The Grosser Feldklappenschrank was assembled with specialized modules that were stacked on top of the Untersatz base units. This page will cover the most commonly used modules.
The most commonly used module was the Connection Unit. Connection Units came in 10 and 50 line variations. Each Connection Unit had signal flaps and corresponding jack sockets so the operator could insert the red plug of the Untersatz to answer a call or insert the black plug to complete a call.
Field telephones were wired into the Connection Units via a 30-pin plug on the rear of the unit. 10 of the pins were not used. The remaining 20 pins carried the a and b phone wires.
The Connection Unit drop flaps would drop to signal the operator of an incoming call. When the operator would insert the red plug into the corresponding jack socket to answer the call, the flap would automatically go up.
It is interesting to note that the 50 Line Connection Unit was produced with an integral 10 Line Conference Call Unit at the top. All jack sockets of the Conference Call Unit were connected in parallel, so multiple callers could be plugged into one circuit. A separate 10 Line Conference Call Unit was also produced.
To provide access to outside lines of the automatic exchange network, 3 Line Rotary Dial Amtszusatz Units were provided. This provided direct dial access to the civilian telephone network.
Large exchanges required the use of 150 Line Parallel Connection Units. These units allowed an operator at one switchboard to connect calls with lines at a neighboring switchboard. The Parallel Connection Units were connected to 10 and 50 Line Connection Units and phone connection terminal strips via 30 conductor cables.
The rear of the 150 Line Parallel Connection Unit had two columns of 30-pin sockets, with 15 sockets in each column. Both columns were wired together, so the 30-pins of the socket in the first row of column 1 connected electrically to the 30-pins of the socket in the first row of column 2, and so forth.
The rear of Grosser Feldklappenschrank large exchanges contained a huge number of interconnecting cables, batteries, and polwechslers.
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WW2 German Field Telephone Equipment 