Cleveland RTA NX Panels
GRS NX (eNtrance-eXit) with moving switch point indicators:

These controlled two interlockings on the airport line

These panels used GRS Syncrostep,
pictured is the office unit.

The B2 code transmitter (upper left) is a
270-code unit. It winds up when the system
is at rest. When a code cycle begins, it
releases and oscillates to generate the code.

Click here to listen to two
control codes being transmitted

Taken from the GRS Syncrostep manual:

"The coded system of remote control described herein is based on the unique principle of using free swings of
two mechanical oscillators - one at each end of the line circuit - to create the steps of a code.
The term SyncroStep is derived from the synchronous stepping thus produced.
The steps are equal units of time in the code cycle. Information is conveyed through
the combination of the presence or absence of energy on the line during each of the steps."

"Syncrostep is a code control system that provides a simple and economical means of remote
controlling large and small interlockings, with a minimum amount of apparatus.
The system has been designed specifically for the control of a single location." - GRS, May, 1952.

It takes about one second to transmit a control and about two seconds to receive an indication.
Equipment for a typical installation

Simplex Stepper Unit (above)

Equipment at field location for a 2-wire
duplex system (right)
Relation of oscillator contacts,
stepping relays,
and steps in a cycle.

Type A relay and base

Type VX polar line relay

Some other SyncroStep machines:

Desk machine

Small desk machine

Floor machine