US&S 501 cTc Timecode Control System

from the Union Switch & Signal "Circuit Control Scheme" (Manual 501)

Possibly the oldest Union System? (April, 1931)

501 is an 8-step, 81-station code system. Cycle time 1-1/2 seconds per code.
The line circuit uses 3-wires: two series circuits throughout the territory with one common return.

Code steps are generated as follows (from the 501 manual):

"The two independent series circuits constituting the line channel are normally energized.
A complete code action, either control or indication, consists of eight successive interruptions
of the line channel. Each of these interruptions may be made in one of three ways; namely, by
opening only one of the series circuits, by opening only the second of the series circuits, or
by opening both of the series circuits.
These three combinations are designated "X", "Y", and "Z", respectively. Thus, it is obvious that
a three-way selection, with this arrangement, can be made on each step of the code."

These pictures are scanned from the manual - they are not the best pictures though:
Front of a 501 cTc machine
Rear of a 501 cTc machine

Coding unit in center,
line unit to the left,
and three storage cabinets
on the floor.
501 Field Equipment

Top L to R:
Line Unit,
Coding Unit,
4 storage units.
Back of units, showing
cable connections