GRS Type-F code used +/- steps to transmit controls and long/short steps to receive indications.
The line circuit is uses three (or four) wires: a control line, an indication line, and a common return.
The control line is in series with the return, as well as the indication line.
Controls are transmitted by reversing polarity of the control line circuit, indications are
transmitted from the field by breaking the indication line.
Because they are two seperate circuits, controls can be transmitted and indications can
be received at the same time at the office - thus it is a "duplex" code type.
From what I can gather out of the book, "F" uses 10 steps. The examples show
6 for station selection and 4 for controls, but as with all GRS systems, it probably
can be re-wired into other combinations.
Of all the GRS code systems, this is one of the oldest (manual printed 1937) and most
complex to understand. Its amazing that this was all invented in the 30's.