly changed from yellow to red all by itself!
An e-mail to Tam Valley's proprietor went unanswered (it turns
out Duncan was on vacation). I called my good buddy and elec-
tronics whiz Geoff. We discussed what might be wrong.
At this point I was confident my panel wiring was correct. In
a previous e-mail Duncan had recommended that I not use
twisted pair wires for the cables connecting the control panel
to the servo controller;
"It would be better to have each wire separated than twisted
together. Twisted pair is propagating differential signals like DCC
but it will promote crosstalk in the case of the Dual 3-Way."
This left me speculating about wire capacitance in the new, (non-
twisted pair) wiring). Given there was only 12' of wire, it seemed
surprising to me wire capacitance could cause such a problem.
